INSTALLATION KNOWLEDGE & TECHNIQUE
NEGATIVE HALF
■ Figure 65. One complete wave cycle showing positive and negative phase.
Destructive Interference
+
■ Figure 66. Destructive Interference.
If two speakers are mounted side-by-side but one is further forward than the other (physical alignment), when the speakers are in phase, the wave from one speaker will interfere with the other to some extent.
■ The two waves are not starting from the same point, even though they are starting at the same time.
■ The sound will be out of phase anywhere from 1º to 359º degrees.
■ In Figure 67 (below), the signal is out of phase by 90º degrees. The result is a new wave that’s both reduced in amplitude (due to interfer-ence) and at a different phase than its two parents. This occurs acousti-cally because you can only wire a speaker polarity to be 0º or 180º degrees, depending on whether or not you reverse the two leads.
In figure 66 (previous page), both speakers are wired identically; however, one is moved ahead or behind the other by 25% of its wavelength (90º degrees is 25%, or one-quarter, of 360º degrees).
■ If a 500 Hz tone is used, the wavelength would be 1130/500 = 2.26 feet.
One-quarter of 2.26 feet is .56 feet.
■ By moving one speaker behind the other by slightly more than six inches, you would have a combined wave that is 90º degrees out of phase.
■ Move it to 1.13 feet apart and the two tones would cancel each other out (180º degrees).
Since we know that the wavelength of higher frequencies is shorter than low fre-quencies, manufacturers will often build home speakers that have the tweeters set back slightly from the woofer so that the voice coils are aligned.
This is often referred to as time alignment, and it compensates for the different sizes (lengths) of the wavelength.
Margin Notes
✍
Time alignment compen-sates for the different sizes (lengths) of the wavelength.+
+ =
=
SUM OF 2 IDENTICAL SIGNALS 90° OUT OF
PHASE
SUM OF 2 IDENTICAL SIGNALS 180° OUT OF
PHASE
■ Figure 67. Sine wave example of front wave 90 and 180 degrees out of phase.
Here’s how you can tell when a four-speaker, full-range system is out of phase (or having most commonly an inverted polarity) by just listening:
1 Balance the system left to right.
2 If the bass is strong on one channel and weak on the other, and the midrange and highs do not sound distinct, go to the amp (or head unit's internal amp) and reverse the speaker lead on one side only.
3 Listen to the system again.
4 Balance left to right to see if the distance or “muddiness” has cleared up and listen to see if the bass seems tighter. If so, the speakers could be out of phase or have an inverted polarity.
Depending on you situation you could have a problem that is corrected simply by reversing polarity electrically (this is a simple fix). If the problem is phase, you could also have to change speaker position or location.
RESONANCE
All objects have a natural tendency to vibrate at certain frequencies.
■ A crystal wineglass, when tapped, will vibrate air to produce a tone some-where between 2 kHz and 6 kHz.
■ The smaller the glass, the higher the pitch of the tone.
If you were to play a tone at the same frequency that the glass produces, the glass would begin to vibrate on its own. This is known as sympathetic vibration and is caused by the natural resonance of the object.
A bass drum will respond to frequencies around 30 Hz to 130 Hz. Playing a tone in this range, and at a high enough volume, will cause the bass drum to vibrate severely.
FREQUENCY RESPONSE
Frequency response is one of the most important sonic measurements for deter-mining quality. Frequency response is the relationship between each individual frequency and its amplitude (this includes refraction and absorption, or con-structive and decon-structive interference). To have a flat response, a system must reproduce all frequencies in the human hearing bandwidth (20 – 20k Hz) with equal amplitude.
Margin Notes
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Frequency response is the relationship between each individual frequency and its amplitude.A waveform that has equal amplitude from low bass to high frequency (full band-width) is considered to have flat response.
■ A waveform with peaks and valleys is uneven in sound and can be annoy-ing to listen to.
■ Irregularities in the midrange areas are quite noticeable – humans are most sensitive at 1k Hz.
■ Peaks are much more noticeable than dips.
Therefore, choosing speakers that reproduce the entire audio spectrum smoothly and without voids is crucial to the performance of the system.
■ The mounting of the speakers is equally critical to ensure the smoothest possible bandwidth performance.
In car audio systems, three things affect the linearity or smoothness of sound:
1 The vehicle’s size and shape cause resonance at certain frequencies and cancellation of others at the seating position.
2 The materials used in the interior – such as glass, plastic and soft fab-rics -- result in either reflections or absorption of the sound (they are either constructive or destructive).
3 Road noise and other ambient noise will mask sounds, primarily in the bass regions. But noise will not effect the linearity of the sound.
An equalizer can help control these external factors, but only after the original acoustic causes have been identified and dealt with.
■ An equalizer should not be used as the only way to fix acoustic problems that could have been taken care of with speaker placement, crossovers, gain adjustments, etc.
Margin Notes
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A waveform that has equal amplitude) from low bass to high frequency (full bandwidth) is considered to have flat response.FREQUENCY POOR RESPONSE AMPLITUDE
20 20K
FREQUENCY EXCELLENT RESPONSE AMPLITUDE
20 20K
■ Figure 68. Example of excellent and poor frequency response.
■ An equalizer should be used to compensate for personal preferences or the differences in recorded material, not an obvious hole in the midbass or a peak in the midhighs.
■ Excessive boosting of the equalizer controls can result in premature curv-ing and can possibly damage speakers.
The human ear is very sensitive:
■ The ear is most sensitive to midrange, which is the human vocal region.
■ At low bass and high frequencies, the ear is less sensitive.
■ High frequency ability deteriorates with age (interestingly enough, women retain their high frequency perception better than men.)
In order to hear flat response, you must often boost the amplitude (volume) of the bass and treble regions and reduce midrange to counteract what is known as Fletcher-Munson curves which depict the uneven frequency response of human hearing.
■ Overcoming this is most commonly achieved with the use of a graphic equalizer. However, remember that the frequency response of any sound system is affected by the environment in which it works.
Figure 69 (below) shows graphs called the Fletcher-Munson Equal Loudness Contours. Using 1,000 Hz as a reference, they illustrate the relative levels that must be produced acoustically to sound equally loud.
Margin Notes
“An intensity level at which sound just becomes audible in an average person with good hearing.”
■ Figure 69. Fletcher-Munson Equal Loudness Contours.
OCTAVES AND HARMONICS
An octave is a musical interval between two tones formed when the ratio between the frequencies of the tone is 2:1 (i.e., a doubling or halving of a frequency).
■ Unless it is an electronically produced pure tone, a musical note will have overtones and harmonics.
■ A musical note has octaves that double the previous octave.
■ A note at a frequency of 440 Hz has octaves at 880 Hz, 1760 Hz, 3520 Hz, etc.
■ Each octave has eight full tones above and below another given tone.
■ Octaves also relate to the vocal range of a singer or musical instrument, as in soprano, alto, tenor, basso, etc.
The fundamental frequency created by most musical instruments – with the exception of synthesizers and pipe organs – is limited to about 8 kHz.
A harmonic is a weaker overtone of the original note (the fundamental frequency) and is responsible for the character of the note.
When a musical note with a complex waveform has a distinct pitch (as opposed to just plain noise), that waveform can be created by combining a set of precisely relat-ed sine waves. These sine waves are callrelat-ed harmonics. We recognize voices on the tele-phone because people sound different due to the harmonic content of their voices.
■ Harmonics occur at frequencies that are multiples of the original note.
■ A note at a frequency of 440 Hz may or may not have harmonics at 880 Hz, 1320 Hz, 1760 Hz, 2200 Hz, etc., and sub-harmonics at 220 Hz, 110 Hz, 55 Hz, etc.
If two instruments have exactly the same harmonic structure and strength, they will sound identical. If only one harmonic is absent or significantly altered, a dif-ference would be discernible to those with excellent hearing.
Speaker basics:
■ A basic speaker has a narrow frequency range, depending on the size of the speaker cone.
■ Large speaker cones naturally produce bass, while small cones produce higher frequencies.
Since a single speaker cone cannot cover all of the frequencies in the audible spec-trum, manufacturers combine a larger bass cone with a small treble driver – or tweeter – to more effectively cover the entire spectrum. The overall intention is to improve the frequency response of the system to give you a flat response and improve the sound quality.
Margin Notes
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An octave is a musical interval between two tones formed when the ratio between the frequencies of the tone is 2:1.✍
A harmonic is a weaker overtone or undertone of the original note (the fundamen-tal frequency) and is respon-sible for the character of the note.Special cabinet-style speakers have been designed to fill in the lower bass region, these regions are difficult for virtually any vehicle to support.
■ Subwoofer systems are intended to accurately reproduce the sub-bass region.
SIGNAL TO NOISE
Signal to noise (s/n) is a ratio that indicates how much audio signal there is in relation to noise, under specific conditions.
■ A high s/n ratio is always preferable to a low s/n.
■ This ratio of audio output level to the level of noise is expressed in decibels (dB).
■ The s/n ratio usually begins at the noise level, whether high or low, and goes to some arbitrary nominal level.
A musical note can be masked in a number of ways. Acoustically, the ambient noise that occurs in the vehicle – as well as the road, wind, and traffic – will com-bine to mask the quieter musical passages. This is referred to as the “noise floor,”
and is concentrated mostly in the bass regions.
■ Many of today’s cars -- particularly the luxury cars -- do an excellent job of lowering the noise floor by insulating the outside noises.
Electrically, the circuit noise present in electronic products will mask the very low-level signals that try to pass. The design of the product generally dictates how noisy it will be.
■ When parts of lower grade (more economical) are incorporated into a design, they will contribute to the thermal noise that infects the musical signal.
■ Quiet passages in the music will be covered by hiss.
Margin Notes
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Signal to noise (s/n) is a ratio that indicates how much audio signal there is in relation to noise, under specific conditions.HARMONICS OCTAVES
FUNDAMENTALS
SUB HARMONIC
220 HZ 440 HZ 880 HZ 1760 HZ 3520 HZ
■ Figure 70. Examples of octaves, harmonics and their fundamental tone.
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A high s/n ratio is always preferable to a low s/n.DYNAMIC RANGE OF A MUSIC RECORDING
Dynamic range is the range of volume, in DeciBels from the softest to the loudest, produced by a source of sounds. The reference is usually a musical selection, or program signal being played.
■ A program is a structured, narrow band signal, while noise is a random, wideband signal.
■ Rock and heavy metal music have a low dynamic range since the dif-ference between the quiet lead guitar solo of 100 dB SPL and a full crescendo of 130 dB SPL is only 30 dB.
■ Classical music can have a quiet flute solo of 60 dB SPL, followed by a crescendo of 110 dB SPL that results in a dynamic range of 50 dB.
Even though the rock selection is louder overall, the classical selection has far greater dynamic range.
HEADROOM
Headroom is one of those terms that are quite common in audio jargon. But ask anyone to define headroom, and they’re at a loss. For once and for all, here is a definition: In an audio device, headroom refers to the difference in levels between the highest level in a given signal and the maximum level that the unit can han-dle without distortion.
■ Obviously, more headroom is desirable.
■ The music can have short peaks that are much higher in level than the average signal level. For example, a musical crescendo consumes a lot of power and can quickly push a system to its limits.
■ These short peaks are not registered by most audio level reading devices and if the musical demand is higher than the system’s ability to track it, the result is severe distortion (clipping) and probable damage.
Think of headroom this way:
■ Imagine you’re jumping on a trampoline in a room with a low ceiling – the consequences could be painful.
■ If you could raise the ceiling (add more “headroom”), then the chance of hitting your head on the ceiling would be lessened.
In terms of a concert, the average sound level (100-110 dB SPL) is the nominal program level.
■ The difference between the highest (peak) levels and the nominal level is the headroom.
■ The classical music in the last section has more headroom than the rock concert.
Margin Notes
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Dynamic range is the range of volume, in DeciBels from the softest to the loud-est, produced by a source of sounds.✍
Headroom in an audio device refers to the differ-ence in levels between the highest level in a given signal and the maximum level that the unit can handle without distortion.■ Classical music is usually much more demanding on a sound system than rock music just for these reasons.
Autosound can be studied in greater depth in the MECP First Class Study Guide.
Section 2
Introduction to Security
When it comes to a security system, the system is only as good as its installation – and you have a direct impact on the quality of the installation. Accordingly, before getting to the basics, always be sure to follow these installation guidelines for security systems:
■ Use caution in determining component locations.
■ If possible, refer to the vehicle’s owner’s manual to ensure the security sys-tem is installed in harmony with the other components on the vehicle (there might be some major differences in the operating characteristics and wiring compared to previous models).
■ Make sure all the connections are solid – use solder whenever possible.
■ Use a multimeter to check all wires – never use an incandescent test light.
Margin Notes
■ Figure 71. Dynamic range and headroom.
30 dB SPL Ambient Noise Level 90 dB Dynamic
Range
70 dB S/N Ratio 20 dB
Headroom
120 30
Maximum Sound Level (Threshold of Pain) 120 dB SPL
(dB SPL) 110 100 9030 80 70 60 50 40 20 10 0
BASIC COMPONENTS OF A SECURITY SYSTEM
Most security systems come with a variety of components that can perform many different functions. The majority of systems you will be working with will include the following parts:
■ Control unit
■ Siren
■ Switch triggers
■ Sensors
■ Engine disable(s)
■ Remote control
■ Accessory output devices
The Control Unit is the main element of all security systems. A control unit has the electronic circuitry necessary to control all the functions of the security system. It has the ability to arm and disarm, monitor triggers, and react to an intrusion.
Most control units also include circuitry designed to enable/inhibit the operation of the engine. In addition, control units can include outputs designed to reflect the status of the system. These can take the form of visual (LED outputs), or Audible sometime referred to as “beepers”.
Margin Notes
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The Control Unit (some-time called the “Brain”) has the ability to arm and disarm, monitor triggers, and react to an intrusion.■ Figure 72. Typical security system.
■ Since control units are the central governing element, they are often called the “brain” of the security system.
■ Control units chirp sirens, blink parking lights, and sound alarms in response to commands or intrusions.
■ More sophisticated systems include multiple trigger inputs that enable you to connect a different area of the vehicle to a different trigger input.
■ This allows the installer to divide up the vehicle into separate
“zones” for the purpose of easier system management and trou-bleshooting.
■ Some systems with multiple trigger inputs include a method to indi-vidually monitor and verify each trigger, sometimes called “diagnostics.”
■ This aids the installer or consumer in detecting and isolating a trig-ger-related problem.
SIRENS
The electronic siren is the most common form of sounding device found as stan-dard equipment in today's automotive security systems. Usually, this device is a self-contained unit consisting of three basic sections or stages:
1 The oscillator or tone generator stage 2 The amplifier stage
3 The speaker or output stage
The typical electronic siren can, therefore, produce its warning sound simply by being connected to the proper power source.
Electronic sirens come in all shapes and sizes, but the most common is the bell or horn shape.
■ The horn shape contributes to overall volume as well as pitch.
■ The outer casings of some sirens are composed of metal, but the great majorities are made from various types of high temperature plastic.
SWITCH TRIGGERS
Switch triggers come in many different forms. The most common is the simple spring-loaded pinswitch. Others include the roller push-button type, the mag-netic reed switch type, metal pressure strip type, and the mercury tilt type. Each of these types has advantages and disadvantages.
There are several types of triggering devices, we will cover just a few of the most common ones:
Margin Notes
■ Spring-Loaded Pinswitch - This type of switch usually consists of a spring-loaded plastic plunger set within a cylindrical metal housing that is threaded at one end.
■ Roller push-button - The roller push-button type is very similar to the spring-loaded pinswitch except that instead of using a straight linear plunger, it uses a ball or “roller” to push against an internal plunger and set of contacts.
■ This feature makes it ideal for applications where a normal spring-loaded pinswitch would shear off, such as uneven surfaces and sliding panels.
■ This switch is typically used to protect truck tailgates, drawers of tool boxes, hoods, and other compartments that slide open.
■ Magnetic Reed Switch - The magnetic reed switch (also called a magnetic proximity switch) uses magnetic force to cause a set of contacts to connect.
This switch basically comes in two parts: a switch and a magnet.
■ The switch contains a set of magnetic “reeds” or two thin flexible slivers of metal, each with a contact at one end.
■ These reeds are enclosed inside a glass tube, insulated from each other and attached to a stationary point at one end.
■ Each reed is connected to a wire, and each wire is run outside the glass tube.
■ The whole switch assembly is typically placed inside a rectangular plastic case.
SENSORS
Motion Sensors - A motion sensor is designed to detect motion – but what kind
Motion Sensors - A motion sensor is designed to detect motion – but what kind