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Hearing Aid Theory

UNREGISTERED VERSION aja at mit.edu
Thu Jun 20 23:33:32 EST 1996


On 15-Jun-96 12:10:29, Jeffrey G. Sirianni (sirianni at UTS.CC.UTEXAS.EDU) said: 

>A friend of mine recently wrote to me with an interesting question.  I am
>opening it up for general discussion.  He is an electrical engineer in
>amplifier design (too bad, huh?) and doesn't know about dB HL, and so he
>uses dB SPL instead.  Comments would be appreciated.

Engineers don't always understand the problems involved with clinical
work, but that doesn't mean they're (we're?) chock full o' bad ideas,
though.  This particular question brings up some good points.

>>I have a theory pertaining to the inabilities of hearing aids to really
>>TREAT HOH patients.

Uh, oh.  I always worry when I hear sentences like this.  Here's why:
Concern #1: "treat".  What does he mean by this word -- letting them
comprehend speech?  Giving them "normal" hearing?  What is he trying
to treat?  Concern #2: "HOH patients".  Make sure he's not lumping
everyone with a hearing loss into one category.

Here's a fun way to demonstrate to him the problems involved with
hearing aid design: Ask him to build an amplifier, then give it to a
co-worker who's been instructed to modify it in any way -- remove
op-amps, change the values of resistors, add circuit elements, short
the outputs to ground, whatever.  Now, your friend's problem is to
design a front-end circuit to compensate for whatever problem his
co-worker introduced into the amplifier.  He can't look at the
amplifier circuit directly, but he can put any signal he wants into it
and display the output on an oscilloscope.  However, he's not allowed
to look at the oscilloscope display -- he has to get somebody from
marketing to look at it, and he's only allowed to ask that person how
"good" the signal looks.  Tackling this problem should give him some
idea what kinds of difficulties hearing aid designers/dispensers face.

>>1. Picture a hearing aid (HA) inserted in a persons ear.  If the unit is on
>>and functioning, a sound that generates a 30 dB SPL signal outside the ear
>>(or at the pinna) is amplified and results in a 60 dB SPL signal inside the
>>auditory canal.  Not knowing much about HA I assume that this is a pretty
>>fair picture of what happens.

>{Send verses on canal resonance theory, recruitment, and half-gain theory}

Okay, here's where I admit my ignorance.  I don't know what half-gain
theory is (at least, not by that name).  Could you send me that verse?

>>2. Very loud sound is usually accompanied by some sort of a physical sense
>>of the sound pressure wave - think about loud bass at a concert; you can
>>feel the bass energy.  If a person was wearing headphones and listening to
>>music at the same level as the live sound, he (or she) would not be fooled
>>into thinking it was a recreation of the event, because the physical air
>>pressure cues are missing.  No smack in the chest thump from the sound
>>pressure waves. The same would hold true for a recording of wind - you'll
>>always know it is a recording because the physical cues are missing.

This may (or may not) be true, but it doesn't explain why listening
through headphones doesn't recreate a live event.  At low listening
levels, the tactile stimulus is well below the perceptual limit (how
good are you at picking up nanometer changes in position?), but
listening over headphones still doesn't give you the same perception
as listening live.  This is generally believed to be due to the
head-related transfer function and pinna cues imposed between the
free-field sound pressure and the input to the middle ear.  I suspect
these cues play a different role in ITE vs. BTE aids, although I
haven't read any studies of this.  An additional point is that people
with hearing loss generally don't have trouble at high SPLs --
possibly because of the vibrations.  

Regarding the sound of wind, I can only offer an empirical
observation.  When I'm riding my bike (so the wind is basically
head-on), the wind is very loud -- I have to turn my head to hear
cars.  When I do turn my head (so the wind is blowing straight into my
ear canal), I can barely hear the wind at all, even though I can still
feel it blowing against me.  This seems to argue that the effect of
tactile cues on auditory perception is much smaller than that of
directionally-dependent pinna filtering cues.

>>3. SO:  My question:  Is it possible that the brain is not fooled by a
>>hearing aid in the same way that the headphone listener is not fooled into
>>thinking he is at a live show?  The ear canal walls are experiencing SPL
>>levels in the 60's, the pinna and face are registering SPL levels in the
>>30s, and the brain still can't decipher what the sound really is. To the HA
>>wearer, the sound is fake, or at least not quite right, because the air
>>pressure sensations don't fit the sound he is perceiving.  Has anyone ever
>>done research into the tactile effects of sound?  Do the air pressure
>>levels on the face and external ear play a part in the location of a sound
>>source?  I kknow that the air presure changes due to normal sound would be
>>minute, but would they still be important?

Arguably the ear canal walls may experience a different tactile
sensation (I'll avoid the use of the term "sound pressure" when
referring to non-auditory sensations) than the rest of the skin;
however, if the hearing aid was fitted properly then the sound
stimulus (SL) should match the skin sensation.  It won't match the
sensation in the ear canal, but evidence from other cross-modality
studies (e.g. the "inverting prism" studies examining the relationship
between the vestibular and visual systems) suggests that we can
compensate for these differences pretty quickly, if we notice them at all.  

>Oh, I see, if one "expects" the feel of a sound based on loudness, doesn't
>this fool the listener?  Well my feeling is that sound is the primary aspect
>in audition, while tactile sensations are secondary.  If you are "expecting"
>the tactile "thump" and it doesn't happen, then you are fooled.  Isn't this
>like the sensation one gets from listening to electrostat speakers (no
>"cone")?

>****************************************
>* Jeff Sirianni, M.A., CCC-A           *
>* Sound Advice / R.G. Delaney, M.D.    *
>* 710 Water Street / Suite 404         *
>* Kerrville, TX  78028                 *
>*                                      *
>* (210) 896-1433                       *
>* (210) 896-1440 FAX                   *
>*                                      *
>* sirianni at uts.cc.utexas.edu           *
>* jgsaudio at aol.com                     *
>****************************************

-- 
A.J. Aranyosi                 Speech and Hearing Sciences Graduate Student
aja at mit.edu      Harvard/M.I.T. Division of Health Sciences and Technology




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