dB(A) and dB(C) data in noise assessments

In a noise assessment you will see a few different types of noise data presented and it can seem a little baffling. Noise is measured in decibels, but not all decibels are the same, that would be far too easy.

dB(A) figures in a noise assessment

For most people, these are considered the main figure. The daily limits of 80 and 85 dB(A) are given as dB(A) so this is usually the first one referred to.

The ‘A’ in dB(A) doesn’t actually mean ‘average’ but if you think of it that way then you won’t be all that far off really.

The human ear doesn’t hear all noises the same and we hear some frequencies more keenly than others. Think of those particular sounds which kind of get in your head, a high pitched shriek. Your ear is hearing those more acutely than others and those sounds are important in how we understand speech.

As you are more sensitive to those sounds, when noise is measured in a noise assessment, the reading is tweaked slightly so the noise meter is recording the noise level in the same way the ear hears it and that kind of adjustment is called dB(A).

dB(A) is what is used to work out an average daily noise exposure, hence if you think of ‘A’ as average its actually also kind of correct.

dB(C) figures in a noise assessment

Starting to get a little nerdy here… dB(C) is another type of weighting or adjustment used when measuring noise risk. In this case we use dB(C) for two things:

dB(C) as peak or impact noise

As well as the daily average limits the Noise Regs have limits for dB(C), these being 135 and 137 dB(C). In this case the dB(C) is being used to measure impact noise - unlike dB(A) the duration of the noise doesn’t matter, it is that moment of immediate impact on the ears which is being measured.

The power of the noise in that instant is enough to damage hearing.

The best example I can think of where this is important is in a packing warehouse. Usually these kinds of places are pretty quiet generally and may have an average dB(A) of say 75 dB(A), so well below the 80 and 85 dB(A) limits in the Noise Regs. However, someone then drags a pallet to where it is needed and rather than lowering it just lets go so it crashes to the ground. That can create a peak of ≈138 dB(C). Do that often enough and it will start to cause hearing problems no matter what the average noise level is.

Another example would be golfists - they wander around outside in a generally low environment, then whacking a golf ball with a titanium golf club can give an instant noise of 140 dB(C) or more. Same with shootists - quiet and then bang, and it is that bang which dB(C) focuses on.

It is normal for the peak dB(C) figure in a noise assessment to be far higher than the average dB(A) - that doesn’t mean there is a problem with the noise levels or the noise assessment itself. A result of say 88 dB(A) and 119 dB(C) for a job is entirely normal. Remember, the limits for dB(C) are a lot higher, at 135 and 137 dB(C).

dB(C) as a means to assess hearing protection

The simplest-yet-still-effective way to assess how good hearing protection is performing is to use the SNR of the hearing protector. SNR, Single Number Rating, is a figure for how many decibels the hearing protector reduces the noise by. What you do is subtract the SNR of the hearing protector from the average dB(C), add 4dB back on for slightly incorrect wearing, and that tells you what the noise level under the protector is.

Important - you use average dB(C) for this, not the peak dB(C) as that will give an incorrect result. More often than not the average dB(C) is in the vague vicinity of the dB(A) average.