Some consideration on the signal-to-noise ratio of condenser microphones

Abstract

From the standpoint of the actual application of the condenser microphone, an evaluating factor may be suitable as the so-called, signal-to-noise ratio. The relation between the optimum condition for sensitivity ( Fig. 2 ) and the optimum condition for the signal-to-noise ratio ( Fig. 9 ) was sought after. As a result of the above, it has been made clear that the optimum condition for sensitivity can be obtained when the distance between the diaphragm and the back electrode is widened until the self-capactance of the microphone and stray capacitance are equal, however, the signal-to-noise ratio is a maximum under different conditions. The above is due to the fact that the noise in a condenser microphone circuit is dependent on the electrostatic capacitance and circuit resistance. If the noise in the amplifier can be disregarded, the signal-to-noise ratio improves when the self-capacitance is large or when the distance between the diaphragm and the back electrode is small and, since it is proportional to the square root of the circuit resistance, it is possible to design in accordance with the balance between the costs of the formation of the distance between the diaphragm and the back electrode and that of the resistor. When the noise of the amplifier cannot be ignored, the signal-to-noise ratio has, with respect to the diaphragm electrode distance, the characteristics of Fig. 9. When the distance between the diaphragm electrode and the back electrode d, back electode area S_b, circuit resistance R, stray capacity C_s and center frequency f_0 and noise level N_&lt( f_0 , 1/3 )&gt in a 1/3 octave band, the signal-to-noise ratio of a condenser microphone becomes the maximum value when the distance between the diaphragm and the back electrode is equal to ε_0 S_b / ( ( 9. 65 × 10^&lt-12&gt / N_( f_0 , 1/3) √&lt R f_0 &gt ) - C_s ). The above occurs when the amplifier noise and resistor noise are equal. From the data cited above when exterior specifications are given, it is possible to design a microphone based on the signal-to-noise ratio.