The acoustic center of laboratory standard microphones

Abstract

An experimental procedure is described for obtaining the effective acoustic distance between pairs of microphones coupled by a free field, leading to the determination of the position of the acoustic center of the microphones. The procedure, which is based on measuring the modulus of the electrical transfer impedance, has been applied to a large number of microphones. In all cases effects due to reflections from the walls of the anechoic chamber and the interference between the microphones have been removed using a time-selective technique. The procedure of determining the position of the acoustic center from the inverse distance law is analyzed. Experimental values of the acoustic center of laboratory standard microphones are presented, and numerical results obtained using the boundary element method supplement the experimental data. Estimated uncertainties are also presented. The results reported confirm values previously defined in an international standard and extend the frequency range.

A comparison of two methods for phase response calibration of hydrophones in the frequency range 10–400 kHz

“…For frequencies well below the TC4034 resonance frequency of 320 kHz, this behavior corresponds to the theoretical phase response of an ideal, infinitely rigid, spherical hydrophone (Luker and Van Buren, 1981). This apparent movement of the acoustic center with frequency is also observed in microphone calibration (Barrera-Figueroa et al, 2006).…”

A New Method for the Determination of the Acoustic Center of Acoustic Transducers

“…E. Matsui completed Ando'sm odel by taking into account the influence of the receivero nt he acoustic field, buthis study focused only on obtaining the free-field correction to the sensitivity of the microphone [9] (not on the position of the acoustic center). More recently,w orks on the determination of acoustic center of microphones have been focused on experimental methods and and numerical method using the boundary element method [4,10] which relies on the determination of the amplitude-based acoustic center (this is coherent with the method used experimentally).…”

“…Several methods have been used for determining the acoustic center of at ransducer: one of them relies on phase measurements [2], butmost of them are based upon the measurement of the deviations of the amplitude of the sound pressure from the inverse distance law [3,4]. These methods are summarized and discussed in reference [5].…”

Hybrid method for determining the parameters of condenser microphones from measured membrane velocities and numerical calculations

“…In an international standard, the acoustic center of a microphone is defined as follows: "For a sound emitting transducer, for a sinusoidal signal of given frequency and for a specified direction and distance, the point from which the approximately spherical wavefronts, as observed in a small region around the observation point, appear to diverge." 12 The acoustic center of LS microphones has been determined using different methodologies: ͑a͒ from the decay of the sound pressure with the distance when a condenser microphone is used as a transmitter and the sound pressure was measured using a probe microphone, 15 ͑b͒ from measurements of the transfer impedance between two microphones at different distances, 3 and ͑c͒ from measuring the decay of the sound pressure with the distance when the sound field is generated by a source of known acoustic center and the decay of the sound pressure is measured with the microphone under test. 16 Method ͑a͒ provided values of the acoustic center that were in good agreement with the expected theoretical results, but there were significant variations due to reflections in the anechoic chamber.…”

“…Furthermore, numerical calculations are sometimes used to complement experimental results at frequencies where the experimental methods might yield unreliable results. [1][2][3][4][5] However, the numerical calculations are usually carried out under a number of assumptions that are not necessarily completely realistic. Several attempts to develop a complete coupled model of a condenser microphone numerically are described in the literature.…”