From: fulk@cs.rochester.edu (Mark Fulk)
Newsgroups: sci.skeptic,sci.med
Subject: Re: Hearing aids (ATTN: Ruth Ginzberg: suggestion for FAQ)
Date: 21 Dec 92 16:41:05 GMT
Organization: University of Rochester

This issue has come up a few times in the last couple of years, so perhaps
it is time for an FAQ entry.  The following information is courtesy of my
wife, a speech pathologist, who had to train with audiologists and sometimes
has to administer hearing tests in her line of work:

NEVER buy a hearing aid from a dealer.  They are only interested in selling
you something expensive, and they are not qualified to prescribe an aid.

If you have a hearing problem, you should discuss it with your doctor
and possibly an ENT doctor (otolaryngologist), in order to rule out
any treatable causes.  Most likely there will not be anything they
can do; they should refer you to an audiologist.  Many ENTs have an
audiologist working in their office.

You want to see an audiologist with certification from the American Speech,
Language, and Hearing Association (ASHA), in the US and Canada; I do
not know the names of other national certification bodies.

The audiologist will spend around half an hour testing your hearing in a
sound-proof booth.  She will also take molds of your ears for the aid.
At the time of your visit, you should make an appointment for a followup
a few weeks after you get your aid.

The followup visit is part of the original audiologist's fee, so don't skip
it in order to save a few bucks; you won't save anything and you will miss
a chance for a potentially important adjustment.

Why should you bother?

1) Modern aids are quite complicated; they can selectively filter, boost,
and shift frequencies.  They are at least as complicated as glasses;
an ill-fitted aid can be worse then no aid at all.

2) Human ears all suffer from a curious defect: the threshold for permanent
damage is well below the threshold for pain (90 dB versus 110 dB).  In a
person with a hearing problem, a level of sound that damages the ears may
not even seem very loud.  A badly-adjusted aid can produce damaging sound
levels, and a badly made ear mold can allow audio feedback, which can easily
reach damaging, even extremely painful, levels.

3) If a pair of glasses fits very badly, the patient quickly notices and
returns to the optometrist.  Unfortunately, a badly fitted hearing aid
can do a great deal of harm long before the patient notices.

4) In New York, and some other states, audiologists are allowed to sell the
hearing aid they prescribe.  The selling price is equal to the audiologist's
purchase price, so no profit is made on the transaction (actually, given
the carrying costs, the audiologist loses money).  If your state allows
this, you should definitely get your aid from the audiologist; going
to a dealer costs more and the dealer may try to pass off an aid that
pays a higher profit margin, saying that it's obviously better, being
more expensive.  Repeated experiences of dealers misfilling prescriptions
is the reason for the New York law.  If you have to go to a dealer to
get your prescription filled, insist that it be filled exactly as written,
and make an appointment with the audiologist for an adjustment ASAP.

----------

And a note on "nerve deafness:"

So-called "nerve deafness" is usually the result of damage, not to nerves,
but to the hair cells of the cochlea.  These cells run in a line in the
inside of the cochlear spiral; the _basilar membrane_ extends from the
other side of the tube to lie against them.  Vibrations in the cochlea
push the membrane, which pushes the hairs (cilia), which cause the hair
cells to develop potentials that are passed on to the auditory neurons.
Very loud sounds cause the basilar membrane to shear off the hairs;
unfortunately, the hair cells never grow new hairs.  Some infections
(measles, sometimes) can also destroy the hair cells.

The usual cause of hearing loss in the hearing population is noise-induced
damage to the hair cells.  Sensitivity to the higher frequencies is most
hurt; people with this loss often have trouble distinguishing among
fricatives (s, f, soft th) and unvoiced stops (p, t, k).

-- 
Mark A. Fulk			University of Rochester
Computer Science Department	fulk@cs.rochester.edu
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