More on Microphones by Michael Williams, (www.williamsmmad.com)   △   <   ∧   >

8 - MEASUREMENT AND THE OPERATIONAL CHARACTERISTICS

Although there are usually many informative characteristics published on the specification sheet of a microphone, the principle measurements that we use to estimate the operational characteristics of microphones, are the on- and off-axis frequency response of a microphone, and its directivity pattern at various frequencies throughout the audible frequency range.

8.1 - FREQUENCY RESPONSE

8.1.3 - The Electrostatic Microphone

The fundamental resonant frequency of a measurement condenser microphone with pressure acoustic coupling is above the upper limit of the audible frequency range from about 20kHz to 40kHz (Figure 32–curve A).
Figure 33 - Typical Amplitude/Frequency Response at Resonance
for a Electrostatic Microphone
without any Constraints (curve A)
with Natural Damping (curve B)
with Additional Damping (curve C)

Unconstrained diaphragm movement will produce a relatively large amplitude of movement at resonance, however the proximity of the back electrode coupled with the small mass of the diaphragm will already introduce considerable damping of the diaphragm movement (Figure 32 - curve B). Additional damping (Curve C in Figure 32), is engineered by piercing small holes in the back plate mainly around the centre area behind the diaphragm as shown in Figure 33, where the amplitude of movement is maximum. The number and diameter of these holes allow considerable control over the coefficient of damping - an almost completely linear high frequency response is possible.


Figure 33 - Condenser Capsule showing Additional Damping of the Fundamental Resonance

Studio-grade pressure microphones on the other hand will have a fundamental resonance frequency somewhere in the range of 3kHz to 8kHz, being predominantly under resistance control. Above the resonant frequency, the response will be inversely proportional to the square of frequency, the response falling therefore at about 12dB per octave. This fall in response can be compensated by the rise in response on-axis due to diffraction i.e. reflection from the front surface of the microphone - sometimes called pressure-doubling. Careful adjustment of the precise frequency of resonance will allow fine tuning of the high frequency response either for a flat on-axis frequency response or for a flat energy response.

The limit to the low frequency response of the condenser pressure microphone is determined mainly by the value of diaphragm/back-electrode capacitance with respect to the series resistance in the polarization circuit as discussed in section 5.3 (The Electrostatic Transducer), but the capillary vent for equalising atmospheric pressure may also be a contributing factor. Figure 34 shows the very wide frequency range and remarkable linearity that is possible for this type of microphone. Studio-grade pressure microphones on the other hand will have a fundamental resonance frequency somewhere in the range of 3kHz to 8kHz, being predominantly under resistance control. Above the resonant frequency, the response will be inversely proportional to the square of frequency, the response falling therefore at about 12db per octave. This fall in response can be compensated by the rise in response on-axis due to diffraction i.e. reflection from the front surface of the microphone - sometimes called pressure-doubling. Careful adjustment of the precise frequency of resonance will allow fine tuning of the high frequency response either for a flat on-axis frequency response or for a flat energy response.

The limit to the low frequency response of the condenser pressure microphone is determined mainly by the value of diaphragm/back-electrode capacitance with respect to the series resistance in the polarization circuit as discussed also in section 5.3 (The Electrostatic Tranducer) but the capillary vent for equalising atmospheric pressure may also be a contributing factor.


Figure 34 - Frequency Response of a Small Diaphragm Condenser Microphone - Schoeps MK2
© frequency response curve published by courtesy of Schoeps

Pressure-gradient condenser microphones normally have a fundamental resonant frequency within the lower-middle of the audible frequency range – anywhere between about 800Hz and 2kHz. They are therefore classified as control by resistance. They do not have any of the irregularities in frequency response which are typical of the moving coil microphone - the correctly damped response curve can show a remarkable linearity throughout the audible frequency range, again with fine tuning of the resonant frequency with respect to diffraction pressure increase, to achieve flat on-axis response or flat energy response. However some low frequency roll-off will be evident, as with any pressure-gradient acoustic coupled microphone system due to the fundamental roll-off in the bass frequencies (the ‘ripple’ below about 40Hz in Figure 35 is due to small imperfections in the bass frequency measurement process).


Figure 35 - Low Frequency Roll-off in the On-axis Frequency Response
of a Cardioid Static Microphone
© frequency response curve published by courtesy of Schoeps