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RESUBMIT:RE: How does a hearing aid help?

Jerry Larson jlarson at op.net
Sat May 10 18:31:06 EST 1997


In article <UPMAIL16.199705102031290514 at msn.com>, rcb5 at MSN.COM ("Ronald
Blue") wrote:

> rom:    daemon at net.bio.net on behalf of Stephen Black
> Sent:   Friday, May 09, 1997 3:00 PM
> To:     neur-sci at net.bio.net
> Subject:        How does a hearing aid help?
> 
> I wonder if anyone would care to take a shot at this one:
> 
> >>>>>>>>Well I am going to try.  Here we go again.
> 


It's been a heck of a long time since I had anything to do with this, and
I'm sure hearing aid technology has advanced quite a bit, but I'll try to
help out.  I'll also refer you to bionet.audiology, where they can help
much more; and there is probably also a newsgroup called
alt.support.deafness or alt.support.hard-of-hearing or
alt.support.hearing-loss  or alt.support.hearing-aids or something like
that.  You could also search the Web, or your local library, for up-to-date
information on hearing aids and hearing loss.

I guess the best way to understand it is that if you have a cover over your
microphone, making the input sound louder will help; this is analagous to a
conductive loss.  If the mike is damaged, it will be less sensitive, and
making the sound louder will help to some degree; but there are limits to
that, since the damaged mike will introduce distortion, and because you
lose dynamic range by losing the quieter sounds.  

> There are two kinds of deafness--nerve deafness and conductive deafness.
> In nerve deafness, the problem is damage within the cochlea, the auditory
> nerve or at some higher level. Conductive deafness is due to impairment
> before this point, usually in the middle ear. The nerve transmission part
> remains ok. 

Well, that's basically correct, though the terminology is archaic. 
"Conductive hearing loss" and "sensorineural hearing loss" would be more
modern and appropriate terminology.  "Deafness" is usually reserved for the
most severe hearing losses, and losses are graded from normal (no loss)
through mild to moderate to severe hardness of hearing, and finally
profound deafness (little or no useable hearing).   In most cases, "nerve
deafness" has nothing to do with the nerve, hence the name change.  Common
diseases, noise exposure, normal ageing, etc. kill off the hair cells
(transducers) in the cochlea; acoustic neuromas and other retrocochlear
nervous system pathologies that can actually affect the acoustic nerve or
auditory pathways are less common.  By the way, the cochlea is tuned from a
high frequency to a low frequency end.  Being smaller and stiffer, and also
closest to the outside, the high-frequency end is much more vulnerable, and
most hearing losses are high-frequency losses.
> 
> So it would seem that a hearing aid would be useful for conductive
> deafness but not nerve deafness, and this is what most elementary
> textbooks say.

Huh?  No, actually that doesn't follow at all.  First of all, there is a
limit to how much of a pure conductive hearing loss you can have.  The
dynamic range of unimpaired humans is about, or slightly more than, 100 dB,
and the greatest possible conductive loss is, I believe, about 35dB. 
However, that would be rare in a developed country, since middle ear
surgery can correct severe middle ear pathology.
So, the point is, if all you have is a conductive loss you hardly need an
aid.  However, a mild sensorineural loss mixed with some conductive loss is
worse than having just the sensory component, and somebody like that would
probably be a good candidate for an aid.

It is true that a significant purely conductive loss would be the easiest
to help with a hearing aid, because the loss would be pretty much
non-frequency specific, and all you have to do is make the sound louder;
the issues of frequency response, phase relationships, distortion, and
dynamic range wouldn't apply.  It is also true that for someone who is
totally, or almost totally,  deaf, a hearing aid can't help much; and
since, as I said, you can't have a really severe loss that is purely
conductive, it is true that the people who really can't be helped all have
sensorineural losses.

 
> >>>>>....consider this, a viberation of the 
> sounds
> >>>>>your are being exposed to under your forearm will increase hearing.
> >>>>>Most likely due to wavelet entrainment.  So any procedure that helps
> >>>>>with wavelet entraiment should help in principle.  Therefore a hearing
> >>>>>aid would help provide the wavelet signal strengthen to help in the
> >>>>>the hearing process.  Consider the large number of nerves cells going
> >>>>>to the hearing system from the brain.  They are doing something.
> >>>>>I would suggest correlational opponent-processing.

Ron-- is this you talking, Ron?  It must be.  You're the only person I know
who is so well-informed that I can never understand anything!  I think this
explanation is too brilliant by half.

Look, whether the aid can help or not depends on what the hearing loss  is,
not its origin.  What you can do, basically, is, you can make the sound
louder.  That helps.  However, it only helps some, not completely,  for a
couple of reasons:

as I mentioned above, there is a finite dynamic range to hearing, and the
growth of loudness is not linear across that whole range.  So you take the
old person with the hearing loss who says, "Speak up" until you speak up
and then says "No need to shout!" Many people with sensorineural losses
have what is called abnormal loudness growth, also called "recruitment"
because of a theory about how it occurs, involving the recruitment of a
different population of sensory cells at higher loudness levels. If you
lose 50 dB of hearing, your remaining dynamic range is only 50dB.  Lose
95dB, and anything you can hear is loud enough to hurt!

Equally important, a damaged inner ear is like a damaged speaker on your
radio.  It has distortion, and turning up the sound doesn't reduce the
distortion.  Part of this is due to altered frequency response, and
theoretically a hearing aid that alters phase and frequency relationships
might help.  In practice, that's not so easy to accomplish, because hearing
is very complex.

When you have a high-frequency loss, you may do OK in a quiet situation,
especially with a male speaker (lower frequency, lots more low-frequency
formants).  When there is noise, distortion, many people speaking at once,
it is much harder for the hard-of-hearing person to process speech, and
it's hard for a hearing aid to help with this.  

Another problem you have with a high-frequency loss is in auditory
location, and bilateral, or crossed, hearing aids, can help with this,
especially when the loss is unilateral.  (A crossed aid is one where the
pickup is on one side, and takes the sound to the opposite ear).
> 
> 
> So it looks as though hearing aids can be of some benefit even in nerve 
> deafness. My question: I don't understand how. If anyone one knows, and 
> particularly if they can supply a good source, I'd be interested to hear 
> about it.

Well, basically by making the sound louder; as I said, that helps, up to a
point, depending on numerous factors starting with the amount of residual
hearing.  There is also a possibility of treating the acoustic signal by
filtering, compression, etc. to reduce noise and distortion and work within
the damaged ear's limitations, and those possibilities are exploited to
some degree.  You need someone who really knows something about modern
hearing aid technology to give you details on this; I'm not the one.

If hearing aids only helped people with pure conductive losses, it wouldn't
be a big business.  Many, or most, deaf children, have hearing aids.  They
don't help all that much, but some help is a lot more than none.  Even if
you can't understand speech, hearing the rhythm of it helps with
lip-reading.
> 
> i can anticipate and reject one possible explanation that may be 
> advanced. This is that if there is only partial damage to the hair cells, 
> hearing function can be enhanced by more intense stimulation of those 
> remaining through hearing aid amplification. But hair cells are tuned to 
> different frequencies, 
> 
> >>>>>Well not exactly correct.  They work together to decode the hearing
> >>>>>experience.  That is why tinnitus results from feedback from the
> >>>>>two systems.  

I don't know about this, but it is basically true that the hair cells are
tuned to specific frequencies.  It's also true that there is a lot of
analysis going on, in the cochlea and in the central nervous system. 
Anyway, you can lose some of the cells, more to the point some of the
response, for a given frequency, so hearing at that frequency is not an
all-or-nothing phenomenon.
> 
> so no matter how strongly those remaining are 
> stimulated, they still cannot bring back perception of the missing 
> frequencies. Is there another explanation?
> 
Frequencies aren't necessarily "missing"; the hearing is less sensitive at
some than others (of course, at worst, it is indeed "missing").  Hearing
losses are (partially) described by curves, called audiograms.  If your
threshold (sensitivity) is normal at 1000 Hz, down 50dB at 2000Hz, and down
90dB at 3000Hz, and hearing is completely absent at 4000Hz, amplifying the
whole signal  by will enable you, perhaps, to hear the 2000Hz part of the
signal that otherwise is too soft.  The problems will be:
--the 1000 Hz part will also be amplified by 50dB, and may be uncomfortably
loud.  This is where amplifying different parts of the frequency spectrum
differently can help, to some degree, but there are limits to that too.
--the loss I've described has a steep slope through the speech frequencies,
which  will probably result in very poor speech comprehension, regardless
how loud you make the sound
--a cochlea as damaged  as that will introduce a lot of distortion.

Hope this helps.

Jerry



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