Transfer function of human head

Hi all,

We are trying to make a hearing aid for profoundly deaf people. It is 
observed that if a sound signal modulated on an ultrasonic carrier is 
conducted through the bones in the human head, it is audible (and even 
understood in some cases) by profoundly deaf people. However, the exact 
phenomenon happening inside the head is not exactly known. We want to 
calculate the transfer function between the excitation position 
(position of the transmitter placed behind the ear) and the cochlea, to 
explain the phenomenon and improve the performance of our device. The 
problem is that we know the input but for the output (the sound actually 
heard), we have to depend on the human subjects and there are lot of 
differences in the data collected from different subjects. We find 
ourself stuck, and shall be extremely thankful if some one could please 
suggest a way to proceed.

Regards,
Ishtiaq.

I. R. Khan wrote:
> Hi all,
> 
> We are trying to make a hearing aid for profoundly deaf people. It is 
> observed that if a sound signal modulated on an ultrasonic carrier is 
> conducted through the bones in the human head, it is audible (and even 
> understood in some cases) by profoundly deaf people. However, the exact 
> phenomenon happening inside the head is not exactly known. We want to 
> calculate the transfer function between the excitation position 
> (position of the transmitter placed behind the ear) and the cochlea, to 
> explain the phenomenon and improve the performance of our device. The 
> problem is that we know the input but for the output (the sound actually 
> heard), we have to depend on the human subjects and there are lot of 
> differences in the data collected from different subjects. We find 
> ourself stuck, and shall be extremely thankful if some one could please 
> suggest a way to proceed.
> 
> Regards,
> Ishtiaq.

This is a rough one, particularly if part of the differences are in the 
nervous system.

I can think of several ways that you could approach this, ranging from 
not at all to wildly invasive.

Not at all invasive would be to establish a reference tone, and compare 
other tones to that (this is a two-dimensional problem, I assume, 
because you'd be considering the frequency of the carrier as well as the 
tone which it is carrying).  You'll still be getting approximate 
amplitudes, and it'd be as boring as hell for your test subject, but 
it'd be a start.  This could be particularly effective if this method 
works on folks with normal hearing without damaging their hearing -- 
then _you_ could be the test subject!

Wildly invasive would be to insert a microphone into someone's cochlea, 
and do your tests.  I hereby volunteer for this, as long as I'm dead 
when you do it*.  This may be something that you could piggy-back on 
some nice volunteer's cochlear implant operation.

Also wildly invasive, but potentially very rewarding, would be to find a 
volunteer who's willing to have probes inserted into his auditory nerve. 
  I have _absolutely_ no idea how much effort this would be, but I would 
assume it's a medical-school research sort of thing rather than a garage 
shop sort of thing (unless you have a very _interesting_ garage).

*  My driver's license says I'm an organ donor, but I'm getting to be a 
pretty high-mileage one, and it's known that my brain will go into a 
flask marked "Abnormal!  Do not use!".

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

I. R. Khan wrote:
> Hi all,
>
> We are trying to make a hearing aid for profoundly deaf people. It is
> observed that if a sound signal modulated on an ultrasonic carrier is
> conducted through the bones in the human head, it is audible (and even
> understood in some cases) by profoundly deaf people. However, the exact
> phenomenon happening inside the head is not exactly known. We want to
> calculate the transfer function between the excitation position
> (position of the transmitter placed behind the ear) and the cochlea, to
> explain the phenomenon and improve the performance of our device. The
> problem is that we know the input but for the output (the sound actually
> heard), we have to depend on the human subjects and there are lot of
> differences in the data collected from different subjects. We find
> ourself stuck, and shall be extremely thankful if some one could please
> suggest a way to proceed.
>
> Regards,
> Ishtiaq.

Wack each subject in the head sharply with a wooden hammer, and take
the Laplace transforms of whatever results. ;)

Seriously,  I vaguely remember that people who work in speech
recognition face this problem often.  I think you are looking for
what's called an HRTF (Head-Related Transfer Function).
http://en.wikipedia.org/wiki/Head-related_transfer_function .  Seems
that people are always trying to come up with better models of the
head, although there seems to be as much interest in speech synthesis
as analysis. As I recall, modeling the head as H(s) is very hard
because:

1.  Head is non-linear.
2.  Head is time-variant.
3.  Transfer function varies widely from head-to-head.
4.  A true impulse would make the subject completely deaf. :)

(etc.)

So you have a non-linear system that is not only time-variant, but
there are many systems under test.

Nevertheless, if you want only one H(s) I would take a brute-force
approach.  Have the each subject swallow a microscopic tethered
microphone and generate white noise and measure the response.  Do this
for various situations, mouth open, closed, background noise, no
background noise (yes, very tedious).

Once you have collected enough data, find the optimum H(s) for
particular listening scenarios from the H(s)'s you measured. The
optimum H(s) could be the MMSE estimate.  You could also make your H(s)
listener-dependent.  Parameterized your H(s) and have the subject
subject tune the device by say, going to the WWW, playing a tone bank,
and putting their mouths over the speaker as the parameters are
determined. 

-Le Chaud Lapin-

Thank you for your very informative and amusing responses. I think every
one is enjoying vacations.

A medical hospital is also involved in this research, but "the bosses"
want to keep surgery out of equation, at least for now. I will look into
your suggestions in detail, but right now I have been asked to study
FDTD method and find if it can be used for getting the transfer function
of head. Can any one please suggest some good book/paper about this
method? Any comments about using it in this problem will also be highly
appreciated.


Ishtiaq.

Tim Wescott wrote:
>
> Wildly invasive would be to insert a microphone into someone's cochlea,
> and do your tests.

An approximation to this could be to fill a human skull with silly
putty or some such material of appropriate density (I assume the
density of the human head can be found :) ) in which a microphone array
is embedded in the anatomically appropriate place. How good would this
approximation be? I have no idea!

C

I. R. Khan skrev:
> Hi all,
>
> We are trying to make a hearing aid for profoundly deaf people. It is
> observed that if a sound signal modulated on an ultrasonic carrier is
> conducted through the bones in the human head, it is audible (and even
> understood in some cases) by profoundly deaf people. However, the exact
> phenomenon happening inside the head is not exactly known. We want to
> calculate the transfer function between the excitation position
> (position of the transmitter placed behind the ear) and the cochlea, to
> explain the phenomenon and improve the performance of our device. The
> problem is that we know the input but for the output (the sound actually
> heard), we have to depend on the human subjects and there are lot of
> differences in the data collected from different subjects. We find
> ourself stuck, and shall be extremely thankful if some one could please
> suggest a way to proceed.

Your solution is, of course, limited to those people whose hearing
impairment is caused by a malfunction in the inner ear. The underlying
assumption seems to be that the cochlea works.

First of all, you need a model for how the cochle works, i.e. how the
movements of the filaments(?) inside the cochlea is transformed to
audible sound.

Second, you need a model for the anatomy of the ear and surrounding
areas. Whether you can get to one of those, is anybody's guess,
even if you can make measurements on some diseased person,
chances are that the different properties of living and dead tissue
will be important here. I wouldn't be surprised if the difference
between flowing and coagulated blood in the tissue, turned out to
be important.

Assuming you can get to a model of the anatomy, use a finite
difference or finitre element method to simulate how the
sound transmitted behind the ear affect the filaments in the
cochlea.

Depending on the wavelengths of the sound you transmit, be
prepared for multiple reflections inside the bone tissue of the
scull and/or the voids of the outer and middle ear.

Rune

I. R. Khan wrote:
> Hi all,
> 
> We are trying to make a hearing aid for profoundly deaf people. It is 
> observed that if a sound signal modulated on an ultrasonic carrier is 
> conducted through the bones in the human head, it is audible (and even 
> understood in some cases) by profoundly deaf people. However, the exact 
> phenomenon happening inside the head is not exactly known. We want to 
> calculate the transfer function between the excitation position 
> (position of the transmitter placed behind the ear) and the cochlea, to 
> explain the phenomenon and improve the performance of our device. The 
> problem is that we know the input but for the output (the sound actually 
> heard), we have to depend on the human subjects and there are lot of 
> differences in the data collected from different subjects. We find 
> ourself stuck, and shall be extremely thankful if some one could please 
> suggest a way to proceed.
> 
> Regards,
> Ishtiaq.


Would an indirect measure be useful? I'm thinking a series of EEG's.

Carrier off, carrier on without modulation, modulated carrier.

Comparison to EEG's of normal hearing subject receiving modulation 
signal thru normal hearing test setup.

"I. R. Khan" <ir_khan@REMOVE.hotmail.com> wrote in news:emvmvn$p8r$1
@aioe.org:

> Hi all,
> 
> We are trying to make a hearing aid for profoundly deaf people. It is 
> observed that if a sound signal modulated on an ultrasonic carrier is 
> conducted through the bones in the human head, it is audible (and even 
> understood in some cases) by profoundly deaf people. However, the exact 
> phenomenon happening inside the head is not exactly known. We want to 
> calculate the transfer function between the excitation position 
> (position of the transmitter placed behind the ear) and the cochlea, to 
> explain the phenomenon and improve the performance of our device. The 
> problem is that we know the input but for the output (the sound actually 
> heard), we have to depend on the human subjects and there are lot of 
> differences in the data collected from different subjects. We find 
> ourself stuck, and shall be extremely thankful if some one could please 
> suggest a way to proceed.
> 
> Regards,
> Ishtiaq.

You could take a psychophysical approach.

Alternatively, you could try to stimulate a DPOE (an otoacoustic emission) 
by stimulating at two frequencies through the bone and recording in the ear 
canal, looking for the third frequency.  

-- 
Scott
Reverse name to reply

"Le Chaud Lapin" <jaibuduvin@gmail.com> wrote in 
news:1167292092.909584.288700@48g2000cwx.googlegroups.com:

> HRTF (Head-Related Transfer Function).


The HRTF does not take bone transmission into account-- just the shape of 
the ear and canal, the separation between the ears, and the acoustic 
shadowing of the head.


-- 
Scott
Reverse name to reply

Richard Owlett <rowlett@atlascomm.net> wrote in 
news:12p7jcag34a7b4e@news.supernews.com:

> Would an indirect measure be useful? I'm thinking a series of EEG's.
> 
> Carrier off, carrier on without modulation, modulated carrier.
> 
> Comparison to EEG's of normal hearing subject receiving modulation 
> signal thru normal hearing test setup.
> 

Yes, an evoked potential would be another viable approach.

-- 
Scott
Reverse name to reply