Environmental coefficients of the free-field sensitivity of measurement microphones

The sensitivity of measurement microphones, both pressure and free field, is affected by changes in the environmental conditions, mainly temperature and static pressure. Static pressure and temperature coefficients for the pressure sensitivity have been the object of previous studies focused on Laboratory Standard microphones and few working standard microphones.

The literature describes frequency dependent values for these coefficients which are used for calibration purposes. However, there is no description of the environmental coefficients of the free-field sensitivity though there have been some implementations that attempt to take care of the differences between the coefficients for the two types of sensitivities.

Measuring the coefficients in a free field poses some challeng; it is not so easy to change neither the static pressure nor the temperature inside anechoic room within the range usually needed for a robust determination of the coefficients. Analysis of data gathered through a sufficiently large number of measurements has been suggested.

This paper proposes an experimental procedure for the determination of the free-field sensitivity coefficients which makes use of the existing facilities used for the determination of the pressure sensitivity coefficients. Results are compared, whenever possible with numerical calculations of the microphones involved, and with alternative procedures based on the knowledge of the pressure sensitivity coefficients, diffraction of the microphone, and the load of the microphone impedance due to the radiation impedance.