The job of the microphone is to convert sound into an electrical signal. As was seen in Chapter 3, sound consists of both pressure and velocity variations and microphones can use either or both in order to obtain various directional characteristics.1
Figure 5.2(a) shows a true pressure microphone which consists of a diaphragm stretched across an otherwise sealed chamber. In practice a small pinhole is provided to allow changes in atmospheric pressure to take place
Figure 5.1 (a) The final sound quality of an audio system is limited by both microphones and
loudspeakers. (b) Sound production must be performed using high-quality loudspeakers on the assumption that the transmitted quality should be limited only by the listener’s equipment.
Figure 5.2 (a) Pressure microphone only allows sound to reach one side of the diaphragm.
(b) Pressure microphone is omnidirectional for small ka. (c) Directional characteristic is more intuitive when displayed in polar form. (d) Velocity or pressure gradient microphone exposes both sides of diaphragm. (e) Output of velocity microphone is a sinusoidal function of direc- tion. (f) In polar coordinates velocity microphone shows characteristic figure-of-eight shape for small ka.
should not be positioned near a reflecting object.
The path-length difference is zero at the wall itself. The pressure zone microphone (PZM) of (b) is designed to be placed on flat surfaces where it will not suffer from reflections. A pressure capsule is placed facing and parallel to a flat surface at a distance which is small compared to the shortest wavelength of interest. The acoustic impedance rises at a boundary because only half-space can be seen and the output of a PZM is beneficially doubled. Figure 5.2(d) shows the pressure gradient (PG) microphone in which the diaphragm is suspended in free air from a symmetrical perimeter frame. The maximum excursion of the diaphragm will occur when it faces squarely across the incident sound. As Figure 5.2(e) shows, the output will fall as the sound moves away from this axis, reaching a null at 90°. If the diaphragm were truly
weightless then it would follow the variations in air velocity perfectly, hence the term velocity microphone. However, as the diaphragm has finite mass then
Figure 5.3 (a) Microphone placed several wavelengths from reflective object suffers comb
filtering due to path-length difference. (b) Pressure zone microphone is designed to be placed at a boundary where there is no path-length difference.
a pressure difference is required to make it move, hence the more accurate term, pressure gradient microphone.
The pressure gradient microphone works by sampling the passing sound wave at two places separated by the front-to-back distance. Figure 5.4 shows that the pressure difference rises with frequency as the front-to-back distance becomes a greater part of the cycle. The force on the diaphragm rises at 6 dB/octave. Eventually the distance exceeds half the wavelength at the critical frequency where the pressure gradient effect falls rapidly. Fortunately the rear of the diaphragm will be starting to experience shading at this frequency so that the drive is only from the front. This has the beneficial effect of transferring to pressure operation so that the loss of output is not as severe as the figure suggests. The pressure gradient signal is in phase with the particle displacement and is in quadrature with the particle velocity.
In practice the directional characteristics shown in Figure 5.2(b) and (e) are redrawn in polar coordinates such that the magnitude of the response of the microphone corresponds to the distance from the centre point at any angle. The pressure microphone (c) has a circular polar diagram as it is omnidirec- tional or omni for short. Omni microphones are good at picking up ambience and reverberation which makes them attractive for music and sound-effects recordings in good locations. In acoustically poor locations they cannot be used because they are unable to discriminate between wanted and unwanted sound. Directional microphones are used instead.
The PG microphone has a polar diagram (f) which is the shape of a figure- of-eight. Note the null at 90° and that the polarity of the output reverses
beyond 90° giving rise to the term dipole. The figure-of-eight microphone
(sometimes just called an eight ) responds in two directions giving a degree of ambience pickup, although the sound will be a little drier than that of an
Figure 5.4 The pressure gradient microphone diaphragm experiences a pressure difference which
polar response will naturally sound drier than an eight, but will have the advantage of rejecting more unwanted sound under poor conditions. In public address applications, use of a cardioid will help to prevent feedback or howl- round which occurs when the microphone picks up too much of the signal from the loudspeakers. Virtually all hand-held microphones have a cardioid response where the major lobe faces axially so that the microphone is pointed
Figure 5.5 (a) Combining an omni response with that of an eight in equal amounts produces the
useful cardioid directivity pattern. (b) Hand-held fixed cardioid response microphones are usually built in the end-fire configuration where the body is placed in the null. (c) Sub-cardioid obtained by having more omni in the mix gives better ambience pickup than cardioid. (d) Hyper-cardioid obtained by having more eight in the mix is more directional than cardioid but the presence of the rear lobe must be considered in practice. (e) Microphones with variable polar diagram are generally built in the side-fire configuration.
at the sound source. This is known as an end-fire configuration shown in Figure 5.5(b).
Where a fixed cardioid-only response is required, this can be obtained using a single diaphragm where the chamber behind it is not sealed, but open to the air via an acoustic labyrinth. Figure 5.6(a) shows that the asymmetry of the labyrinth means that sound which is incident from the front reaches the rear of the diaphragm after a path difference allowing pressure gradient operation. Sound from the rear arrives at both sides of the diaphragm simultaneously, nulling the pressure gradient effect. Sound incident at 90° experiences half the path-length difference, giving a reduced output in comparison with the on- axis case. The overall response has a cardioid polar diagram. This approach is almost universal in hand-held cardioid microphones.
In variable directivity microphones there are two such cardioid mechanisms facing in opposite directions as shown in (b). The system was first devised by the Neumann company.2 The central baffle block contains a pattern of tiny holes, some of which are drilled right through and some of which are blind. The blind holes increase the volume behind the diaphragms, reducing the resonant frequency in pressure operation when the diaphragms move in anti-phase. The holes add damping because the viscosity of air is significant in such small cross-sections.
The through-drilled holes allow the two diaphragms to move in tandem so that pressure gradient operation is allowed along with further damping. Sound incident from one side (c) acts on the outside of the diaphragm on that side directly, but passes through the other diaphragm and then through the cross-drilled holes to act on the inside of the first diaphragm. The path-length difference creates the pressure gradient condition. Sound from the ‘wrong’ side (d) arrives at both sides of the far diaphragm without a path-length differ- ence.
The relative polarity and amplitude of signals from the two diaphragms can be varied by a control. By disabling one or other signal, a cardioid response can
Figure 5.6 (a) Fixed cardioid response is obtained with a labyrinth delaying sound reaching
the rear of the diaphragm. (b) Double cardioid capsule is the basis of the variable directivity microphone. (c) Sound arriving from the same side experiences a path-length difference to create a pressure gradient. (d) Sound arriving from the opposite side sees no path-length difference and fails to excite diaphragm.
sport is one application. Figure 5.7 shows the use of a conventional cardioid microphone fitted with a parabolic reflector. Only wavefronts arriving directly on-axis are focused on the microphone which is rendered highly directional. Parabolic units are popular for recording wildlife, but they are large and clumsy.
Figure 5.8(a) shows that the shotgun microphone consists of a conventional microphone capsule which is mounted at one end of a slotted tube. Sound wavefronts approaching from an angle will be diffracted by the slots such that each slot becomes a re-radiator launching sound down the inside of the tube. However, Figure 5.8(b) shows that the radiation from the slots travelling down the tube will not add coherently and will be largely cancelled. A wavefront approaching directly on axis as in Figure 5.8(c) will pass directly down the outside and the inside of the tube as if the tube were not there and consequently will give a maximum output.
Contact microphones are designed to sense the vibrations of the body of a musical instrument or the throat of a speaking human to produce an elec- trical signal. In fact these are not microphones at all but are accelerometers. Figure 5.9 shows that in an accelerometer a mass is supported on a compli- ance. When the base of the accelerometer is vibrated, the mass tends to lag
Figure 5.7 Parabolic reflector with cardioid microphone is very directional but fails at low
Figure 5.8 (a) Shotgun microphone has slotted tube. (b) Off-axis sound enters slots to produce
multiple incoherent sources which cancel. (c) On-axis sound is unaware of tube.
Figure 5.9 Contact microphone is actually an accelerometer. Suspended mass lags body vibration
actuating sensor.
behind, causing a relative movement. Any convenient transducing mechanism can be used to sense the movement.