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Dear List,

I have added a few comments to the following edited postings by
Peter and Lars.  At the bottom you will find my summary response.

Jont Allen, /June 29, 1997/

> Peter Meijer wrote:
> Date:         Mon, 16 Jun 1997 09:28:02 +0200
> Subject:      The vOICe to AIM
>                                             June 16, 1997
> I'd like to thank all of you who responded, publicly
> or privately, to my inquiry on auditory models (subject
> was "Re: Time and Space"). So far, it seems that consensus
> about the validity of any model for the kinds of auditory
> profile analysis that I'm after is rather unlikely.
> ...
> sounds, I have now run a few experiments using Roy Patterson's
> AIM model. First draft results can be found on my new page at
>    http://ourworld.compuserve.com/homepages/Peter_Meijer/aumodel.htm
> ...
> The vOICe image-to-sound mapping. I hope this will in the
> longer run help bridge the gap between the available large
> body of "microscopic" knowledge of auditory processing and
> perception and the "macroscopic" level of complex sound
> ...
> Results from other auditory models may be added at a
> later stage when other models become available to me.
> Best wishes,
> Peter Meijer
> ------------------------------------------------------------------------
> Lars Bramslow (lbramslow@bk.dk) wrote:
> Date:         Tue, 17 Jun 1997 16:32:00 +0200
> Subject:      Cochlear vs. psychoacoustic models
> Dear List,
> The recent discussion concerning cochlear vs. psychoacoustic models
> deserves further attention, and I wish to share my experience using the
> two approaches.
> For my Ph.D. project (1991 - 1993) I wished to use some auditory model
> as a front-end to an objective measure of sound quality, with special
> focus on hearing aids.  This meant that the model besides normal hearing
> should be able simulate a hearing loss, given by an audiogram.

Isn't it also important to simulate recruitment and
the variation, if any, of the critical ratio with level?

> I needed a practical "engineering" model that could take real signals,
> with SPL calibration, recorded in free field, and process them into some
> kind of auditory "spectrogram", i.e. excitation, firing pattern,
> specific loudness etc.
> The cochlear models seemed relevant with models of active outer hair
> cells for increased tuning and sensitivity, and basilar membrane
> mechanical model, so I started out implementing such a filter chain,
> similar to ideas from Allen and Kates.  However, as soon as I wanted to
> model a particular hearing loss or even make sensible estimates of the
> many parameters in the model, I was helpless.  The literature on
> auditory physiology does provide tuning data for animals only, who had
> been impaired by a toxic or by extreme noise exposure, but this was
> still a long way from an ordinary sensorineural hearing loss!  I was
> also in doubt how to implement some kind of loudness summation, to come
> up with sones.
> So, back to the literature.  The classic psychoacoustic model is the
> Zwicker school, however the procedure for deriving excitation patterns

What about Fletcher and Munson's 1933 loudness model paper,
Munson and Gardner's 1950 forward masking loudness model paper, and
Steinberg and Gardner's 1937 model of recruitment?
These give ample information to do what you want, dont they?

> with upward spread of masking is complicated and graphical (ISO532B),
> and there is no direct information on the filter shape, let alone useful
> information on how to model a hearing loss.
> The answer came from Cambridge.  In several papers, Moore and Glasberg,
> described their model of auditory filters, the roex-filters, WITH level
> AND hearing-loss dependency WITH real numbers.  The filter bands are
> ERB's (not critical bands) and the frequency scale is E (not Bark).

I guess I still dont know the difference between an ERB and a critical band.
Can somebody help me?

> They even provided FORTRAN listings, in case of doubt (believe it, it's
> hard to implement something based on a paper alone).

This is an excellent starting point if you are trying to make
objective measures as applied to hearing aids. If you want to
model the cochlea, this model doesn't help much.

> The model structure then looked like this:  Short-term FFT (I know this
> could be better to avoid the fixed time resolution, i.e. some sort of
> wavelet), followed by various corrections for outer ear, middle ear and
> equal loudness contours, followed by a level-dependent bank of
> roex-filters, whose shape also depended on hearing loss, followed by
> Zwicker's loudness models to estimate specific loudness (son/ERB).  Some
> other output variable could have been chosen, but specific loudness made
> sense in order to include the elevated absolute thresholds.  The whole
> thing was adjusted and fitted according to significant psychoacoustic
> reports from the literature (in 1993).
> The resulting model was able to predict common experiments well:  1 kHz
> masking patterns at various levels, uniformly exciting noise, impaired
> frequency selectivity, loudness growth for normal hearing and hearing
> loss, and equal loudness contours (ISO 226).  There was some

This is the data that was used to set the parameters of this
model. I should hope that it would do a good job.

> underestimation of upward spread of masking and consequently also
> loudness, at high levels.
> Temporal properties were not simulated, nor investigated due to time
> limitations in the project.
> The model acted well as a pre-processor for a neural network and managed
> to predict subjective ratings of sound quality by hearing-impaired and
> normal-hearing listeners well.
> Conclusion:
>  I think cochlear models are useful to model and study the basic
> cochlear mechanisms, but far from useful to make "measuring instrument"
> models.  There are newer models that can simulate a certain percentage
> of inner- and outer-haircell loss along the basilar membrane, but how
> does one estimate this from a regular audiogram.  Granted, the
> psychoacoustic model to some extent is a "black box" simulation, but it
> works in real life!

Are you saying that the cochlear models dont work in real life?

> I hope I haven't generated a "flame" by writing this, but this is an
> important issue to our list.  For those interested, I can provide a
> literature reference list.
> Sincerely,
> Lars Bramslow
> ------------------------------------------------------------------------
> Peter Meijer wrote:
> Date:         Wed, 18 Jun 1997 15:50:37 +0200
> Subject:      Re: Cochlear vs. psychoacoustic models
>                                             June 18, 1997
> Dear Lars,
> I agree with much of what you say, and like to elaborate
> a little on it.
> Indeed there will always be good reasons for having different
> types of models - both cochlear and psychoacoustic, both
> ``physical'' and ``functional.'' In fact, it all depends on
> the purpose one has in mind to use a model for. If the target
> is a detailed understanding of low-level auditory mechanisms,
> then go for the physical route, applying brute-force computing to
> solve the many (partial) differential equations. If the target is
> efficient simulation, e.g., needed to pull through many complex
> sounds at or near the (hearing) system level, then opt for a
> functional model, even a ``black-box'' one if it is too hard
> to derive a simplified but still accurate functional model from
> the physical one (and quite often it *is* too hard).

On this we can agree. It is much harder to model the basic
physics that to make a nonlinear-regression model.

> An advantage of functional models is also that it is often
> easier to switch off particular effects, allowing one to

I think it is easy to switch-off effects in a physical model too,
assuming you programed it yourself.

> trace which modelled non-ideality causes a certain observed
> (functional) effect at the macroscopic level. With physical
> models it is often far more difficult to create, say, a given,
> measured, nonlinear distortion level: functional models are

I dont think so. It is easy, if you know how.

> sometimes even more accurate than physical model in representing
> observable effects, because many parameters in a physical model

and sometimes physical models are more accurate.

> are often only approximately known, while in a functional model
> distortion itself may be a parameter. The opposite can also
> happen, of course, with (too) simple functional models being
> (much) less accurate than the physical ones. Furthermore,
> functional models may have poor predictive value for ``new''
> auditory phenomena: they are better for application areas in
> which the range of relevant auditory phenomena is already
> known/spanned. In other words, functional models can be
> quite good at ``interpolation'' of data sets, but are often
> poor at ``extrapolation.''

This is the whole point of doing a physical model.
Peter, I couldn't agree more.

> ...
> Once the physical and functional models give similar
> outcomes under a wide variety of data sets (sound waves),
> not just few example cases - or they must be very carefully
> selected, confidence will grow that each type of model is
> good for its particular target area (e.g., understanding
> versus efficiency).
> I haven't seen a systematic approach to this so far.

  We havent had the models, or the compute power.

> [A moment ago the posting from John Beerends came in
>  while I was finishing this text. I think his detailed
>  comments are consistent with what I remarked above
>  about the possible high accuracy of functional models.]

These models represent an important steping stone, however
not everybody agrees that they are the final word. "high
accuracy" may be too strong a term.

> Best wishes,
> Peter Meijer
> ------------------------------------------------------------------------

 Jont Allen responds:

What problem are you trying to solve? Do you want to build a
model of the auditory system, or make a new hearing aid?
I sure didnt try to build a better cochlear model while working
on a hearing aid design, I was too busy.

The basic research in how the cochlea and auditory system works
is a difficult puzzle. We do not all agree on how it works.
But without differing opinions, we cannot make progress.

The purpose of a cochlear model is to obtain a basic understanding of
what is going on, not provide a simulation that can be used to
simulate a hearing aid.

The Cambridge (Moore et al.) model is based on Zwicker. The Zwicker model
is based on Fletcher and Munson (1933). As far as I can tell, the
Fletcher and Munson model is based on Wegel and Lane (1928).

The user of a model should not try to "compete" with the designer
of a good cochlear model, nor should the modeler try to compete
with the hearing aid designer. Each has an important place, and
plays a role.

Jont Allen, /June 29, 1997/