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Noise - Occupational - Hearing Protection

This section contains information relevant internationally and the directory can be consulted for suppliers of hearing protection.  The HSE's site has some information on hearing protectors; the FAQ on tinnitus (ringing-in-the-ears) can be read here; newsgroups dealing with hearing problems can be found here.

Ear (or hearing) protection amounts to either ear muffs or ear plugs, and selection depends on a variety of factors including

  • The nature and level of noise, muffs generally being more effective than plugs.
  • Compatibility with work and other protective equipment; muffs being more bulky are not as easy to wear with helmets and safety specs etc.
  • It is important that ear plugs, especially more rigid variants are correctly fitted to the particular employee.
  • Individual preference - if employees are given a choice of suitable protectors, there are more likely to wear them.

Predicting Hearing Protector Effectiveness
If having read the following and other guidance you do not feel competent to perform the calculations, details of consultants can be found on the directory.

As with convention means of noise control, in general the higher the frequency, the more effective any given protector will be.

To determine the reduction in noise level (at the ear), the performance of the protector is compared with the frequency characteristics of the noise in question. Protector manufacturers are required to provide standard information (BS ISO EN 4869-1:1995) including:

  • mean and standard deviation attenuation values at each octave-band centre frequency from 125 Hz to 8 kHz (63 Hz is optional);
  • assumed protection values at each centre frequency [the mean attenuation minus the standard deviation at each frequency]
  • H - High, M - Medium and L - Low frequency attenuation values in accordance with BS EN ISO 4869-2: 1994
  • Single Noise Rating - SNR value in accordance with BS EN ISO 4869-2: 1994

The H, M and L and SNR values are based on the mean attenuation values and standard deviations and are provided to allow simplified calculations of the effective A-weighted sound pressure levels (see below)

There are three methods of calculating the degree of protection; for general industrial noise all three give very similar results. The simpler methods become less accurate where the noise is dominated by tonal noise, particularly at low frequencies. Suggestions for Instrumentation can be found on this site . Considering each method in turn.

1. Octave Band - This is the most accurate prediction method, but requires the most detailed noise measurement and involves the most complicated method of calculating the LAeq at the ear. The procedure is as follows

  • Measure the noise in octave band terms plus the overall LAeq "outside".
  • Subtract the octave band assumed protection value for the ear protector
  • Add the A-weighting correction factors in each octave band
  • Convert this sum back to an overall dBA level - LAeq "inside"
  • The protection in terms of dBA is the "outside" LAeq minus the "inside" LAeq.

2. HML - This is the preferred method in the absence of an octave-band spectrum. Two simple measurements of the sound pressure level are combined to produce three figures. The procedure is as follows

  • Measure the noise in terms of the LAeq and the LCeq.
  • The PNR [predicted noise level reduction] is calculated from one of two formulae:

            If LCeq - LAeq is less than or equal to 2:

            PNR = M - [(H - M)/4]x( LCeq - LAeq - 2)

            If LCeq - LAeq is more than 2:

            PNR = M - [(M - L)/8]x( LCeq - LAeq - 2)

  • The PNR is subtracted from the LAeq to give the assumed level at the ear in dBA.

3. SNR - This method is used with a single measurement of the sound pressure level, in terms of LCeq; the SNR is subtracted from the LAeq to give the assumed level at the ear in terms of the LAeq.

Hearing Protector Attenuation - Practice
Many people do not benefit from the predicted protection. There are many reasons including, correct fitting, deterioration of defenders with use, and the length of time the protectors are actually used. Ear plugs are particularly susceptible to poor fitting and can have near zero performance if not correctly fitted. If ear plugs are poorly fitted, 5 dBA is probably the best reduction that can be expected.

The duration of use point is fundamentally important and yet often ignored. The nature of the decibel scale means that in a noise level of say 100 LAeq, if very efficient protectors are only worn for half the 8-hour working day, the daily noise dose would be 97 LAeq, 8hours, i.e. little benefit.


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