Fundamentals

The first acoustical measuring instruments were purely mechanical, such as the Rayleigh-disk for the measurement of the sound particle velocity, the Helmholtz-resonator for sound analysis with the human ear as indicator or the Kundt’s tube for the determination of wavelengths.

Modern acoustical metrology uses largely electroacoustical instruments. The general principle is based on the converting of an acoustical quantity via an electroacoustical transducer (sound receiver) into a corresponding electrical quantity, which then may be amplified, reprocessed with frequency weightings and then the result displayed or recorded.

Typical electroacoustical transducers are the microphones which can convert the acoustical quantity – normally the sound pressure into an electrical signal which then can be further evaluated.

Special condenser microphones are used worldwide as standard microphones because of their metrological properties like broad band frequency and dynamic range, long time stability, and relatively less dependent on temperature, humidity and air pressure. They are of two types LS1 and LS2, where LS stands for “Laboratory Standard” and the following number defines the mechanical construction of the microphone.

 

Calibration of Microphones

BEV applies the reciprocity measuring method as a primary method for the calibration of standard microphones. The basic principle of this method is the reciprocity of the electroacoustical transducer (microphone). The pressure sensitivity M [V.Pa-1] of a microphone is defined as the quotient of the microphone open circuit output voltage U [V] and the sound pressure p [Pa] uniformly distributed over the microphone diaphragm surface.

Using pair-wise combinations of the microphones, the product of the pressure sensitivity of two microphones is defined as a function of the acoustic transfer impedances. Depending on the number of microphones under test –three microphones are possible at maximum – a number of equations are defined with the same number of unknowns. Finally, the sensitivity of each microphone is determined taking into account the various thermodynamic correction factors.