A previous blog explained how the SNR (Single Number Rating) figure of a CE approved hearing protector was used to predict the amount of hearing protection provided. It also suggested exercising caution using the SNR for the prediction if the offending noise was excessive in high or low frequencies. Only one value, the equivalent continuous ‘C’ weighted sound pressure level was required for the prediction exercise.

For noise content rich in high and low frequencies it may be preferable to use the HML method instead for predicting the hearing protector attenuation, but the sound level meter that is measuring the noise must be capable of at least capturing ‘A’ weighted and ‘C’ weighted sound pressure levels (if it has no facility for taking octave band measurements).

The HML of a hearing protector (CE approved) specifies 3 values, viz. High (H), Medium (M), and Low (L) which usually accompany the product packaging, and these values are derived from the octave band attenuation performance of the hearing protector using an ISO standard.

This HML method takes into account the spectral characteristics of the offending noise when measuring the equivalent continuous ‘C’ (L_Ceq) and ‘A’ (L_Aeq) weighted sound pressure levels. Computation of (L_Ceq – L_Aeq) shall be applied to the next stages of processing for predicting the attenuation. Prediction of the hearing protector attenuation is a two stage process, where firstly the Predicted Noise Reduction (PNR) is determined as follows:

The second stage of the process is:

L_Aeq’ = L_Aeq – PNR

where L_Aeq’ is the noise level predicted ‘underneath’ the hearing protector. The HSE recommend adding 4dB to this noise level to allow for ‘real world’ factors such as improper wearing of hearing protection and interference (particularly when wearing earmuffs) from other forms of headwear such as glasses and earrings.

As an example, for a hearing protector (where H = 25dB, M = 18dB, L = 13dB) in a noise field where L_Ceq = 103dB, and L_Aeq = 104dB, we have (L_Ceq – L_Aeq) = 103dB – 104dB = -1dB (which is numerically less than 2dB).

Therefore, from equation …(2) above, PNR = 18dB – [0.25(25 – 18)(-1 -2)]dB = 18dB – (-5.25)dB = 23.3dB

Hence L _Aeq’ = 104dB – 23.3dB = 80.7dB (or 81dB to the nearest whole dB).

Allowing for the ‘real world’ factor of 4dB, an 85dBA noise level is predicted ‘underneath’ the hearing protector. You may find it more convenient to use the HSE Hearing protection calculator spreadsheet on http://www.hse.gov.uk/noise/calculator.htm, under the “HML” tabbed worksheet.

Important! The L_Ceq measured quantity is different to an L_Cpeak one also seen on sound level meters and should not be confused with one another. The latter is a ‘C’ weighted absolute peak measurement where high peak levels may present a risk to hearing from a physical damage point of view. The selection of appropriate hearing protection against such damage uses a prediction method (which also uses the HML data) that is different to the one described in this posting.

It should be emphasised that the octave band method of predicting attenuation is regarded as the one that’s most accurate. To use this method your sound level meter must have the capability to measure noise in octave bands.

Essel Acoustics has the skills, experience and qualifications to carry out a noise risk assessment which will include recommending the appropriate hearing protection (if its use cannot be avoided) for staff working in your environment, to fulfil regulatory requirements.