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Frequency Band Filter
A frequency band is all the frequencies between an upper and lower frequency
limit.
When a signal is passed through a frequency band filter the signal remains
unaltered for the selected frequency band whilst all other frequencies are
filtered out.
A common frequency band filter on sound meter instrumentation is an octave
band filter. Here the frequency band is one octave (1/1) and the upper and
lower limits are a factor of 2 apart.
The octave band can be further divided for greater resolution into one third
octave bands (1/3) where the upper and lower limits are a factor of 2
1/3
apart.
When one octave or one third octave bands are selected, the frequency that
describes the band is the centre frequency.
In general sound meter instruments of higher specification have frequency band
filtering and various Castle sound meter models have this facility.
To determine the degree of protection given by hearing protection,
measurements using octave band analysis are generally required, however
another method known as the HML method is also available allowing lower
specified sound meter models without frequency band filtering to be used for
this very important procedure. See also ‘Hearing Protection’.
Hearing Protection
Hearing protection is generally more effective at higher frequencies, hence the
performance of a given ear defender in a given noise environment must take
into account the frequency content of the noise source.
Hearing protectors are either ear muffs (or defenders), which cover the ears, or
ear plugs, which are inserted into the ear canals. Within these two groups
there are several sub-divisions. Ear muffs can have several degrees of
attenuation, whilst the ear plugs can be of a variety of materials, both disposable
and re-usable.
Ear Muffs:
These consist of a cup shaped cover over each ear, held in place by a spring
loaded headband. To ensure a good seal around the ear, the cups are edged
with a cushion filled with liquid or foam. The degree of attenuation is affected by
the material of the cup and its lining and the quality of the seal around the ear.
