Uncertainty and Signal-to-Noise Ratio for Unsteady Background Noise

This article presents methodologies that can be used to evaluate the impact of unsteady background noise for standard acoustic tests. When sound or noise emitted by a unit-under-test is measured according to acoustical test standards in the presence of background noise, background corrections are necessary. However, the use of a background correction factor is valid only when the signal-to-noise ratio (SNR) of the sound source is above a lower-limit specified in acoustic test standards. Therefore, the testing of increasingly quiet devices is becoming problematic because low SNRs are significantly affected by background noise variations (i.e., unsteadiness, defined as a background with change, variation, or interruption). This study investigates and introduces two methodologies that address effects of background variations. As the first methodology, the uncertainty in the background correction is evaluated and discussed. The second methodology is a different SNR metric, zero loudness SNR, in order to provide acceptable tolerances during standard acoustic tests that use loudness as a rating method. The two above methodologies are presented with 200 real-world ventilation device testing conforming to ISO, ANSI, and AMCA standards. It is shown that the combined use of the zero loudness SNR and the uncertainty of the SNR enables to assess both background unsteadiness and the impact of low SNRs. The method is found out to be useful acoustical tests of devices such as ventilating fans suffering from low SNRs.