On experimental determination of the random-incidence response of microphones

The random-incidence sensitivity of a microphone is defined as the ratio of the output voltage to the sound pressure that would exist at the position of the acoustic center of the microphone in the absence of the microphone in a sound field with incident plane waves coming from all directions. The random-incidence correction of a number of laboratory standard microphones has been determined experimentally. Although the measurement procedure seems to be straightforward, some practical and fundamental problems arise: (i) Reflections from the mounting rig contaminate the measured frequency response, and whereas some of these reflections can be removed using a time-selective technique, others coincide with the direct impulse response and consequently cannot be removed in the time domain and thus affect the accuracy of the estimate; (ii) the accuracy of the estimate is relying on the rotational symmetry of the microphone and depends on the angular resolution. The effect of the angular resolution has been compared with the analytical solution of the scattering and diffraction around a solid sphere. Numerical calculations supplement the experimental results. Although the procedure has only been applied to laboratory standard microphones, it is not restricted to such microphones and may be applied to other types of measurement microphones.