Optical measuring and calibration techniques for microphones

Abstract

Microphone measurement techniques are normally based on acoustical excitation and electrical derivation of either the voltage or the current output of the transducer. With condenser microphones, electrical excitation can be used as well. Calibration of microphones based on acoustical excitation therefore is quite time consuming, and sensitive for several errors originating in inaccurate sound-field properties and environment conditions. Measurements of the microphone membrane velocity with optical means like the laser Doppler vibrometer technique offer an accurate, touchless method to derive the local movement at almost each point of the membrane. Excitation of the membrane can be accomplished by direct feeding of the transducer (due to its reversible construction), which provides highest accuracy for the derivation of either current or voltage at the terminals of the transducer. From the knowledge of the vibration characteristics and the excitation properties, the ‘‘differential transducer sensitivity’’ defined at each surface point can be derived. With numerical methods, like the discrete solution of the Helmholtz equation or BEM, the radiated sound can be calculated, and finally the overall microphone radiation pattern and sensitivity can be calculated. The described techniques were applied to different types of microphones. The measurement and calculation results will be shown.