Selecting the correct hearing protector for a specific noise environment requires calculating the residual A-weighted sound level at the worker's ear after the hearing protection is worn. Too little attenuation leaves workers over-exposed; too much attenuation (over-protection) creates communication difficulties and safety hazards by masking warning signals. The EU PPE Directive, ISO 4869, and most national occupational noise regulations specify three accepted methods for checking hearing protector adequacy: the SNR method, the HML method, and the Octave Band method.
The SNR is a single attenuation value printed on the hearing protector packaging. To apply the SNR method, measure the noise level using C-weighting (LCeq), then subtract the SNR value to obtain an estimate of the A-weighted noise level at the ear. The SNR method is the least accurate because it assumes the noise spectrum is similar to the test spectrum used to derive the SNR. It is appropriate for screening purposes only.
The HML method uses three attenuation values — H (high frequency), M (medium frequency), and L (low frequency) — to account for the spectral character of the noise. Both A-weighted and C-weighted measurements are needed. By comparing LCeq and LAeq, a correction factor is applied to select either H or M/L values for the calculation.
The octave band method uses the measured noise spectrum (in octave bands from 63 Hz to 8 kHz) and the HPE's mean attenuation and standard deviation in each octave band. The A-weighted level at the ear is calculated band by band and then summed logarithmically. This method is required when noise is tonal, when unusual spectra are present, or when regulatory frameworks demand the highest verification accuracy.
SoundPLANmanda's occupational noise mapping software automates exposure calculations and helps identify which zones require hearing protection and what attenuation level is needed. View the occupational noise glossary | Back to occupational noise mapping