| Résumé | Sound pressure level measurements are key points to determine whether there is a breach of noise limits. Any measurement made without the knowledge of its uncertainty lacks signifi- cance. For this reason, an open measurement system has been developed at the National Re- search Council Canada for sound pressure level measurements with the focus on measurement uncertainties. For uncertainty evaluation of such a system, the most difficult task is determining the uncertainty contribution of its acoustic front end, or its microphone and preamplifier assem- bly. The acoustic frond end can be modelled as a linear time-invariant system. Once the fre- quency response of the acoustic front end has been measured, its effect on an arbitrary signal can be analyzed. However, measured microphone frequency response does not include phase in- formation, as no country in the world yet has Calibration and Measurement Capability (CMC) for microphone pressure sensitivity phase. With the completion of key comparison CCAUV.A- K5, this situation will change in the near future. Now is the time to determine the uncertainty contribution of acoustic front end using the phase information available, such as that in CCAUV.A-K5. In this paper, a method for the evaluation of the uncertainty contribution of acoustic front ends is presented. The acoustic front end is first modelled as a linear time- invariant system. Its measured frequency response at discrete frequencies is then curve-fit to ob- tain the response covering the entire frequency domain. The output of the acoustic front end is simply the convolution of the input with the acoustic front end's impulse response. The uncer- tainty contribution is finally calculated by propagating the uncertainties of the acoustic front end output at every time instance. Examples are given for typical acoustic front ends with various types of acoustic signals. |
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