The influence of microphone vents on measurements of acoustic intensity and impedance

Abstract

The pressure equalization vents of conventional microphones introduce bias errors in measurements of the sound intensity. Two error terms in the estimated active intensity are derived using a low-frequency model of microphone. The first error term is associated with the lower-limiting frequency of the microphones ͑1 to 2 Hz͒ and is proportional to the reactive sound intensity. It is shown here that the difference between limiting frequencies of the two microphones causes the second error term to be proportional to the mean-square pressure and to be of comparable importance at low frequencies. The amplitudes of errors due to the vents are analyzed in a quasistanding wave. Unlike the first error, the second can be corrected at the same time as the phase error between measurement channels, and several correction techniques are examined. For the error proportional to the reactive intensity, a correction method in a standing wave tube is suggested, using the joint measurement of several other energetic quantities. Examples of the influence of these errors on typical parameter values for half-inch microphones are presented in the case of measuring the acoustic impedance of materials using a two-microphone probe.

Sphygmomanometer Dynamic Pressure Measurement Using a Condenser Microphone

“…In fact, it increases with decreasing frequency, because the fraction of stiffness due to the reactive pressure in the internal cavity, becomes smaller as it is equalized through the vent. The low frequency sensitivity increase was smaller for microphones having a low fraction of air-stiffness [35][36][37].…”