Subject: Cochlear vs. psychoacoustic models From: =?iso-8859-1?Q?Lars_Bramsl=F8w?= <LBRAMSLOW(at)BK.DK> Date: Tue, 17 Jun 1997 16:32:00 +0200Dear 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. 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 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). They even provided FORTRAN listings, in case of doubt (believe it, it's hard to implement something based on a paper alone). 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 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! 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 ------------------------------------------------------------------------ Lars Bramslow Bruel & Kjaer Skodsborgvej 307 DK-2850 Naerum DENMARK tel (front desk) +45 45 80 05 00 tel (direct): +45 45 80 78 55 (tone) 2663 fax: +45 45 80 40 82 email: lbramslow(at)bk.dk ------------------------------------------------------------------------