”Real-world” attenuation
of muff-type hearing protectors:
The effect of spectacles
Frank Lemstad and Roald Kluge
Sinus as,Sandvigå 24
N-4007 Stavanger, Norway
ABSTRACT
A study has been carried out to investigate how much
spectacles influence the attenuation of ear muffs Measurements
have been performed using broad-band stationary noise
on real persons with four different safety spectacles.
It is concluded that spectacles introduce a very significant
reduction in the attenuation. Depending on the spectacle
and person, the effect can be from “moderate” to “severe”.
Particularly important is the reduction at low frequencies
(150-400 Hz). This is due to the side bars introducing
a leakage that makes the ear muff act as a kind of Helmholtz
resonator with a frequency 200-300 Hz (depending on muff
volume). However, also at 3-5 kHz the opening seem to
produce a resonance which – for
some spectacles and persons – can reduce the attenuation significantly.
The “spectacle effect” varies a lot from
person to person and spectacle to spectacle. However,
it is very consistent over the three different Peltor
muff models tested. For optimum attenuation a good fit
and thin side bars (particularly small width) is essential.
Unfortunately, many safety spectacles have rather thick
bars which may produce almost disastrously poor muff
attenuation – even with the very best muffs available.
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