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5 - ELECTROMAGNETIC AND ELECTROSTATIC TRANSDUCERS

5.3 - THE ELECTROSTATIC TRANSDUCER

The electrostatic or condenser microphone was invented by Wente in 1917 ^[4]. He describes a transducer system where a flexible electrically charged metallic diaphragm forms one plate of an electrical condenser, the other electrode being a rigid back-plate structure, thereby creating a condenser of variable capacity. Figure 19 shows just such a cross-section of the capsule of an electrostatic or condenser microphone capsule.


Figure 19 - X-Section of an Electrostatic Transducer
Nowadays, we can distinguish two types of microphones using the electrostatic principle to generate an electrical signal proportional to the sound wave pressure:

the condenser microphone, where the microphone capsule receives polarization via the electronic circuit of the microphone, which itself is powered from an external source such as a separate mains powered DC source or battery, or directly from the mixing desk as a ‘phantom’ on the microphone cable

the electret, where an electrostatic charge is produced by modifying the molecular structure of one of the electrodes by electrostatic induction during manufacturing process - the electrode retains its charge when mounted in the capsule, and produces an opposite charge in the facing electrode of the microphone condenser assembly. Most professional electret microphones use back electrode polarization, whereas front electrode or diaphragm polarization is mainly used on the more inexpensive non-professional microphone capsules.

In a condenser microphone, the capacity formed by the diaphragm and the back electrode is charged or polarized by the application of a DC voltage in series with a high resistance. If we apply an initial polarizing voltage to this capacity there will be a migration of electrons from one electrode to the other, resulting in a positive charge on one electrode and a negative charge on the other.

The relation between the capacity of the condenser (C), the charge (Q) and the voltage (V) is:



The time taken for the capacity to attain maximum charge (and consequently for the flow of electrons to cease) will depend on the value of resistance in series with the charge circuit. This charge time will condition the possibility of electron flow following any change or variation in the condenser capacity due to displacement of the diaphragm. If the resistance is very high in value, then the time constant for the charging circuit will be so long that we can consider that the charge on the condenser is maintained constant - the voltage across the condenser will then be inversely proportional to the variation of capacity. Movement of the diaphragm under the action of the sound wave pressure will vary the distance between the diaphragm and the back electrode, and thereby vary the capacity of the condenser. This capacity is inversely proportional to the distance between the electrodes – the smaller the distance between the electrodes the higher the value of capacity. The voltage between the electrodes of the condenser will therefore be proportional to the movement of the diaphragm as shown in Figures 20 and 21.


Figure 20 - Compression Produces Voltage Decrease


Figure 21 - Decompression Produces Voltage Increase

The electrical polarization circuit is absent in an electret microphone (one of the electrodes being pre-polarized by modification of its molecular structure), so the series resistance in this case can be considered as theoretically being infinite in value. The value of the capacity (formed by the diaphragm and the back electrode) and the resistance in the polarization circuit will determine the charging time constant, and therefore the limit to the low frequency response. The beginning of low frequency roll-off for a condenser microphone can be calculated as follows :

The capacity of a 1 inch capsule is about 50pf - with a series resistance of 4G&#937 the roll-off frequency will be about 0.8Hz.

Stray capacitance, both inside the capsule and due to the coupling to the microphone pre-amplification input, must be reduced to an absolute minimum. It is electrically in parallel with the capsule capacitance and will therefore act as a voltage divider, attenuating the voltage generated by the capsule. The coupling capacitor blocks any polarization voltage biasing the following pre-amplification input stage, and must therefore have the highest possible internal resistance to DC.

A consequence of the polarization voltage which is applied to the capsule assembly, is to produce an inward displacement of the microphone diaphragm, due to electrostatic attraction between the charge on the back-plate and the opposite charge on the diaphragm. This effect will obviously set a limit to the magnitude of the polarization voltage, as well as the minimum distance between the back-plate and diaphragm - too high a polarization voltage or too small a distance between the back-plate and diaphragm will increase the possibility of the diaphragm ‘sticking’ to the back electrode. Sensitivity is also proportional to the magnitude of the polarization voltage, however an increase in sensitivity will unfortunately be, to some extent, at the expense of the low frequency response. Three small-diaphragm electrostatic microphone capsules are shown in Figure 22.


Figure 22 - Three small diaphragm electrostatic condenser microphones capsules
Schoeps MK4, AKG CK1 and Sennheiser MKH 406
Sennheiser ^[5][6] have developed a condenser microphone system where symmetrical electrodes are placed on each side of the diaphragm. This reduces distortion by eliminating the asymmetrical diaphragm displacement and therefore the associated asymmetrical variation of the acoustical impedance behind the diaphragm. The tension on the diaphragm in this design is also reduced to such an extent that the fundamental resonance frequency is similar to an electrodynamic microphone with a corresponding increase in sensitivity and reduction in signal to noise ratio. In addition to the usual acoustic damping techniques, equalisation is also applied in the pre-amplification stage to produce a flat frequency response. However careful design of the front plate electrode is necessary in order to reduce the effect of this electrode, as an acoustic obstacle, on the on-axis frequency response.

[4] 1917 – "A condenser transmitter as a uniformly sensitive instrument for the absolute measurement of sound intensity" by E.C.Wente, Phys. Rev. 10 – 63

[5] 1981 – "New Investigations on Linearity Problems of Capacitive Transducers"; by Manfred Hibbing, Sennheiser electronic, Germany, AES 68th Convention,Hamburg, Germany – preprint 1752

[6] 1985 – "Design of a Low Noise Studio Condenser Microphone" by Manfred Hibbing, Sennheiser electronic, Germany, 77th AES Convention, Hamburg – preprint 2215