A Pressure-Step Method for Determining the Low Frequency Sensitivity of Measurement Microphones at High Sound Pressures

The European Community (EC) "Noise at Work" Directive sets limits for the exposure of workers to audible noise including exposure to high-level impulses. The action level for peak sound pressures is 200 Pa. Further, the proposed EC "Physical Agents" Directive, has suggested the introduction of additional action levels at peak sound pressures of 112 Pa and 600 Pa. Microphones and sound level meters, however, are calibrated using sound pressures of about 0.1 Pa and their linearity can only readily be tested up to about 20 Pa. There is no current traceability path for measurements at these higher levels.

This paper presents the development of a new method of calibrating the low frequency response (∼ 125 Hz) of a microphone at sound pressures up to 600 Pa, using a pressure step as an input signal to the microphone.

Background to the work is given, and calibration results are provided for three Brüel and Kjær microphone types, the 4134, the 4136, and the 4176. The results are not wholly unexpected, with both the 4134 and 4136 microphones showing good linearity over the whole pressure range and agreement at low sound pressures with calibrations by independent methods. The 4176 microphone shows non-linear behaviour above 100 Pa.

The proposed method is able to support the development and application of improved international standards specifying the performance of microphones and sound level meters.