Uncertainty due to instrumentation for sound pressure level measurements in high frequency range

In many countries, high frequency noise (ultrasonic noise) is included in the list of factors harmful to health in a working environment. For that reason it is necessary to assess risk arising from this type of noise. A previous paper showed that there is no clear and complete information on the factors influencing the result of a measurement of the sound pressure level for the frequency range above 20 kHz. This article presents the influence of measuring instrumentation on the result of a measurement of sound pressure in the range of 10-40 kHz. It discusses the factors associated with adjustment and calibration of the metre/analyser using a sound calibrator; the factors associated with the microphone's frequency response in the free field; the factors associated with uncorrected errors of the metre/ analyser, resulting from its operation; and the factors resulting from the effects of various physical factors (static pressure, temperature, humidity). It also presents the standard uncertainty values for all the above-mentioned factors, and determines the overall uncertainty values for individual frequency bands. Finally, the article outlines the need for introducing, into the measurement result, corrections associated with the microphone's protection grid and the data from a periodic metrological check.