Effect of Cavity Resonance on the Frequency Response Characteristic of the Condenser Microphone

The effect of the cavity before the membrane in the usual condenser microphone is to cause an increase of pressure at the membrane over that which exists in the undisturbed (free) sound-field, particularly at high frequencies. An approximate theory of the effect is given and a specimen case is calculated. In this case the pressure ratio reaches a maximum of 2.5 at 3000 cycles. Experimental values obtained with the Rayleigh disk are compared with the computations and are in general agreement. A modified design for a condenser microphone without a diaphragm cavity, and adapted for spherical mounting, is described. The temperature coefficient of sensitivity due to the unequal thermal expansions of a duralumin membrane in a steel mounting was found to be 0.6 per cent per degree C. The importance of taking the cavity and reflection effects into consideration in the construction of curves showing the overall fidelity of broadcast transmitters is stressed on account of its intimate relation to the design of radio receivers for uniform frequency response.