Measuring noise (2) 'A' weighting

In the measurement details that are commonly associated with frost fans, the noise levels are typically produced as dBA L10.

We have explained a bit about what the ‘dB’ part means earlier. Now we will explain a bit about the ‘A’ part. Again (and sorry to harp on) this is a fairly tricky subject for some and this information will not make you an expert. But it may help lift the fog a little.

In the measurement of Sound levels, A-weighting is the most commonly used of a family of curves defined in the International standard IEC61672:2003.

Sound level, loudness, and sound pressure are not the same things; indeed there is not even a simple relationship between them, because the human hearing system is more sensitive to some frequencies than others, and furthermore, its frequency response varies with level, as has been demonstrated by the measurement of equal-loudness contours. In general, low frequency and high frequency sounds are perceived to be not as loud as mid-frequency sounds, and the effect is more pronounced at low pressure levels, with a flattening of response at high levels. Sound pressure level meters (SPL meters) therefore incorporate weighting filters, which reduce the contribution of low and high frequencies to produce a reading that corresponds approximately to what we hear.

A-weighted decibels are abbreviated dB(A) or dBA. When acoustic (calibrated microphone) measurements are being referred to, then the units used will be dB SPL referenced to 20 micropascals = 0 dB SPL.

While the A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many sound level meters, it does not really give valid results for noise because of the way in which the human ear analyzes sound. The A-weighting system is used commonly in roadway noise and aircraft noise analyses. Humans are considerably more sensitive to noise in the region of 6 kHz than to tones of equivalent level (see ITU-R 468 weighting for further explanation).

A-weighting is also in common use for assessing potential hearing damage caused by loud noise, though this seems to be based on the widespread availability of sound level meters incorporating A-weighting rather than on any good experimental evidence to suggest that such use is valid.

Beware… the distance of the measuring microphone from a sound source is often "forgotten", when SPL measurements are quoted, making the data useless.

In the next instalment we will give some common variations of noise measurement variations including the ‘L10’ reference.