The best designed moving-coil microphones have open-circuit voltage responses to sounds of random incidence that are within 5   dB of the average response over the frequency range between 40 and 16,000   Hz. Sound pressures as low as 16   dB SPL and as high as 140   dB SPL re 20   μPa can be measured. Changes of response with temperature, pressure, and humidity are believed to be, in the better instruments, of the order of 3–5   dB maximum below 1000   Hz for the temperature range of 10–100°F, pressure range of 0.65–0.78   m   Hg, and humidity range of 0%–90% relative humidity.

The electrical impedance is that of a coil of wire. Below 1000   Hz, the resistive component predominates over the reactive component. Most moving-coil microphones have a nominal electrical impedance of about 300   Ω. The mechanical impedance is not high enough to permit use in a closed cavity without seriously changing the sound pressure therein.

To connect a dynamic microphone to an amplifier, a stepping-up transformer is required, which is usually contained within the microphone housing.

Behind the diaphragm the acoustical circuit is more complicated. First there is an air gap between the diaphragm and the magnet that forms an acoustic compliance and resistance (see Fig. 5.9b). This air gap connects with a large back cavity that is also an acoustic compliance. In the connecting passages, there are screens that serve as acoustic resistances. Also, the interconnecting passages form an acoustic mass. The large air cavity connects to the outside of the microphone through a narrow pressure-equalizing tube, which prevents static displacements of the diaphragm due to variations in atmospheric pressure. It also attenuates the output of the microphone at the very lowest frequencies because the sound arriving at the rear of the diaphragm via the tube cancels that at the front. However, for simplicity, we shall ignore this tube during our analysis because its effect is only evident well below the working frequency range of the microphone, although in some designs it is tuned to resonate with the back cavity and thus boost the low-frequency output rather like a bass-reflex port in a loudspeaker, a topic which is covered in more detail in Chapter 7.

The complete acoustical circuit behind the diaphragm is given in Fig. 5.12. The acoustic compliance and resistance directly behind the diaphragm are C _AG and R _AG respectively, the acoustic resistances of the screens are R _AS , the acoustic mass of the interconnecting passage is M _AS , and the acoustic compliance of the large back cavity is C _AB .

The electromechanical circuit (mechanical-admittance analogy) for the diaphragm and voice coil is given in Fig. 5.13. The force exerted on the diaphragm is f ˜ D , and its resulting velocity is u ˜ D . Here, M _MD is the mass of diaphragm and voice coil; C _MS is the compliance of the suspension; L is the inductance of voice coil; and R _E is the electric resistance of the voice coil. Z _EL is the electric impedance of the electric load to which the microphone is connected. The quantity e ˜ 0 = B l u ˜ D is the open-circuit voltage produced by the microphone. There will also be some mechanical resistance due to the suspension, but this is generally very small compared with the acoustic resistance R _AS so we will ignore it.

To combine Figs. 5.11–5.13, the dual of Fig. 5.13 must first be taken; it is shown in Fig. 5.14. Now, to join Figs. 5.11, 5.12, and Fig. 5.14, all forces in Fig. 5.14 must be divided by the area of the diaphragm S _D and all velocities multiplied by S _D . This can be done by inserting an area transformer into the circuit. Recognizing that U ˜ D must be the same for all three component circuits, we get the circuit of Fig. 5.15 for the moving-coil microphone.

At very low frequencies, Fig. 5.15 reduces to Fig. 5.16a. The generator p ˜ B is effectively open circuited by the three acoustic compliances C _AF , C _AG , and C _AB , and the mechanical compliance C _MS of which only C _AB and C _MS have appreciable size. Also, all of the resistances and reactances of the masses are small compared with the reactances of C _AB and C _MS S _D ². Hence, e ˜ 0 is very small. This region is marked (a) in Fig. 5.17, where we see the voltage response in decibels as a function of frequency. In region (a), the response increases at the rate of 6   dB per octave increase in frequency.

As the frequency increases (see Fig. 5.16b), a highly damped resonance condition occurs involving the resistance and mass of the screens behind the diaphragm, R _AS and M _AS , and the diaphragm constants themselves, M _MD and C _MS , together with the compliance C _AB back of the back cavity. This is region (b) of Fig. 5.17. A highly important design feature, therefore, is a resistance of the screens R _AS large enough so that the response curve in region (b) is as flat as possible. The damping is so great that it makes more sense to define this region by two break frequencies ω _L and ω _U , which define the upper and lower limits of this region rather than a single resonance frequency ω ₀. These are defined by

The lower frequency ω _L marks the beginning of the 6   dB/octave low-frequency roll-off, which occurs at 55   Hz in Fig. 5.17, where the response is 3   dB less than that in the flat region. The upper frequency ω _U occurs somewhere near the upper limit of region (b). At the resonance frequency ω ₀, the mass and compliance elements cancel each other's reactances leaving just resistive element R _AS . At this frequency, the midband sensitivity is given by

In the case of the microphone shown in Fig. 5.9, the sensitivity is 2.4   mV/Pa or −52   dBV/Pa.

Above region (b) (see Fig. 5.16c), a resonance condition results that involves primarily the mass of the diaphragm M _MD and the stiffness of the air immediately behind it, C _AG . This yields the response shown in region (c) of Fig. 5.17. The resonance frequency ω _C at the center of region (c) is given by

Because the air gap is so small, the viscous air flow therein has a damping effect on this resonance, as represented by R _AG , which is important for keeping the frequency response reasonably flat. The large value of R _AS damps the antiresonance of C _AG with M _AS , which would otherwise produce a suck-out in the frequency response.

Finally, a third resonance occurs involving primarily the acoustical elements in front of the diaphragm (see Fig. 5.16d and region (d) of Fig. 5.17). The resonance frequency ω _D at the center of region (d) is given by

Because there are three reactive elements in the circuit (M _AA   +   M _AH , C _AF , and M _MD ), the response then drops off at the rate of −18   dB per doubling of frequency. Of course, this is the open-circuit roll-off and a steeper rate is likely to occur if the microphone is loaded with a capacitive cable that will resonate with the coil inductance at some frequency. A step-up transformer also has a limited bandwidth, although through careful design this need not compromise the performance of the microphone. The low-frequency response depends on the inductance, which is maximized through the use of a generous core size and an ample number of turns. The high-frequency response is extended through the use of a split bobbin to reduce the interwinding capacitance and several interleaving primary and secondary sections, which reduces the leakage inductance.

These various resonance conditions result in a microphone whose response is substantially flat from 50 to 20,000   Hz except for diffraction effects around the microphone. These diffraction effects will influence the response in different ways, depending on the direction of travel of the sound wave relative to the position of the microphone. The usual effect is that the response is enhanced in regions (c) and (d) if the sound wave impinges on the front of the microphone at normal (perpendicular) incidence compared with grazing incidence. One purpose of the outer protective screen is to minimize this enhancement.