Free-field response of sound level meters

Abstract

Within specified ranges, sound level meters should be able to measure a frequency-weighted sound pressure level at a given point, regardless of the direction of sound incidence and the frequency of the sound. Acoustical characteristics of sound level meters are specified in draft IEC 61672 as a function of frequency for plane progressive sound waves of steady sinusoidal signals under free-field conditions. The free-field frequency response of a microphone in a specified reference direction is modified by (a) sound reflected from the case of the sound level meter, (b) the apparatus that supports the instrument under test, and (c) the frequency response of the system that processes the signals from the microphone. From the simulated influence of sound waves reflected from an instrument equipped with a microphone having a FLAT frequency response, it is shown that linear frequency steps should be preferred to logarithmic frequency steps for evaluation of reflection effects. Test signal frequencies are calculated from the quotient of the speed of sound by the distances from the reflection point on the case to the microphone reference point. Measurement results are shown and comments noted regarding the procedures in the proposed standard for testing acoustical characteristics.