The cochlear amplifier

[ingrast]

Last Monday, August 21, I attended the first lecture of an audio seminar by the Multimedia interdisciplinary working group here at the University. This one and the next lecture scheduled for Aug. 28, are devoted to the human ear and auditory system, and I thought it may be of interest to share, given the fact the level is both accessible and up to date, and of course of central concern for those involved with audio technologies.

The most striking observations for me at least, can be summarized as follows:

The cochlear mechanical response (it is an elongated, coiled organ which works as a spectrum analyzer), is substantially nonlinear. This implies significant production of harmonic and intermodulation products.

Related with above, there is an actual distributed acoustic amplifier, formed by specialized cells similar to the actual sensorial ones, but dedicated to sense vibrations and regenerate them. This feature allows for both about 50 dB increase in sensitivity, and for a sharpening of frequency discrimination well above what the Basilar Membrane should in a purely passive mechanical way. This amplification, which requires energy supply readily available, obviously only works on living tissue and the protein involved (prestin) was characterized as recently as year 2000.

All this system in turn, apart from generating stimuli upstream to the auditory cortex (afferent pathway), is in turn heavily controlled and modulated by downstream (efferent pathway) signals responsible for compression among other features.

If there is interest on this issue, I will be posting noteworthy facts from this seminar, as well as keep this thread obviously open to welcome contributions.

Rodolfo

P.S. Moderators may choose to move this thread to whatever forum they deem better suited.

[SY]

This is a perfect place for it. Very interesting; please do add stuff when it's discussed.

[ingrast]

The support slides of the first lecture (in spanish) can be found here. There is also relevant material for review i found in a cursory search.

From Physiological Reviews by Robles et. al. Excellent compilation of different facts and research findings.

From Stanford U. a course including fine diagrams and fairly up to date (2002) facts. (go to syllabus for lectures download).

Among other things, the Robles review mentions the measured -in vivo- mechanical intermodulation distortion diminishes with increasing SPL. I wonder whether this has something to do with the fact (at least for me) that low level music tends to be muddier, and upong bringing up volume, definition and transparency seem to pop out much better.

Rodolfo

[Lord Manhammer]

I'm all ears! Seriously, please post more; for a forum supposedly dedicated to audio, there is depressingly little information on the workings of the ear.

[Sch3mat1c]

The thing that people always claiming "ohhh but the ear has 10% THD!!!!!11" keep forgetting is that obviously, a 1kHz sinewave does NOT sound like it has any 2kHz, 3kHz, 4kHz, 5kHz, ... elements. A pure tone is clearly distiguishable from a distorted tone. NOT ONLY that, but if 10% were true, nobody would notice 1%, maybe even 2% THD. Which isn't true (as I recall...I don't have a THDometer to find out myself). Thus, as it is an established fact that the cochlea distorts, the brain MUST counter it in a forward fashion, passing linear sound to the conciousness.

Hmm, to the conciousness eh? That's where you freaks oughta start... tap right into the brain. -50 to +200dB with no pain or compression. Or into the memory center and you can enjoy a flawless performance at any time!

Tim

[Stocker]

There was a very good, very long article on this topic linked to IIRC in a thread on this site. Aside from the increasing amplification with lower SPL, there was an interesting side note about how the first researchers used corpses to test hearing response... the sounds got so loud that the bodies were literally being moved by the SPL before electrical signals they could measure were produced...

kind of gory, in retrospect...

[Sch3mat1c]

...Loud enough to wake the dead?

[MBK]

Actually the first hint that the ear works as an active amplifier apparently came from Thomas Gold in the 60's. It was his thesis and it wasn't well received... Thomas Gold had a lifetime production of theories well ahead of his time, sometimes confirmed, sometimes (as of this time) refuted, in fields as diverse as biology, astrophysics, cosmology, and geology, to name a few...

(oops, just saw he's cited in the review article, and it was... 1948, gasp)

[ingrast]

Originally posted by MBK
....
(oops, just saw he's cited in the review article, and it was... 1948, gasp)

Yes, in fact the lecturer last Monday mentioned there is a Nobel prize photograph where if I don't remember wrong, Von Beckesy was the one awarded on the basis of his extensive research but on the premises of a passive sensory mechanism, and in the same frame was an unhappy Gould looking at it. At last justice was made.

Gould - and engineer by profession - had no basis to support his hypotheses, but from an engineering standpoint that there was no other way except some active operation for the actual hearing performance to be achieved. He was right.

Rodolfo

[tschrama]

Originally posted by Sch3mat1c
The thing that people always claiming "ohhh but the ear has 10% THD!!!!!" keep forgetting is that obviously, a 1kHz sinewave does NOT sound like it has any 2kHz, 3kHz, 4kHz, 5kHz, ... elements. A pure tone is clearly distiguishable from a distorted tone.

Tim

sorry, but you're just plain wrong ...

Actually, the non-linear mechanics and electrophysiology of the inner ear organ indeed does make a loud sine sound like a distorted sine.

I have read in a book about the history of acient european music, that harmonics and scales were invented by monikken who very carefully listen to their own ears IMD and HD.


This non-linear mechanics and electrophysiology of the inner ear is being used every day in countless hospitals to check the hearing of childs suspected of having hearing problems: no non-linear products = something is wrong.