Using the Concept of Constant Divergence of the Sound Pressure Level for Determining the Free-Field Sensitivity of Microphones by Comparison

This paper shows how to use the constant divergence of the sound pressure level in the field of an acoustic source for determining the free-field sensitivity of microphones by comparison. For the free field calibration by comparison when using simultaneous excitation, the International Standard IEC 61094-8:2012 recommends placing the two microphones at points where the sound pressure is the same. The paper shows that this severe constraint can be avoided by using the constant divergence of the sound pressure level between different locations in the field of the acoustic source. The paper discusses the principle of the constant divergence of the sound pressure level and shows practical ways for determining the free-field sensitivity of microphones by using this concept. Along with theoretical approach and experimental results, the paper presents the work performed at Scantek, Inc. for developing acoustical test methods based on the constant divergence of the sound pressure level.