Microphones are normally fairly small so that they will have minimal effect on the sound field they are sampling. There is a limit, however, and it is difficult to manufacture studio quality microphones smaller than about 12 mm (0.5 in) in diameter. As microphones operate at higher frequencies, there are bound to be certain aberrations in directional response as the dimensions of the microphone case become a significant portion of the sound wavelength. Diffraction refers to the bending of sound waves as they encounter objects whose dimensions are a significant portion of a wavelength.
Many measurements of off-axis microphone response have been made over the years, and even more theoretical graphs have been developed. We will now present some of these.
Figure 3.12 shows polar response diagrams for a circular diaphragm at the end of a long tube, a con-dition that describes many microphones. In the diagrams, ka 2a/, where a is the radius of the diaphragm. Thus, ka represents the diaphragm circumference divided by wavelength. DI stands for directivity index; it is a value, expressed in decibels, indicating the ratio of on-axis pickup relative to the total pickup integrated over all directions. Figure 3.13 shows the same set of measurements for a
fIgure 3.12
Theoretical polar response for a microphone mounted at the end of a tube. (Data presentation after Beranek, 1954.)
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microphone which is effectively open to the air equally on both sides. It represents the action a ribbon microphone, with its characteristic “figure-eight” angular response.
Figure 3.14 shows families of on- and off-axis frequency response curves for microphones mounted on the indicated surfaces of a cylinder and a sphere. Normally, a limit for the HF response of a micro-phone would be a diameter/ ratio of about one.
fIgure 3.14
On- and off-axis frequency response for microphones mounted on the end of a cylinder and a sphere.
(Data after Muller et al., 1938.) fIgure 3.13
Theoretical polar response for a free microphone diaphragm open on both sides. (Data presentation after Beranek, 1954.)
75 In addition to diffraction effects, there are related response aberrations due to the angle at which sound
impinges on the microphone’s diaphragm. Figure 3.15(a) shows a plane wave impinging at an off-axis oblique angle on a microphone diaphragm subtended diameter which is one-fourth of the sound wavelength. It can be seen that the center portion of the diaphragm is sampling the full value of the waveform, while adjacent portions are sampling a slightly lesser value. Essentially, the diaphragm will respond accurately, but with some small diminution of output for the off-axis pickup angle shown here.
The condition shown in Figure 3.15(b) is for an off-axis sound wavelength which is equal to the sub-tended diameter of the microphone diaphragm. Here, the diaphragm samples the entire wavelength, which will result in near cancellation in response over the face of the diaphragm.
referenceS
Beranek, L., 1954. Acoustics. New York: J. Wiley.
Muller, G.G., et al., 1938. The Diffraction Produced by Cylindrical and Cubical Obstacles and by Circular and Square Plates. J. Acoust. Soc. Am. 10, 6–13.
Robertson, A., 1963. Microphones. New York: Scientific American Books.
fIgure 3.15
Plane sound waves impinging on a microphone diaphragm at an oblique angle. Microphone diaphragm subtended diameter equal to /4 (a); microphone diaphragm subtended diameter equal to (b). (Data after Robertson, 1963.)
77 The second chapter on microphone technology by John Eargle focuses on the electrical aspects of
microphone operation, including phantom powering, microphone transformers, mixing console microphone preamplifiers, microphone splitters, cable losses, ground loops, RF microphones, and the latest so-called “digital” microphones.
The powering of microphones is a big subject in itself. Dynamic microphones, in which a coil attached to the diaphragm moves in the field of a permanent magnet, do not require external power, but they are not generally considered to be in the first line of quality. Capacitor microphones do need power, and the standard method is “phantom powering” which applies 48V to the hot and cold signal con-nection through two 6k8 resistors. This puts a strict limit on the amount of power that a microphone can draw, restricting the flexibility of design for the electronics. To address this problem, work on set-ting up a “super phantom” standard with 2k2 resistors is proceeding. John also describes the unloved and obsolescent technology known as T-feed or T-powering.
Virtually all high-performance microphones are capacitor types that require a substantial polarizing voltage on the capsule diaphragm. The capsule polarizing voltage is often in the range 60–65 Volts, and since this is greater than the phantom voltage, some sort of DC-DC conversion is required. Low-voltage power is also required for buffer stages to drive the loading of a connecting cable and a mixing console input.
A capacitor capsule has an extremely high output impedance, equivalent to a very small capacitor—in fact the highest impedance I have ever had to deal with. A while ago I designed the electronics for an capacitor microphone, and to avoid bass loss the biasing resistor had to have the astronomical value of 10 gigaOhms. This exotic component came in a glass encapsulation that had to be manipulated with tweezers—one touch of a finger and the insulation properties of the glass were fatally compromised.
In general, microphone output levels are low. Back in the day (say pre-1978) step-up microphone transformers with ratios of up to 1:10 were widely used to get the signal up to a level suitable for low-noise amplification. While this technology had its advantages, the cost and weight of the transformers, and their non-ideal linearity, made a transformerless input amplifier a very desirable thing to aim at.
The difficulties of getting the noise low enough and the linearity good enough were at first formidable, but after a good deal of work (some of which I did) they have since become almost universal.
Microphone pads, or attenuators, are used when the output is too high for the mixing console input to cope with; this typically happens when you put a microphone inside a kick-drum. If switchable pads are built into the console, they are at the wrong end of the microphone cable and degrade both noise performance and common-mode rejection. I might modestly mention that I invented the “padless”
microphone input amplifier in 1987 to eliminate these problems, and got a patent for it. Looking at the mixer market today, the idea seems to have caught on.
John concludes this chapter by examining “digital” microphones. These are of course only digital after a certain point. The sound pressure impinging on the diaphragm and the movement of the diaphragm itself remain defiantly analog. The digital-to-analog conversion takes place later and the data is sent down an AES-3 cable.
Audio Engineering Explained
Copyright © 2009 by Elsevier Inc. All rights of reproduction in any form reserved.
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2010