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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

8.1.1 - The Electrodynamic Moving Coil Microphone
The fundamental resonant frequency of a moving coil microphone is in the mid frequency range, from about 1kHz to 8kHz. If the microphone diaphragm and its associated coil were free to vibrate almost without constraint, it would show a typical amplitude resonance curve with respect to frequency as shown in Figure 24 (curve A).
Figure 24 - Typical Amplitude/Frequency Response at Resonance
for a Electrodynamic Moving Coil Microphone
unconstrained resonance (curve A ),
partially damped resonance (curve B),
and maximum workable damping (curve C)

In practice, movement of the diaphragm, which is maximum at the resonant frequency of the microphone, will produce compression or decompression of the air-space in- between the diaphragm and the surface of the magnet pole-pieces, and consequent air flow through the gap between the coil and the pole pieces of the magnetic circuit which will cancel out to some extent this change in pressure. The surfaces within the narrow air gap introduce a quantity of resistance or friction to this air flow - the alternating air flow produces viscous forces that oppose the movement of the diaphragm – this will in turn introduce a certain amount of damping on the movement of the diaphragm as shown in Figure 23 (curve B).

Any movement of the voice coil will, at the same time as producing an electrical signal proportional to the diaphragm movement, also produce a small reverse electromagnetic force proportional to the amplitude of movement. This ‘back emf’ will also tend to oppose the movement of the coil and thereby contribute to damping excessive movement of the diaphragm at resonance.

Further damping (curve C in Figure 23) may be introduced by increasing the air flow friction through the air gap region of the magnet. It is common practice to introduce some form of fibrous material such as a felt pad into the air flow just behind the air gap in the magnet as shown in Figure 25.
Figure 25 - Additional Damping with Felt Pad behind the Voice Coil
The combined effect of air-flow friction in the air gap and in the fibrous pad, and the back ‘emf’ has the effect of reducing the amplitude of movement of the diaphragm and coil, and therefore reducing the excessive voltage generated at resonance. Further correction of the amplitude of movement at resonance is not possible without introducing an even further and undesirable reduction in the overall output voltage sensitivity. However the frequency response is still far from being satisfactory. The only remaining artifice left to the designer is the introduction of low frequency and high frequency boost with the help of some resonating cavities.

Low frequency boost is achieved by means of a Helmholtz resonator in the body of the microphone and coupled to the diaphragm through a central coaxial tube as shown in Figure 26.

Figure 26 - Low and High Frequency Helmholtz Resonators

The amplitude of resonance must be critically controlled to achieve as near as possible flat frequency response whilst reducing to a minimum the long decay tail associated with any resonating cavity. Again some fibrous material introduced into the coupling tube will easily control the damping coefficient. High frequency resonators depend very much on the specific design of the diaphragm, the magnet pole plate and annular ring plate, and also the front protection grid structure of the microphone.

The departure from an overall flat frequency response curve will therefore be characterized by four main aspects of design:

• The degree of fundamental resonant frequency damping
• The pressure increase due to diffraction
• The fall-off in response at the extremities of the damped resonance curve
• The profile of the bass frequency boost
  and the high frequency resonating cavities as shown in Figure 27

Figure 27 - Resonant Cavities used to extend the Overall Frequency Response
of a Moving Coil Microphone
To this we must add a certain number of irregularities due to the inherent vibration of the diaphragm itself as it vibrates in various partial mode resonances. A typical frequency response curve is shown in Figure 28


Figure 28 - Electrodynamic Microphone (Beyer M88 no 23307)
© frequency response curve published by courtesy of Beyer
We should therefore expect to see a limited band frequency response – low frequency roll-off below the bass resonator frequency, and high frequency roll-off above highest high frequency resonator. The mid and upper frequency response is usually somewhat irregular due to these high frequency cavity resonances and diaphragm partial mode resonances There is also usually a marked difference in response above and below about 800Hz (the precise frequency being a function of diaphragm diameter).

The increase in mid-frequency response is often used to produce the specific ‘presence’ effect in a vocal microphone – or more generally a brighter or more metallic sound in a studio microphone. But no matter what the usage, the source of the effect is the coefficient of damping of the fundamental resonance of this resistance controlled microphone system coupled with the on-axis pressure increase due to diffraction/reflection from the diaphragm.