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Another effect of very high impedance is excessive loss of high frequencies over cables of more than a few feet long. Cost is low, but few are used today except for specialised applications such as vibration measurement. 33 Capacitor A thin, plastic diaphragm coated with aluminium or gold is stretched over a shallow cavity having a flat metal backplate. A capacitance is thereby created which depends on the spacing. It is usually between 20-30 pF, but can be from 5-75 pF. c. voltage of from 9-100 V, but usually 48 V, charges the capacitor and electrostatic attraction pulls the diaphragm, keeping it taut.

Off-axis response is normal. The effect is not obtained with ribbon transducers. dB 40 30 20 10 100 200 J_ 500 Ik 2k Fig. 42. A typical response at 0° and 90° incidence. 5k 10k 44 Direct/diffuse fields The distance at which a microphone is placed from the sound source affects not only the output and to some extent the frequency response, but also the proportion of direct to diffuse sound it receives. Direct sound decreases with distance, but diffuse sound resulting from multiple reflections is fairly constant.

Capacitor No polarising voltage is needed for these although a 12 V supply is required. m. oscillator circuit is tuned by the capacitor, thus producing frequency modulations in sympathy with the sound. An integral detector converts these to audio which is fed out via the cable. Apart from a lower supply voltage, the advantage is avoidance of electrostatic tension of the diaphragm and the risk of flashover. c. supply restricts their use mainly to recording and broadcast studios where the highest quality is required at any cost.