Stapedius reflex and safety

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Tony - 24 May 2007 13:57 GMT

I am involved in controlling the exposure to high sound levels of radio and
TV artistes. These people are listening on headphones to a mixture of
sounds, of which the loudest is their own voice. The law on high sound
levels is of course the same for this type of sound as it is for industrial
noise, and employers don't want to be sued in the future for exposing people
to an excessive and illegal sound level. However when the sound of their
own voice is controlled to below the legal limit, many performers complain
that it is not loud enough for them to judge their own performance. In the
case of a professional singer, the level that they can legally be given on
headphones is probably lower than they would get without the headphones!

I have read that the middle ear acoustic reflex reduces the sound level to
the inner ear when speaking or singing, by a considerable amount. This
implies that the risk to hearing is much lower when the source of sound is
the person's own voice. So, should the legislation allow a higher sound
level for this situation? And what would be a good reference book or paper
about this?

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Tony W
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Heinz Weissing - 25 May 2007 14:48 GMT

Hallo Tony,

it was long ago that I dealt with this matter, but as far as I know the
stapedius reflex is no reliable means to reduce the effects of excessive
noise exposure. It works only at the beginning of a high-level noise
exposure, but gets tired later (the stapedius muscle will slacken and thus
loose tension). Moreover, the stapedius reflex is able to reduce sound
transmission to the inner ear only in a certain frequency range - for the
lower frequencies that are associated with wider displacement of the
ossicles, if I remenber correctly.

So the middle ear acoustic reflex is nothing to rely on. I am sure that its
effect, if any, has already been included in the accepted noise level
limits, e. g. in Germany 85 dBA for a daily exposure of 8 hours or 94 dBA
for 1 hour.

At the time when I read about the acoustic reflex, there was no clue that
the reflex intensity somehow depends on the source (sound from the outside
compared with a singer's own voice).

Heinz

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Natural Science Forum / Physics / Acoustics / May 2007

Stapedius reflex and safety