Lynn Olson's terrific article about the basics of speaker design and construction. PART 1
This is the first section of an article published in the Vol. 3, Issue 6 of Positive Feedback magazine. It is my *personal* view of the art of
designing high-end loudspeakers, and is not intended as any kind of "bible" for folks new to high-end audio. Since I've gotten a lot of questions about the kind of thing that appears in Positive Feedback, it's probably simplest to demonstrate by reposting a complete article in this forum.
---snip---* Looking Over My Shoulder, Part I ---snip---* by Lynn Olson
* Introduction
Where did I come from? Where am I going? Who am I?
These ancient questions of philosophy are the first questions you must ask yourself if you are serious about designing audio equipment. These
questions repeat themselves in only slightly altered form: What is the history of the art of sound?
What is is the true potential of the future? And what, really, do you want to do?
* The Future
If you relax and make a mental journey to the far future, it is easy to
imagine the perfect loudspeaker. It would made of an immense number of tiny point sources that would create a true acoustic wavefront (or soundfield).
Resonances due to massive drivers, cabinets, or frames would be a thing of the distant past. A myriad of waveform distortions (harmonic,
intermodulation, crossmodulation, frequency, phase, and group delay) would be utterly absent ... the sound would be literally as clear as air itself.
This perfect loudspeaker would be made of trillions of microscopic coherent light and sound transducers, integrated with signal processing circuits all operating in parallel. (This is similar to present-day military
phased-array radars, with tens of thousands of tiny antennas with integrated electronics subsystems.) It would be constructed by a
combination of nanotechnology and genetic engineering and operate at the molecular level, appearing simply as a very thin film when not in
operation.
Just for a moment, imagine a thin-film mirror for 3-dimensional sound and images with a near-infinite random-access memory that is also connected to all other mirrors; it would be "transparent" in a lot more ways than one. * The Present
Whew! Now that we've glimpsed perfection it shows just how far we have to go in 1994. Here's a partial list of the major problems we face now:
* 2-speaker stereo falls woefully short of a true acoustic wavefront, producing a phasey, unrealistic image of small size that causes listening fatigue for many people (particularly non-audiophiles). The virtual image is unstable with respect to listener position, spectral energy
distribution, and room characteristics.
Even a simple central mono image has been shown to suffer from deep
comb-filter cancellation nulls between between 1kHz and 4 kHz, which is why a solo vocalist sounds different coming from a single mono speaker and a
conventional stereo pair. It now appears that 2-channel stereo requires a minimum of 3 speakers to faithfully represent the tonal quality of
centrally located sound sources, such as vocalists.
* Large amounts of harmonic, intermodulation, and crossmodulation distortions combine with mechanical driver resonances to concentrate spectral energy at certain frequencies. Driver damping techniques usually improve spectral characteristics (the frequency response curve looks better as a result) but do not provide much improvement for the underlying breakup modes, so the distortions may actually be spread over a much broader
frequency range. The narrowband nature of resonant distortions in
loudspeakers is why a single-frequency THD or IM measurement is useless; it takes an expensive tracking-generator type of measuring system in order to
create a usable frequency vs. harmonic distortion graph.
In order to eliminate these resonant distortions, the driver diaphragm needs to have a density equal to air and absolutely uniform acceleration over the entire surface at all frequencies. As you can imagine, we are nowhere close to meeting this criterion. As a result, all speakers have tonal colorations ranging from subtle to gross, with some types of
colorations present at all times, and other types of colorations appearing only at high or low levels. A reviewer's preferences in music can easily mask the presence of these problems, so make sure to get a second (and third) opinion.
* Standing-wave resonant energy is stored in drivers (all types, except for "massless" exotics), cabinets, and in the listening room itself. The
unwanted mechanical energy must be quickly removed in two ways: rigid, low-loss mechanical links to the earth itself (a rigid path from the magnet to stand to floor to ground), and also dissipated as heat energy in
high-loss, amorphous materials such as lead, sand, sorbothane, etc. The energy that is not removed is slowly released from every single mechanical part of the speaker, each of which has its own resonant signature.
In any real speaker system, regardless of operating principle, there are hundreds of standing-wave resonances at any one time, which are gradually released over times ranging from milliseconds to several seconds. These resonances continually overlay the actual structure of the music and alter the tonal color, distort and hide the reverberent qualities of the original recording, and deform and blur the stereo image.
In speakers that measure "textbook-perfect", this type of "hidden" resonance is the dominant source of coloration. This is also the reason that 1/3 octave pink-noise measurement techniques have fallen out of favor, being replaced by much more sensitive techniques such as TDS, FFT, MLSSA, and others.
* Radiation patterns shift dramatically with frequency, and change sharply at crossover points; in addition, the radiation pattern is further deformed by diffractive re-radiation at every sharp cabinet edge (regardless of
cabinet size or type - this includes planar types of loudspeakers).
reverse-phase phantom sources that combine with the direct sound from the actual driver. These secondary phantom images create significant ripples in the midrange response (up to 6 dB) and create delayed sounds which
interfere with the timing cues necessary to perceive stereo images. These
dispersion problems are audible as room-dependent colorations, harsh or dim midrange and treble, fatiguing stereo, and a "detenting" effect that pulls
images in towards the loudspeaker cabinets.
* This list only covers some of the problems of contemporary loudspeakers. There are other problems, not as severe, but still quite audible to a
skilled listener. These problems occur in all loudspeaker types - dynamic direct radiator, horns, ribbons, electromagnetic planar, electrostatic planar, you name it. They all have lots of THD and IM distortion
concentrated at certain frequencies, they all store and release significant amounts of resonant energy, and they all have frequency-dependent dispersion further degraded by diffractive re-radiation.
This is why I treat claims of "perfection", or of a "major breakthrough", with a big grain of salt. Does it address even one of the serious flaws cited above? Not too often. By contrast, what we're really seeing is a steady, progressive improvement in materials technology and big steps in measurement technology and computer modelling. (I, for one, am grateful, since it's been a major improvement compared to what I had to work with at Audionics in the mid-Seventies!)
* The Past
Modern speakers are far better than the speakers of the Fifties (which I still remember). Very few people had full-blown Altec "Voice of the Theatre" A-5 systems, 3-way Bozak B-305's, or Klipschorns. Most "hi-fi nuts" had to put up with University, Jensen, or Electro-Voice 12" coaxial drivers in big plywood boxes with a single layer of fiberglass on the rear wall. A large cutout served as the vent, resulting in boomy, resonant boxes tuned much too high, with 6 to 12 dB peaks in the 80 to 150 Hz region. (Have you ever heard a restored jukebox?)
The coax, or worse, triax drivers went into paper cone breakup at 200 Hz and above, cavity resonances (due to the horn element mounted in the cone driver) at 800 Hz and above, horn breakup throughout the working range of the short horn, and phenolic diaphragm breakup at 8 kHz and above. A "good"
driver of this type usually had a plus/minus tolerance of 4 to 8 dB, and it took a lot of judicious pen damping to get it to measure that well. It wasn't for nothing that early hi-fi acquired a "boom-and-tweet"
reputation. The sound quality was closer to modern-day autosound, or an un-renovated neighborhood theatre, than a modern system. The tube electronics helped smooth out much of the shrillness, but they couldn't rescue the truly mediocre phono cartridges and loudspeakers of the day. (Even so, the first-generation Quad, the RCA LC-1A, the Tannoy, and the Lowther compare very well with modern systems ... but these were quite rare at the time.)
I find it interesting that second-generation transistor amplifiers like the
Dyna 120, Crown DC-300, and Phase Linear 400 were favorably compared to the classic vacuum-tube Dyna Stereo 70 and Marantz 9 in the early Seventies.
(Even by J. Gordon Holt's "Stereophile" magazine!) That tells us a lot about the resolving power of the best speakers of the early Seventies. Progressive improvements in speaker design now reveal the actual sonic quality of these second-generation transistor amplifiers as quite
repellent, while the "freshened-up" tube amplifiers sound as good or better than many expensive transistor amplifiers made today.
It does make you wonder about the resale value of the current crop of
multi-thousand dollar transistor amps and D/A converters - what will they sound like on speakers ten years hence? By then we'll be listening to
speakers with evaporated diamond diaphragms and other materials with very low distortion and stored energy, a long-overdue high-fidelity digital
storage medium with 24-bit depth and 96 kHz sampling, and the possibility of replacing conventional 2-speaker stereo with something more natural. You can be confident the sound will be far more transparent and realistic than what we're accustomed to now.
* Where Do You Start Your Design?
What kind of sound do you like? People really do hear in quite different ways, and different people assign importance to different qualities of sound. Some audiophiles value timbral (or "tonal") qualities above all else, treasuring the sound of their favorite instruments or voices; some like a sense of immediacy, directness, and emotional impact; some like the sensation of an immense 3D space; and others like a see-through
transparency, a palpable "you are there" quality.
It helps a lot if you know what's important to YOU, what you tend to dismiss, and what you don't hear at all. We all tend to unconsciously define the bounds of "reality" by our own personal perceptions and thoughts, but this just isn't so. Reality is far, far larger than any
personal set of boundaries. That's why getting a second and third opinion is so important.
Since all speakers have serious flaws in the absolute sense, it's up to you to select the qualities that are most important, and most believable, to you personally. "Perfect Sound Forever" is an arrogant marketing slogan, not a realistic goal for an artist. For one thing, the materials to build anything of the sort just don't exist. (Unless you've found a way to
generate a controllable room-temperature plasma. If you have, you'd better talk to the Department of Energy first.)
* Major Schools of Speaker Design
Since all designers are forced to choose on a subjective basis, there is no single "right" or "wrong" way to design a speaker. If anyone tells you that, it might be amusing to investigate their personal beliefs a little
further and see if they worship at a church of religious fundamentalism or the invisible church of "scientific" fundamentalism.
Where do I stand personally? Well, I'm certainly not a fundamentalist! Oh, you meant speaker design!
I care about to spectral flatness, very low energy storage, minimizing IM and FM distortion, and low diffraction, in that order. Subjectively, I seek out an elusive quality I call "the bloom of life"... that rare
"You-Are-There" sensation of being in the presence of musicians creating beauty.
In the section that follows, I'll describe the various paths that designers must choose as they make their way to sonic perfection.
PART 2
Positive Feedback magazine. It is my *personal* view of the art of
designing high-end loudspeakers, and is not intended as any kind of "bible" for folks new to high-end audio. Since I've gotten a lot of questions about the kind of thing that appears in Positive Feedback, it's probably simplest to demonstrate by reposting a complete article in this forum.
---snip---* Pulse Coherent (3D-Imaging) Dynamics
Duntech, Thiele, Spica, and Vandersteen systems fall in this group. The designer takes expensive steps to control diffraction, offset the drivers for a coherent arrival pattern, and usually employs a first-order (6 dB/Oct) crossover. Some, such as Spica, may use 3rd (18 dB/Oct) or 4th order (24 dB/Oct) Gaussian or Bessel crossovers.
This is the only type of dynamic to offer accurate pulse reproduction, sometimes even bettering exotic electrostats or ribbon loudspeakers. However, the audibility of phase, and pulse distortion, is quite
controversial in the engineering community, with the more conservative engineers feeling it is a waste of time and money to ensure accurate pulse reproduction.
In a typical pulse-coherent design, the drivers are asked to be well controlled 2 or more octaves out of their normal operating ranges, so
power-handling and IM distortion are compromised. Expensive drivers are required to partly overcome this problem, along with accurate resonance correction in the crossover. Controlling the radiation pattern with
first-order crossovers and offset drivers is difficult; as a result,
speakers of this type may sound quite different sitting and standing up. The yardstick for this type of design is the spatial quality of the stereo image; if it isn't signicantly better than any other type described below, the design is not a success, since that is the primary potential benefit of this design approach.
* Flat Response (Objective-Design) Dynamics
Most British and Canadian speakers fall in this group. They have very flat spectral responses, with the British paying more importance to the 1 or 2
importance to the frequency response averaged over a forward-facing
hemisphere. These design priorities have been arrived at by BBC broadcast professionals and NRC listening panels respectively, using statistically verifiable double-blind listening tests.
This school of design is most closely identified with an "objective"
engineering-oriented philosophy. Not by accident, the engineers with the most advanced academic credentials tend to design speakers using this philosophy. These folks are not going to be sympathetic to exotic wires, resistors, capacitors, the directly-heated triode mystique, or anything not audible to a double-blind listening panel.
To its great credit, the BBC is known as the first group to accurately
measure and identify sources of driver and cabinet resonances in the early Sixties, and many British speakers still excel in this area. Since audible resonances may be as far as 20 dB below a conventional sine-wave response curve, the BBC was the first organization to identify and measure
colorations that were completely missed by the conventional sine-wave or 3rd-octive pink-noise measurements. These types of measurements are now considered a standard part of FFT, TDS, or MLSSA systems.
Objective-design crossovers are usually 3rd-order Butterworth or 4th-order Linkwitz-Riley, which offer the flattest, most accurate response curve and the best control of out-of-band IM distortion (at the expense of pulse
distortion and overshoot).
Laurie Fincham of KEF deserves credit for designing the first
computer-optimized crossover using acoustically-accurate rolloff curves combined with driver resonance correction (in the early Seventies). This crossover optimizing technique is now available at far less cost by using a 386, 486, or 586 PC with programs such as XOPT or LEAP. Competent designers are now expected to be knowledgeable in the design of
acoustically-accurate crossovers, regardless of their design philosophy. Recent British designs also focus on a high-quality mechanical ground path to the floor (exotic stands). Objective-school designers usually ignore
pulse response, diffraction control, and subjective areas such as
capacitor, inductor, and wire quality. In contrast, research is focussed on steadily improving driver quality, cabinet resonance control, and accurate pair-matching in production.
* Minimalists (Subjective-Design) Dynamics
Some Italian, Scandinavian, English, and American speakers fall in this group. The crossover is extremely simple, sometimes consisting of one capacitor to protect the tweeter (no, I don't know how Sonus Faber protects their tweeter without a capacitor). Drivers and crossover components are of the very highest quality, along with exotic wire and cabinet materials.
Measurements usually play a minor role in the development of this type of speaker. Since this design philosophy leaves driver resonances uncorrected and accepts the resulting frequency and pulse response aberrations produced by the minimal crossover, compatibility may depend strongly on the sonic flavor of the rest of the audio chain. Personally, I find these types of
speakers somewhat colored, but with an exciting, dynamic, and involving sound.
* Horns and High Efficiency Dynamics
Most Japanese high-end speakers fall in this group, as well as a handful of French, Italian, English and American systems. They use design philosophies derived from Western Electric theater speakers, Paul Voigt's Tractrix horn, or a combination of both. Nearly all horn systems are at their best when used with low-powered, sometimes single-ended, Class A directly-heated triode amplifiers, with transistor amps, even Class A types, frequently giving very poor results.
Horns typically have extremely low THD, IM, and FM distortion, reasonably flat response, and sharp cutoff characteristics at both ends of the
frequency range, which may be fairly narrow. Historically, horns were usually beset by intractable problems with impulse response, diffraction, and smooth dispersion, which is why most high-end designers (in the West) avoided horn systems, leaving them to the studio monitor and PA markets. In the past decade, though, Dr. Bruce Edgar in the US, and others in Japan, have made very significant improvements in horn design for high-end and ultra-fi audio, which are just beginning to be recognized by magazines like Sound Practices. According to the movers and shakers in the American ultra-fi renaissance of the 90's(Joe Roberts, Herb Reichert, and Mike LeFevre), these new horns, and the Edgarhorn in particular, are in a class
of their own, superior to ribbons, electrostats, planars, exotic dynamics, etc.
My own opinion? Well, I feel that the trio I mentioned above have pretty good taste, so it's entirely possible I'll also like the new horns. Right
now, I can't say anything until my own set of Edgar midrange horns arrive and I design them into a full-range speaker system. Stay tuned.
* Electrostatic Planars
A few English, American, and Japanese companies make this class of speaker, which I have to admit are old-time favorites of mine. A well-designed
electrostatic offers the most linear and completely uniform diaphragm motion of any class of loudspeaker (and low IM distortion), as well as the potential for the best pulse response. The first Quad electrostatic is the most famous example of a speaker decades ahead of its time.
There is a downside, of course, and that is very low efficiency, an extremely reactive amplifier load, restricted dynamic range, fragility, limited bass, and a tricky room-sensitive dipolar radiation pattern that becomes quite directive at high frequencies. These problems are not easy to solve, particularly the large-panel dispersion, which is not an asset, but a serious problem for stereo imaging.
The old Quad pioneered the most widely used solution, a side-by-side 3-way system, using progressively narrower panels for the higher frequencies. The new Quad uses a complex phased array system which approximates a spherical radiator. The Martin-Logan uses an unusual cylindrical panel relying on
curved damping pads stretched across the perforated high-voltage stators. Problems still remain, though. All of the electrostats I have measured show moderate resonances below 200 Hz (primary room-diaphragm resonance) and multiple sharp resonances above 8 kHz (non-homogenous diaphragm motion and
standing waves in the HV stators or metal grill-frame assembly).
In short, wonderful midrange and depth perspective, and good-but-not-great at the frequency extremes, reasonable-to-fair stereo imaging, and somewhat limited dynamic range.
A few are made in the USA, being represented by Apogee, Magnepan, Eminent Technology, and others. These fall in two classes: the true ribbon, which
is a very thin corrugated aluminum "voice coil" hanging freely like a
streamer in a side-by-side magnetic field, and the magnetic-planars, which are sheets of stretched Mylar or Kapton film with the "voice coil" either printed or glued on the film.
Magnetic-planars use arrays of magnets on the back side of the film (not good for distortion) or on both sides (operating in push-pull, but also creating a cavity between front and rear magnet pairs). The arrays of magnets provide a somewhat uneven drive field, so the uniformity of
diaphragm motion is not in the same class as an electrostat. Then again, HV arcing is not problem, so the magnetic-planars can play a lot louder than their electrostatic cousins.
The magnetic-planars have weak magnetic coupling, which is much lower than a dynamic driver due to the large magnet spacing and shorter length of wire in the gap. In a dynamic driver, a high "BL-product" means a strong
magnetic field in the gap (the "B") and a long helical voice coil immersed in the field (the "L"). Dynamics with a high BL-product provide the tightest amplifier-speaker coupling, which is why they are sensitive to amplifier damping factor and wire resistance. By contrast, in a
planar-magnetic, damping is mostly provided by the stretched film and the air load, and little by the amplifier. Both impedance and efficiency are
lower than dynamics, and attempts to raise both by increasing the amount of aluminum wire on the film usually degrades the transient response.
The true ribbon is free of the stretched film resonances and obstructing magnets of the planar-magnetic, so it offers outstanding pulse response, uniform drive, and a good approximation of a line source, but the
efficiency and impedance are both phenomenally low and it is not usable as a woofer due to the small area. Most practical ribbons either use a
stepdown transformer or ask the amplifier to drive a 1/2 ohm load (not a joke, unfortunately).
Magnetic-planar sound quality is usually midway between a good dynamic and an electrostatic, with a significant freedom from cabinet colorations at
mid and high frequencies. Radiation pattern is similar to an electrostatic, which means problems with imaging and bass, with the minimal amplifier
damping making some rooms unusable. This type of speaker is probably the most room-sensitive of any type, requiring the listener to choose between smooth bass and accurate imaging in some rooms.
These kinds of speakers aren't my cup of tea, but I know many people who really enjoy the neutral, relaxed type of sound they can offer. In
addition, a true ribbon offers some of the best treble around, superior to dynamics or electrostats, exceeded only by the "massless" exotics.
* Hybrids
Each type has a quite distinctive sound, and some, naturally, are hybrids. This gets pretty tricky when the dispersion patterns are different, along with different IM distortion spectra. It helps if the original designer is the one making the decisions, because a consistent philosophy is then being used for the entire system (we hope). Even so, if the designer is
unfamiliar with the strengths and weaknesses of each type, the result can be the typical disjointed "hybrid sound", with the crossover region quite obvious.
The most common blunder seems to be high-performance electrostats unhappily mated to low-performance woofers in thin-walled closed-box cabinets. At
least there's a precedent for this, I guess, going right back to the AR1-W and the JansZen 130 tweeter.
* "Massless" Exotics
One day, I'd like to design one of these myself. The "massless" speakers fall into this category ... Ionovac, Magnat, and Plasmatronics (what a
name!) They DO sound exotic, and measure the same. No resonances at all, and accurate pulse and frequency response up to 100kHz or more. Low distortion too ... like an amplifier. Actually, the "diaphragms" do have mass. But it's not much. It's the same as air, so the acoustic coupling is 1:1. Efficiency is a little difficult to state, though, since the output tube plates of the power amplifier are providing a high voltage that directly modulates a conductive gas.
I remember hearing the Plasmatronics at the 1979 Winter CES, and I must say I've never heard a tweeter that even came close to that one. They darkened the room, and you could see this weird purple glow through the grill cloth
that looked for all the world like a gassy triode ... but it was the tweeter! It glowed and pulsed with the music!
The rest of the speaker, though, was a pretty mundane paper-cone setup in a huge cabinet ... oh well. Even so, the Plasmatronics was a wild thing, a
glimpse of the future, a SR-71 Blackbird next to a bunch of Cessna 172's. Not too surprisingly, the designer was a plasma physicist at Los Alamos Labs. Talk about being ahead of your time! This was 12 years before anyone was talking about turning swords to plowshares, or in this case, Dr.
Teller's atomic toy into the next breakthrough in audio!
The real-world problems? Well, if you ionize air (by using RF heating) you strip apart oxygen-2 and get oxygen-3 (ozone gas). Not very healthy, and illegal in the USA. The Plasmatronics ionized helium gas, which got around that problem, but it required a fresh tank of helium every month (I'm
serious!). I remember seeing a full-size helium tank, gauges and all, in a special compartment inside the subwoofer enclosure.
* Summary
In Part 1, I covered the basics of the art of speaker design from a
personal perspective, briefly illuminating the past, the future, and the
present, and went on to describe the many camps that now exist in the ranks of speaker designers and manufacturers.
In Part 2, I'll discuss the different qualities of dynamic drivers, the way they affect the sound of the total loudspeaker, and the effect they have on the sound quality of the listener's high-fidelity system.
PART 3
The second in a series called "The Soul of Sound" where I wear my professor hat. These articles are my *personal* view of designing high-end speakers. I expect this article to be thoroughly out of date in two years or less ... so enjoy it while it's fresh!
---snip---Looking Over My Shoulder, Part II
by Lynn Olson * Drivers
The speaker driver determines the ultimate potential of the entire loudspeaker, and plays a dominant role in the sound of the entire high-fidelity system. As mentioned in the first part of the series, there is no perfect driver at the present state of the art, and that goal is many decades away, since it requires a driver with a density equal to air, completely uniform motion at all frequencies, and no distortion of any kind.
We have a long road ahead of us ... but take heart! Major advances in
materials sciences are happening right now, with many improvements due to occur in this decade. I confidently expect we'll be seeing breakthroughs
every 2 or 3 years at the present rate of progress.
This is thanks to major advances in computer modelling of mechanical behaviour and the research conducted in the aerospace, automotive, and sports/recreation industries for lightweight, high-performance materials to replace costly and heavy traditional materials. We now have Kevlar, carbon fiber composites, and forged aluminum drivers; we can expect synthetic diamond, ultralow density aerogel-silica glasses, new types of metallic and carbon monocrystals, and new classes of composite drivers before the turn of the Millenium.
* Why Drivers Sound Like That
The major challenge facing the driver designer is combining uniformity of motion (rigidity) with freedom from resonance at mid and high frequencies (self-damping). This is the major tradeoff in speaker systems of all types (except the "massless" group discussed in the first part of this series).
There are additional problems introduced by cavity resonances and magnetic non-linearities, which are discussed later.
* Uniform Motion
Rigidity means accelerations from the voice coil are accurately translated into cone or dome acceleration over the entire driver surface; this
distortion and a transparent, "see-through" quality to the sound. Audiophiles usually describe this type of sound as "fast", much to the
dismay of measurement-oriented objectivist engineers. "How can can a mid or woofer possibly be fast, since the crossover limits the pulse risetime to a
fifth, or tenth, of what any tweeter can do?" This quickly leads to what diplomats call a "full and frank exchange of views", in other words, a mutual exchange of misunderstandings.
As usual, both sides are right, and both sides are wrong. They're just
speaking about different things. What the audiophile is actually hearing is uniform cone motion; this phenomenon can be measured by the absence of IM distortion, a flat frequency response in the working range, and good pulse response with a clean and quick decay signature.
Well, that's great, you might think, just make the cone, or dome, or whatever as rigid as possible. How about a metal, like bronze, perhaps? That's nice and strong, and it can be formed into nearly any shape. You can see the direction this is taking. Bells are made out of bronze. Another problem raises its head .... resonance! After all, why does a bell, or any other rigid metal, ring so long, for many thousands of cycles?
The answer has two parts, one obvious, one not so obvious. First, the metal is rigid, and formed in a shape that increases the rigidity even further. Second, the only path for the bell to release mechanical energy is to the air itself, which takes a long, long time, since the density of air and bronze are quite different, resulting in very weak coupling, and very little damping by the air load. This leads us to another desirable property for the speaker driver, which is ...
* Self-Damping
We also want the voice coil to stop the cone or dome, not have the cone or dome play a tune all by themselves. Unfortunately, the most rigid materials (traditionally metals) have very little self-damping, resulting in
vibrations of very long duration (high Q). One way to control the problem is to extend a heavy rubber surround partway down the cone, and pay a lot of attention to the damping behaviour of the spider and surround materials.
At the present, though, even the best Kevlar, carbon-fiber, or aluminum designs show at least one high-Q peak at the top of the working range, requiring a sharp crossover, a notch filter, or preferably both to control the peak. Unfortunately, this peak usually falls in a region between 3 and 5 kHz, right where the ear is most sensitive to resonant coloration.
Self-damping results in an absence of coloration, as well as contributing to a relaxed, natural, and unfatiguing quality. Interestingly, many
audiophiles (and reviewers, too!) are unaware of the particular sound of driver-material resonance, calling it "amplifier sensitivity", "room
sensitivity", or similar term that seems to point away from the loudspeaker itself.
There are highly-reviewed (by the large-circulation "underground" magazines) 2-way speakers that use 7" Kevlar drivers crossed over to
metal-dome tweeters. Technically, these loudspeakers operate with uniform motion over the range of both drivers; in practice, though, the crossovers are hard pressed to remove all of the energy from the Kevlar breakup region between 3 and 5 kHz.
The reviews of these particular 2-way speakers go on at considerable, and amusing, length about the trials in finding an amplifier that "revealed" the full quality of the loudspeaker. In reality, the reviewer was forced to use an amplifier that was particularly free of coloration in the region where the Kevlar driver was breaking up. Since most audiophiles and reviewers are unfamilar with the direct sound (and measurements) of
commonly-used raw drivers, they can't evaluate how much "Kevlar sound", or "aluminum sound", remains as a residue in the finished design.
This is a problem, by the way, that plagues all current 2-way Kevlar, metal, or carbon-fiber loudspeakers ... at the current state of the art, the 6.5" or 7" drivers are forced to operate right up to the edge of their working ranges in order to meet the tweeter in a moderate-distortion frequency range.
If you lower the crossover frequency, tweeter IM distortion skyrockets, resulting in raspy, distorted high frequencies at mid-to-high listening levels; if you raise the crossover frequency, the Kevlar breakup creeps in, resulting in a forward, aggressive sound at moderate listening levels, and complete breakup at high levels (unlike paper cones, Kevlar, metal, and
carbon fibers do not go into gradual breakup).
This presents the designer with a tough choice: rough sound in the entire treble region, or the characteristic Kevlar forwardness, which can at times actually give a snarly sound to the speaker system. At the present, the best choice is a fourth-order (24dB/Oct.) crossover with a sharp notch tuned to the Kevlar resonance.
I should add, by the way, that I like Kevlar and carbon-fiber drivers very much ... but they are difficult drivers to work with, with strong resonant signatures that must be controlled acoustically and electrically.
As mentioned above, rigid cones have advantages, but are difficult to damp completely. A different approach is to use a cone material that is made from a highly lossy material (traditionally, this was plastic-doped paper,
but this has been supplanted by polypropylene in most modern loudspeakers). The cone then damps itself, progressively losing energy as the impulse from the voice coil spreads outwards across the cone surface. The choice of
spider and surround are then much less critical.
This type of material typically measures quite flat and also allows a
simple 6dB/Octave crossover; personally, though, I don't care for the sound of most polypropylene drivers, finding them rather vague and
blurry-sounding at low-to-medium listening levels. Without access to a B&K swept IM distortion analyzer, I have to resort to guesswork, but I strongly suspect that this type of cone has fairly high IM distortion since it is
quite soft. In addition, it is quite difficult to make a material that has perfectly linear mechanical attenuation; in practice, distortion creeps in when you actually want a progressive attenuation of energy over the surface of the cone.
I think what is really happening is similar to many soft-dome tweeters; the cone is actually breaking up throughout the entire frequency range, but the heavy damping hides this from the instrumentation (but not the ear). To overcome this subjective effect, the best drivers of this type (Dynaudio, Scan-Speak, and Vifa) are actually composites, adding silica, talc, or metal dust to the plastic, which significantly improve rigidity without losing the characteristic polypropylene smoothness.
Even though the dust cap in a mid/woofer (or the dome in a tweeter) looks pretty harmless, the space between dustcap and the polepiece of the magnet creates a small resonant cavity. One example of this was the (in)famous KEF B110 Bextrene midbass driver dating from the early Seventies (as used in the BBC LS 3/5a).
Although this driver was probably the one of the first high-quality midranges available, it also had a number of problems, such as low
efficiency, limited power-handling, a broad one-octave peak centered at 1.5 kHz (corrected by the crossover), and group of 3 very high-Q peaks centered around 4.5 kHz (only slightly attentuated by the BBC third-order
crossover). These upper peaks, which reviewers mistakenly ascribed to the tweeter, were also very directional, which is typical of dustcap
resonances.
The popular tweeters of the 1970's, including the Audax and Peerless 1" soft-domes, also had similar resonances between 9 and 16 kHz, which were partially damped by a felt pad nearly filling the space between the dome and the polepiece. Since the soft-domes were much more lossy than the stiff B110 dustcap, the resonances were much broader and only 1 to 3 dB in magnitude ... but they were still there, and they were responsible for some of the fatiguing quality noticed by attentive listeners.
Not surprisingly, the problems were much worse in the phenolic, fiberglass, and hard paper domes used in the more mundane speakers of the day. (Ah yes ... who can remember such paragons of excellence as the BIC Venturis? The Cerwin-Vegas? The Rectilinears? The JBL L100's? In a prior life, I actually had to sell these awful things! "Wait'll you hear 'Dark Side of the Moon' on these babies!")
Returning to the present, the best midbass and tweeter drivers now sidestep this problem in two ways: a vented polepiece assembly, used by the
Scandinavian manufacturers Dynaudio, Scan-Speak, Vifa, and Seas; and a bullet-like extension of the polepiece, which replaces the midbass dustcap entirely, used by the French manufacturers Audax and Focal.
The Dynaudio Esotec D-260, Esotar T-330D, and Scan-Speak D2905/9000 tweeters are the most notable examples of using a vented polepiece loading into a tiny transmission-line to damp the backwave from the tweeter dome;
their use in the Sonus Faber Extrema and ProAc Response Threes is a comment on how successful this technique can be.
By contrast, the Focal T120 and T120K, which use a rigid fiberglass or Kevlar inverted dome directly above an undamped polepiece cavity, show a series of high-Q resonant peaks at the top of their operating range, which are caused by the resonant cavity coupling to the first breakup region of the rigid dome.
I must admit I was rather puzzled by the public acclaim for these drivers when they first came out; I didn't like the way they sounded, and I wasn't too impressed by their measurements.
>From all accounts, though, the new Focal titanium-dome T120Ti and titanium-dioxide T122Ti-O2 are excellent, and I liked what I heard when I auditioned a speaker that used the Focal T120Ti at a recent Triode Society meeting.
Magnetic Non-linearities
Most audiophiles are aware that loudspeaker drivers are inductive; after all, the voice coil is wound around a ferrous polepiece, and that's how you make an iron-core inductor (or "choke"). Not as many audiophiles know about the myriad of problems this creates.
If the inductance were constant, like an air-core inductor, there would be no problem; just adjust the crossover design to allow for it (using a
simple R-C network) and off you go. Unfortunately, this is an iron-core inductor, and much worse, the inductance varies with the position of the voice coil.
The varying inductance has profound consequences, since the inductance is actually a important factor in determing the upper rolloff frequency of the driver, and its resulting acoustic delay (relative to the tweeter). Vary
this inductance, and the rolloff frequency and acoustic delay move along with it. When does this happen? Whenever the driver moves a significant proportion of the linear region of voice-coil travel, which is less than you'd think. In the excellent 8" Vifa P21W0-12-08, this linear region is only 8 mm (plus/minus 4 mm either way). A more typical figure for linear travel would be 6 mm for most 8" drivers, and 1 to 3 mm for most midranges.
Play some deep bass, and the effects of inductance modulation begin to show, creating IM and FM distortion over the entire frequency spectrum. This is a genuine problem for 2-way systems and 3-way systems using a low midrange crossover; it means that any time you can actually see the drivers move, there are quite significant amounts of IM and FM distortion. What does this sound like? You can expect a loss of resolution that is depends on the bass content of the program material, which may be masked by problems in the power amplifier (such as output transformer saturation or inadequate power supplies).
Are there solutions? Yes. The best drivers from Scan-Speak (SD System) and Dynaudio (DTL-System) plate the polepiece with copper to short out eddy currents induced within the magnet structure by the voice coil. The
specification that gives this away is the voice coil inductance.
The 8" Scan-Speak 21W/8554, probably one of the best 8" drivers in the world, has an inductance of 0.1mH, which is far lower than the 8" Vifa
P21W0-20-08, which has in inductance of 0.9mH. Both are excellent drivers; the Scan-Speak, though, is almost certainly going to have more transparent sound when asked to carry bass and midrange at the same time.
The inductance figure also has a another hidden meaning; remember, the upper rolloff frequency of the driver is the combined function of the mechanical rolloff and self-inductance of the voice coil. If you calculate the electrical rolloff frequency by using the VC inductance and the DC resistance, a few drivers have an electrical rolloff well above the
measured acoustical rolloff. This is desirable; it means that the interaction between the two rolloff mechanisms is going to be small. Other drivers (and this is true of most drivers) are going to have an electrical rolloff well below the measured acoustical rolloff. How is this possible? The mechanical system actually has a broad peak which is masked by the self-inductance of the voice coil. This is not good; any change in
either the mechanical system or the electrical system is going to strongly modulate the frequency and transient response.
This, by the way, is the same kind of problem found in the old
moving-magnet phono cartridges. Most moving-magnets (typically Shure and Stanton) were mechanically peaked, then rolled-off electrically by the
combination of cable capacitance and cartridge inductance. Not
surprisingly, this type of cartridge usually sounded much less transparent than its high-end moving-coil brethren, which had less than one-tenth the inductance and a much flatter, more accurate mechanical system.
In the section that follows, I'll show you how you can make your own
decisions about which drivers you like (and second-guess the manufacturer, reviewer, and your audio friends).
* Selecting A Driver
I use a method that's so crude it might sound kind of dumb; I put the driver on large, IEC-sized baffle (135cm by 85 cm) and listen to it. No crossover, no enclosure, and if it's a tweeter, not very loud at all. I listen to pink noise (to assess the severity of the peaks that may appear
in the sine-wave and FFT waterfall measurements) and music (to get a sense of how much potential resolution the driver posseses).
This does take an educated ear, though, since you have to listen around the peaks that the crossover might notch out, and not hold the restricted
bandwidth against it. However, this listening process tells you a lot about how complex the crossover has to be, particularly if you remember that the crossover can never totally remove a resonance ... it can just make it a
lot more tolerable.
In addition, I very carefully assess the results of the MLSSA PC-based measurement system (using the same IEC baffle), looking at the: 1) Impulse Response. (How fast does it settle to zero? Is there chaotic hash in the decay region or is it a single, smooth resonance? Are there two or more resonances?)
2) The Group Delay vs. Frequency Response. (How ragged is the frequency range above the first breakup? Can it be fixed in the crossover?)
3) The Waterfall Cumulative Decay Spectrum. (Can I accept the resonances that can't be fixed in the crossover? If crossover correction is required, how complex is it going to be?)
the broad, low-level colorations that may appear here?)
Listening and measurements are equally essential. Both give only a partial picture of the actual driver. Even the finest modern audiophile system will have very serious sonic deficits 5 years from now; measurements provide a reality-check on colorations that present-day equipment may not reveal. In turn, the MLSSA system can point out troublesome colorations to listen for; some are much more audible than others.
The thoughtful designer is obliged to be as careful as the craftsman (or craftswoman) who lavishes a full measure of care and attention on even the hidden parts of their creation.
---snap---This is a revision of an article published in Vol. 4-2 of Positive Feedback magazine, a publication of the nonprofit Oregon Triode Society located at: 4106 NE Glisan
Portland, OR, 97232 USA
For reprints, back issues, T-shirts, or subscription information, call John Pearsall at: (503) 234-4155
--
>From Lynn T. Olson
PART 4
The second in a series called "The Soul of Sound" where I wear my professor hat. These articles are my *personal* view of designing high-end speakers. I expect this article to be thoroughly out of date in two years or less ... so enjoy it while it's fresh!
---snip---* Types of Drivers
It helps when you start listening and comparing to have a good grasp on the basic characteristics of the driver, so you can determine if it is a good
example of its type. By listening carefully and examining all of the relevant specifications, you can find out just how well the driver
designers solved the problems of making a good driver. * Paper Cone Drivers
This dates back to the original Rice & Kellogg patent in the late Twenties. Paper ranges in quality from the worst TopTone clock/radio speakers to the superb Scan-Speak 5" cone/dome midrange used in the Theil line of speakers and the SEAS 6.5" midbass used in the Wilson Audio WATT. This oldest of materials is actually a composite structure, and changes properties
significantly when it doped with an appropriate plastic (the choice of doping is invariably a trade secret of the driver vendor). The doping is
quite important, since paper undergoes significant alterations with changes in humidity and time if it is left undoped; the doping stabilizes the
material and typically improves the self-damping.
Strengths are: Good-to-excellent self-damping, potentially excellent resolution and detail, very flat response potential, and a gradual onset of cone breakup. It can be used with low slope linear-phase crossovers without much trouble. Paper is a material that sounds better than it usually
measures ... this is an asset, not a disadvantage.
Weaknesses are: Not as rigid as the Kevlars, carbon fibers, and metals, so it lacks the last measure of electrostatic-like inner detail. Doesn't go as loud as the materials above, but the onset of breakup is much more gradual. Paper-cone drivers usually require modest shelving equalization in the crossover for best results.
Paper is not as consistent as synthetics, so pair-matching isn't quite as exact, which may affect imaging, depending on the precision and quality of manufacture. Properties may change with time, even with doping.
Best Examples are: Scan-Speak 8640 5" cone/dome midrange, with linear response out to 13kHz, very low distortion, excellent pulse response, and excellent inner detail.
SEAS 6.5" midbass (as used in the Wilson Audio WATT, but possibly modified).
I have heard from several sources that Kurt Mueller makes the best paper cones and surrounds, which are used by Scan-Speak, Seas, Vifa, and others, as well as many high-quality proprietary drivers made in Britain and the United States.
* Bextene Cone Drivers
This is an acetate plastic derived from wood pulp, not petrochemicals, and is always damped by a layer of doping material to control the strong first resonance it displays around 1.5 kHz. It was originally developed by the BBC in 1967 to replace paper with a more consistent and predictable
material for monitoring purposes. It came into widespread use in the early Seventies, with the typical audiophile speaker using a 8" KEF or Audax Bextrene midbass driver with an Audax 1" soft-dome tweeter.
The BBC-derived designs always employed equalization to flatten the
Bextrene driver in the midband; the most famous (or infamous, depending on whether you were the listener or the designer) driver was the KEF B110 used in the BBC LS 3/5a minimonitor.
Bextrene has been replaced by BBC-developed polypropylene, which gives a much flatter response, does not require a layer of doping material, and provides a 3-4 dB increase in efficiency due to the decrease in cone mass. Bextrene is now considered an obsolete material by nearly all speaker designers.
Strengths are: Consistent batch-to-batch, excellent potential imaging (by mid-Seventies standards). Inner resolution higher than most paper cones. Weaknesses are: Very low efficiency (85 dB at 1 meter), requires a strong notch filter in the midband, "quacky" coloration by modern standards, sudden, unpleasant onset of breakup at not-so-high levels, and numerous resonances at the top of the working band.
Best Examples are: None. Modern designers are not willing to tolerate the complex notching and shelving equalization required to make these drivers acceptable.
These came into common use in the early Seventies with the introduction of the Peerless 1" soft-dome (remember the tweeters of the original Polk
speakers?), followed by the superior Audax 1" tweeter, which found its way into many British and American designs during the Seventies and early Eighties.
These designs fell into disfavor with the introduction of titanium and
aluminum domes and the Focal inverted-fiberglass domes in the mid-Eighties, which swept the Audax-class soft-dome drivers off the audiophile market. In the last two years, the soft-domes have made a surprising comeback with the introduction of the Dynaudio Esotec D-260, Esotar T-330D, and
Scan-Speak D2905/9000 1" tweeters, which compete on even terms with any metal-dome around. These new designs combine sophisticated
transmission-line back-loading with new dome profiles and new coating materials. As a result, they have the sonic resolution and detail of the best metal domes without the characteristic 22 to 27 kHz resonance. Strengths are: Intrinsic self-damping and potential for extremely flat response and first-class impulse response. Potential for natural, open
sound without intrusive and fatiguing resonances, a valuable quality when listening to many digital recordings.
Weaknesses are: The older class of soft-domes had a dull sound with a
hard-to-pin-down fatiguing quality. Many had quite limited power-handling and required a high-slope 18 dB/Octave crossover to minimize IM distortion. The high-profile dome, required for rigidity, has more restricted HF
dispersion than the competing metal-domes, which have flatter profiles. The new units mentioned above do not have these faults, however, with the exception of dispersion.
Best Examples are: The Dynaudio Esotec D-260, Esotar T-330D, and Scan-Speak D2905/9000 1" tweeters.
* Soft-Dome Midranges
These things are dogs! I've listened to the AR-3, AR LST, ADS systems,
Audax 2", and the Dynaudio D-52 soft dome midranges, and they barked, they snarled, they chewed the rug, and made a mess on any loudspeaker they
approached! They measure flat, all right, but they sound opaque, fatiguing, strongly colored, and 2-dimensional.
The first problem is that soft-dome midranges have a limited bandwidth resulting from restricted linear excursion (1-2 mm typical) and do not gracefully tolerate even a 500 Hz crossover, operating best over a limited 800 Hz to 3200 Hz range.
A second problem is that they are quite prone to side-to-side rocking modes, since there is no spider combined with the surround to force the movement into a linear back-and-forth motion.
A third problem is that the doped silk dome is just, well, too soft for the job it has to do in the power band of the midrange.
The newer class of cone-domes, such as the 5" Scan-Speak 13M/8636 and 13M/8640, and the 5" Dynaudio 15W-75, are another story. These 3 drivers are actually constructed as high-precision cone drivers, not midrange
domes. The only thing they have in common with the soft domes is a large dustcap, which does act as a dome at high frequencies.
This class of driver has much more excursion, much lower distortion, and much wider frequency response than the older soft-dome midranges. The cone-dome drivers are capable of realistic and transparent sound. They are described in more detail in the other sections, since they use Kevlar,
paper, and polypropylene respectively.
Another "special case" is the professional-grade ATC 3" dome with an integral short horn. This driver uses a dual spider to eliminate the
rocking problem that plagues most soft-domes, reducing the IM distortion very significantly. Ron Nelson (of Nelson-Reed) recommended this driver as one of the very best midranges around, and I take his recommendation seriously. This is a very expensive driver (around US$300/each) and needs to be hand-selected so the resonant frequencies of the left and right
channels match.
Strengths are: None. Metal-dome midranges have some potential, but they require sharp crossovers on both ends with an additional sharp notch filter at high frequencies to remove the first (and worst) HF breakup mode. Note: This does not apply to the ATC driver or the cone-domes.
Weaknesses are: High distortion, fatiguing sound, high crossover frequency, limited bandwidth, limited power-handling, and misleading frequency
response measurements. It takes a detailed swept IM distortion measurement and laser holography to get the goods on these drivers. Note: This does not apply to the ATC driver or the cone-domes.
Best Examples are: ATC 3" professional-series - a totally different animal than the usual soft-domes. About 4 times as expensive, though. The Dynaudio D-54 in a Edgarhorn is reputed by the "Sound Practices" group to be the
finest midrange in the world ... yours truly has a pair on order, so I'll let you know.
* Polypropylene Drivers
This material was developed by the BBC in 1976 (my dates may be off) as a replacement for Bextrene. Since it is intrinsically highly self-damping, a correctly designed polypropylene driver is capable of flat response over its working range without equalization. In addition, it typically attains efficiencies of 88 to 91 dB at 1 meter, which is a significant improvement over Bextene.
This material has become nearly universal, since it requires a minimum of hand treatment to assemble a loudspeaker - the only difficult problem was finding adhesives that would stick to polypropylene, and that problem was solved in the beginning of the Eighties.
This material is used in speakers ranging from mundane rack-stereo trash to the first-rank ProAc Response Threes and Hales System Two Signatures. The cone profile and additional materials added to the polypropylene mix
strongly determine the ultimate quality of this type of driver. Strengths are: Very flat response if correctly designed, very low coloration, good impulse response, crossover can be as simple as one capacitor for the tweeter, good efficiency, and gradual onset of cone breakup. The best examples can be as transparent as the best paper-cone drivers, which is an excellent standard.
Weaknesses are: Not quite up to the standard of transparency set by the rigid-cone class of drivers and the planar electrostatics. Many poly
midbass units do not mate well with the popular metal-dome tweeters, with differences in resolution that can be audible to the skilled listener. Not the best choice for woofers 10 inches or larger unless the polypropylene is quite thick and reinforced with another, more rigid, material. Large woofers are better served with stiff paper or carbon fiber.
Best Examples are: The Scan-Speak 18W/8543 7" midbass, as used in the ProAc Response Threes, is probably the finest polypropylene driver in the world. Another closely-ranked contender is the Dynaudio 17W-75 Ext. 7" midbass, as used in the Hales System Two Signature.
The Vifa P13WH-00-08 5.5" midbass/midrange unit is another excellent performer, well suited for midrange or minimonitor use. It is unique in
having a textbook-flat midrange combined with a completely smooth 2nd-order rolloff.
* Metal-Dome Tweeters
Advances in German metallurgy (at Elac and MB) resulted in thin profile titanium and aluminum domes in the mid-Eighties, with drivers from several vendors in Germany, Norway, and France now available. This type of driver can offer very transparent sound, rivaling the best electrostatics if
correctly designed.
The downside is the lack of self-damping, with aluminum coming ahead of titanium in being better behaved in the ultrasonic region. At the present, all metal-dome drivers have significant ultrasonic peaks, ranging in magnitude from 3 dB (excellent) to 12 dB (not so good).
There's controversy about the significance of this peak, since the
all-wise, all-knowing, all-seeing potentates of Philips and Sony have gone to great lengths to make sure that none of our wonderful new "perfect sound forever" recordings will ever have any musical information above 20kHz. Maybe one day we'll finally have a digital system designed with
high-fidelity in mind ... something with an upper limit of at least 32 kHz and a true resolution of 20 to 24 bits.
Strengths are: Uniform piston action right up to the HF resonance,
correctly designed. Dispersion is typically excellent, since the metal domes have flatter profiles than soft-domes.
Weaknesses are: Potential for (I hate to say it) "metallic" coloration
caused by the HF peak intermodulating with the inband sound. Some early designs have restricted power-handling. If significantly overloaded,
breakup distortion is obvious over the whole frequency range.
Best Examples are: Vifa D25AG-35-06 1" aluminum dome, which is even better with the plastic phase disk removed. This dome has a vented pole piece, so
power handling is quite good, and the ultrasonic peak is only about 3 dB even with the phase disk removed. Also, the brand- new Focal T122Ti-O2 is reputed to be outstanding.
* Rigid Drivers Aluminum
The first rigid drivers to find limited use in high-fidelity applications
were the small Jordan Watts 4" aluminum-cone units. The hand manufacture, high price, and low efficiency limited the market for these drivers, and
very few appeared in the United States (I have heard of them by reputation but have never auditioned them personally). Some new British speakers are also using 5" and 7" aluminum-cone midbass drivers, which tend to have very low efficiency and require a notch filter in the crossover.
Expanded-Foam
The next generation were the expanded-foam bass units, with the KEF B139 being the most famous examples. This class of driver offered piston-band operation through the midbass, but suffered from very low efficiency, limited power-handling, and severe high-Q resonances in the midband. (It was not generally known that the B139 had a 12dB peak at 1100Hz with a very high Q. Many reviewers blamed the midrange for problems that were actually caused by the B139 not having a notch filter in the crossover.)
They were quite popular in 3 and 4-way transmission-line systems in Britain (IMF) and the United States (Audionics) in the Seventies.
I remember working with the KEF B139, B110, Richard Allen 7", and front and rear KEF T27's in my first commercial design, the Audionics TLM-200 4-way system. My baptism into the mysterious arts of speaker design went as
follows:
Charlie, my boss: "Hey Lynn! You remember what Laurie Fincham was talking about when we visited KEF last year? All that stuff about impedance
correction and frequency response target functions?"
Yours Truly: "Well, I didn't write it down, but I remember some of it." Charlie: "Great! You can do the crossover for this!" ... pointing at a
massive six-foot-tall loudspeaker with the 4 aforementioned drivers. The previous designer had left town without warning, leaving Audionics with a monster transmission-line complete with drivers and no crossover.True story, folks.
Carbon-Fiber (The Audio Professor Returns to the Topic)
The next generation were the Japanese carbon-fiber units, which made their first appearance in the pro studio monitor (prosound) 12" TAD units with very high efficiencies and very high prices (around $300 each in 1980).
Carbon fiber prices have now dropped, and Vifa and Audax make good examples of this type of driver. The Japanese make lots more of them, having
pioneered the technology, but they have been difficult to obtain if you are a non-Japanese small-run specialist manufacturer.
These drivers have true piston action, outstanding bass and midbass response (the best I have ever heard), but also have nasty, chaotic breakup modes at the top of their range. Removing these breakup modes requires a sharp slope and one or two very sharp notch filters (this type of driver and filter is used in the top-of-the-line Linaeum speaker).
Although I very much dislike drivers that require filters as complex as this (after doing the TLM-200, I vowed I would *never again* design a 57-component crossover), I must admit the Vifa 8" and 10" carbon fiber woofers are the only direct radiators where I've actually felt, not heard, tactile bass.
Kevlar
Kevlar drivers made their appearance in the mid-Eighties with the French Focal and German Eton lines, with the Eton having superior damping due to the higher-loss Nomex honeycomb structure separating the front and rear Kevlar layers. The Eton and much newer Scan-Speak Kevlar drivers now share the limelight as the worlds pre-eminent high-tech drivers.
A unique and quite desirable property of the latest Scan-Speak Kevlar drivers is a smooth rolloff region above the usual Kevlar peak. All of the other Kevlar drivers (that I have measured and listened to) have chaotic breakup regions; the Scan-Speaks are the only ones that appear
well-controlled in this region, which is certain to provide a significant improvement in smoothness and transparency compared to other types. Composite
Audax has made a surprise reappearance in the high-end market with an unusual composite technology, called HD-A. This is an acrylic gel
containing a controlled mix of grain-aligned carbon-fiber and Kevlar fibers. The initial factory measurements show a striking combination of
piston-band behaviour combined with minimal high-frequency peaking and a smooth rolloff above that point.
The Future
Evaporated diamond coatings are now available at low cost with a recent Russian breakthrough (cheap enough to coat floppy disks!), and I very much hope that Scan-Speak and others pick up this technology quickly. As I said earlier, things are changing fast.
* Overall Strengths & Weaknesses of Rigid-Class Drivers
Strengths are: Best available transparency, imaging, and depth presentation of any type, equalling or exceeding electrostats if carefully designed.
High efficiency, high peak levels, and very low IM distortion in the best examples. This class of drivers is considered at the state of the art by many designers, and this field is expected to advance quite rapidly as material technology advances.
Weaknesses are: Older designs have severe peaking at the top of the working band, and nearly all have a uncontrolled chaotic breakup region above the high-frequency peak. This would cause fatiguing sound over the long run and a compression of depth perspective and "air".
Any loudspeaker that does not use a correctly designed notch filter with a Kevlar or carbon-fiber driver can be considered faulty; since the HF peak does not lend itself to correction with a conventional low-pass filter, and will be quite obvious to any listener familiar with the sound of an
unequalized Kevlar or carbon-fiber driver. Unpeaked rigid drivers are not currently available ... stay tuned, this will probably change in less than a year.
Although these types play quite loudly, the onset of breakup can be quite sudden and unpleasant, akin to clipping in a amplifier. Some Kevlar and carbon-fiber drivers require an extremely long break-in period (>100 hours) to soften the fibers in the cone and the spider; this is a fault, since it
indicates the materials may not be mechanically stable with extended use. Best Examples are: The Scan-Speak 13M/8636 5" midrange, 18W/8544 7" midbass, and 21W/8554 8" bass drivers. These are the only Kevlar drivers that have reasonably well-behaved rolloff regions above the high-frequency peak.
The Scan-Speak drivers also have vented pole-pieces that are copper-coated, reducing inductive types of IM distortion by tenfold or more. These drivers are probably at the pinnacle of rigid-driver technology (in Spring of 1993 at least).
The Audax HM130Z0 5.25" midrange, HM170Z0 6.5" midbass, and HM210Z0 8" bass
drivers in the HD-A series also look very interesting.
The German Etons also bear close watching, since the manufacturer is continuing to work on techniques to maintain the cone rigidity while improving the self-damping characteristics.
* More Coming Attractions
they make extensive reference to construction drawings of the cabinet,
suggested placement in the room, and lots of MLSSA measurements. The two articles (The Soul of Sound, Part III and IV) describe an efficient (92dB @ 1 meter) 2-way transmission line suitable for use with triode amplifiers of modest power (8 to 22 watts work just fine). Look for these articles in Vol. 4-4 and 5-1 of Positive Feedback magazine - back issues are still available, as far as I know.
If I can convert the Macintosh PICT artwork to something that will survive the uuencoding process and figure out an appropriate FTP site, I may post these in the future. In the meantime, if you good folks on the Net are
interested, I could post my review of the Reichert 300B and the Ongaku in this forum. (The review articles are entitled "The Outer Limits" and are *not* the Authorized Version from StereoPriest, Absolution Sound, or the Audio Cleric.)
---snap---This is a revision of an article published in Vol. 4-2 of Positive Feedback magazine, a publication of the nonprofit Oregon Triode Society located at: 4106 NE Glisan
Portland, OR, 97232 USA
For reprints, back issues, T-shirts, or subscription information, call John Pearsall at: (503) 234-4155
--
>From Lynn T. Olson
A series of meditations and thoughts on the differences between tube and transistor amps
Yah sure, you betcha, it's time for another Positive Feedback article ... a preview from the next issue, Vol. 5, Issue 2. This is a continuation of the "Soul of Sound" series on the art of high-end audio design.
Readers who are collecting the entire series might notice the absence of Part III and Part IV ... well, those two parts are the construction
articles for the Ariel high-efficiency transmission-line loudspeaker, and I'm still working on the politically correct way to transmit the rather large graphics files over the Net (I may set up an FTP site here at
Teleport). If you just can't wait to find out about the Ariels, you're probably better off subscribing to the magazine, which is chock-full of tweaks, reviews, construction articles, and off-the-wall pieces of all kinds.
---The Soul of Sound, Part V by Lynn Olson
This month's column is a collection of meditations on the art of audio
design. They arise from a series of questions from fellow OTS members and e-mail that appeared in my Internet mailbox - which is:
For those who haven't followed these electronic discussions, here they are in print. I also have a bonus feature at the end of the column, a
newly-arrived little brother for the Ariel. *Frequently Asked Questions*
Q: Why do tubes sound different from transistors? Is this due to "euphonic" second harmonic distortion?
I don't think "euphonic distortion" exists. Granted, some sonic flaws are a lot more tolerable than others, but I've never found any class of
distortion or signal bending that makes the sound more transparent, more real, and more lifelike.
People have been making gizmos like Aphex Aural Exciters, delay circuits, and "T-function shaping" for decades now, and all they really do is make music sound more like commercial FM radio. To my ears, at least, when I remove a known problem (such as a resonant mode in a cabinet, a standing wave on a driver cone, a problem in a power supply, or improve the
linearity of an amplifying element) it pretty much always sounds better ... more real, more truthful, more expressive, etc. In short, fixing
problems makes it sound truer to life.
circuits, despite worse overall distortion? The answer doesn't lie in circuit simplicity, since if that were true, first-generation transistor units like the Dyna PAT-4 and the Marantz 7T preamps would be audio classics. Well, they're not, and a quick listen on even a half-decent
system tells the tale.
I don't like measurements that only apply to the entire "black box", be it a speaker, an amp, a CD player, or whatever. They tend to mislead both the reviewer and the buyer. The measurements that tend to be the most
meaningful are ones you can make on elements of the black box ... an active device, a single driver, the way the crossover behaves far
off-axis, a single reflection at a cabinet edge, the frequency vs. source impedance of a power supply, etc.
In other words, measurements are primarily useful as a tool for the designer to isolate problems, not as a go-nogo signal for the reviewer or buyer. The reason is pretty simple: when you work at the component level, you can actually isolate cause-and-effect, and deal directly with the
cause of the problem. When you get the vector sum of all of the stuff in the black box, the measurements get a lot less useful, unless the designer really made a big mistake.
Case in point: I've lived with the Audio Note Ongaku SE211, as well as the Kassai PSE 300B and the Reichert SE 300B. I've also had access to my trusty Audionics CC-2 (not a bad transistor amp), a modern multi-kbuck Class A transistor unit, and a souped-up Dyna Stereo-70. They all sounded different, particularly to non-audiophile friends.
The Ongaku, by far, had the worst THD and power measurements ... 22W at 3% distortion. It also made the Ariel sound better than any electrostat I've
ever heard ... in fact, the best sound I'd heard in many years. It
certainly sounded better than anything I heard at the 1994 Winter CES. So what's going on here? Maybe THD is simply measuring the wrong thing. I have a hypothesis I want to explore. Any of you who have access to
serious measuring equipment are invited to join in. The hypothesis is based on a very simple premise:
If it sounds much clearer, more natural, more true-to-life, that implies a problem (or type of distortion) has been removed. (Even if the problem