Evaluating the Specific Sound Level from Plant and Machinery in High Residual Sound Environments

Abstract

BS 4142 requires the selection of periods within a sound pressure level time-series to calculate the specific sound level from the ambient and residual sound levels. This is usually done in the time-domain by monitoring and/or controlling the specific sound source(s); however, there are instances where it is necessary to evaluate the specific sound level at a remote location without any control over the operation of the specific sound source(s), which may make it difficult to obtain a representative specific sound level where there are high levels of residual sound. BS 4142 directs practitioners to carry out measurements closer to the source and then use a calculation method to estimate the specific sound level at the assessment location(s). For some types of specific sound sources, it may be possible to increase the accuracy of the specific sound level evaluation using two microphones. This paper presents a method that uses two microphones to acquire ambient sound data simultaneously at the receptor and closer to the source. Coherence is used to identify the components present in the signal measured at the receptor that are also present in the signal measured closer to the source. This approach allows the specific sound level to be evaluated when the difference between the overall A-weighted ambient and residual sound levels is less than 3dB.