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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,replica watches 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, omega replica 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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