I am going to be honest, this is news to me. I was advised that the SNR level *is* the reduction in decibel level. for example 100-32=68dB which is ok (or if anything, slighty too much).

I am not a hearing expert, and only follow what I have been advised by our noise surveillance contractor. Have they advised me wrong?

Edited by user 13 September 2021 14:36:23(UTC)  | Reason: Not specified

It is a calculation to correct between the A weighted measurements taken in the workplace and the C rated values applied as SNR. On the HSE calculator is a relativley straight forward table:

 dB(A) 85-90 select SNR 20 or less

dB(A) 90-95 select SNR 20-30

dB(A) 95-100 select SNR 25-35

dB(A) 100-105 select SNR 30 or more

So for a background 100dB(A) you want an SNR of 30 or more. Just be careful not to over egg the protection as it could make the operators feel isolated.

The HSE web-page https://www.hse.gov.uk/noise/goodpractice/hearingrealworld.htm states:

Accounting for 'real world' factors

It is very likely that under conditions of real use, hearing protectors will give lower protection than predicted by manufacturers' data which is obtained from standardised tests. The difference between manufacturers' data and 'real-world' protection is due to factors such as poor fitting and wearing of spectacles or other personal protective equipment. You should account for this 'real world' protection by 'derating' the protector by 4 dB. This means, having followed one of the standardised methods for calculating the attenuation provided by a hearing protection device, assume that the sound level at the ear when hearing protection is worn will be 4 dB higher than would be predicted by the method. In this way you will get a better indication of the protection that users are most likely to get, and can select appropriate hearing protection accordingly.

The derating does not apply to the assessment of hearing protector performance against peak noise.

The 4 dB derating described above is regarded as an appropriate factor to bridge the gap between manufacturers' data and real-world factors, without introducing further complexity to the prediction of hearing protector performance. You will still be able to select an appropriate hearing protection device for the character of the noise, and hearing protectors that show better repeatability in standardised tests will still be distinguishable.

Not sure where they get this 4dB from, but if you simply adopt this 4dB error on the actual value of SNR then IMHO, you will be providing suitable protection.

i.e.  You measure you rnoise levels as 110dB and you provide ear protection with SNR = 32dB.

Either 110 + 4 = 114dB + SNR protection of 32dB = exposure of 82dB, or 

110dB - SNR of (32 - 4 = 28dB ) = exposure of 82dB

Hope that makes sense

Allowing for the correction factor raised by Evans38004

For 100 db(A) you are looking for a product with an SNR of 30 - 26 giving "at ear" values of 74 - 78

Originally Posted by: ballyclover

I may well be wrong but this was the way i was shown years ago, That you take away 5 from the ear plug rating, devide by 2 and take that away from your main decible reading.  Some say subtract 7 but ive used 5.

Absolutely no idea where you got this calculation from - 5 is closer to the HSE adjustmemt of 4.

Very likely over protecting with the divide by two before subtraction.

Sounds like a merger of two methods.

To apply the SNR protection calculation, sound levels must be C weighted (dBC, not dBA). 

HSE's calculator is ideal for this: www.hse.gov.uk%2Fnoise%2Fhearingcalc.xlsm&wdOrigin=BROWSELINK">hearingcalc.xlsm (live.com)