Caltrans Online Training-Highway Noise Measurements and Instrumentation

First, let’s define what we mean by ambient and background noise levels. Ambient noise is the combined noise level of all noise sources present at the moment of a measurement, including all noise sources of interest. Background noise is the combined noise level of all sources present at the moment of measurement, excluding the noise sources of interest. Why is this distinction between ambient and background noise important? This may best be illustrated with a common example. When we measure highway noise, our source of interest is obviously the traffic on the highway. And when we’re close to the highway, we assume that we are measuring only the highway traffic noise. But is this really true? Our instrument cannot selectively exclude certain noise sources and therefore always measures the noise from all sources combined. In other words, it measures the ambient noise level. In this case, we can say that the ambient noise equals the background noise plus the highway noise. The background noise includes the noise from local streets and typical community noise sources. As is the case with any equation that includes three parameters, if we know two of the parameters, we can calculate the third. However, in the “Fundamentals” training module, we learned that if two noise sources generate noise levels that differ by 10 decibels or more, the lower noise level does not contribute to the combined noise level of both sources. So, if the background noise level is at least 10 decibels lower than the ambient noise level, the ambient noise level can be assumed to be controlled by noise from the highway. For the moment, let’s stay on the topic of ambient noise and move on to the next slide.

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