Why You Hear What You Hear

 

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Supplements for Chapter 21

Mechanisms of Hearing

 

Check your hearing range.

Listen to chirp2.wav, and time it in seconds from the first sound to the disappearance of the sine tone at high frequency. The tone goes linearly from 220 to 18,000 Hz. Use good earphones or earbuds,

Your hearing cuts off in frequency approximately according to

max high frequency = r 18,000/30 Hz,

where r is the number of seconds you hear the tone. If it reaches 18,000, of course it could go all the way to 20,000 or slightly more. If so, you are likely less than mid-20's in age.

Links

Must - watch video: Auditory Transduction by Brandon Pletsch (below)

 

 

Very good article (download the free PDF) about early theories of hearing: http://asadl.org/jasa/resource/1/jasman/v20/i4/p591_s2?bypassSSO=1

A good, comprehensive reference on cochlear mechanics is found here.

Excellent graphics and detail about the workings of the basilar membrane from Oxford University

PHYSCLIPS introduction to the workings of the ear.

More graphic detail on inner and outer hair cells

Nomenclature:

Hair cells as resonators: the Underwater Ukulele

Hair cells are small, and immersed in liquid - i.e. strongly damped. There is no way they act as little resonators with a high Q capable of giving us our keen frequency resolution capabilities. Even the relative large, heavy ukulele strings in the underwater (!) example below have a Q of 5 at best.

Herman Helmholtz loved the idea that hair cells were little resonators capable of explaining frequency resolution. He should have known better. We now know we achieve our frequency resolution by active neural feedback, (see "Dancing outer hair cell", next) although the precise mechanism for this is not known; see Why You Hear What You Hear for discussion. As discussed there, it seems a high Q physical resonator-hair cell would interfere with our fast time resolution of transient sounds; this is checked by an experiment suggested in Why You Hear What You Hear.

Underwater Ukulele sonogram at the left; the same Ukulele in air at the right. Note the strong damping in water, even stronger for the higher harmonics, that never really get going. The sounds are here.

 

Beating and Roughness

A demonstration of beating and roughness can be downloaded for the Wolfram CDF Player at

http://demonstrations.wolfram.com/BeatingAndRoughness/

You can also make your own demos easily using MAX Partials,

Discover your own hearing response:

http://www.phys.unsw.edu.au/jw/hearing.html

Sound localization

Some great facts and insight about sound localization in complex vs. pure tone sounds.

The file here is a stereo recording of white noise with the exact same track in both channels, except that one channel has been delayed by 0.001 s. Both channels are equally loud- is that what you hear? Try removing one earbud, and then the other after replacing the first. YOU MUST USE EARBUDS

Concealed Hearing Devices of the 19th Century

Fascinating web history at this website of the Washington University School of medicine.

Auditory Chimeras

Figure out what these researchers are talking about - when you do, its pretty interesting! See the Chimera spectrograms for one way to understand what is going on. A lot of speech can be decomposed into an envelope inside of which is the waveform detail. The key question is the role of the envelope as opposed to the detail. This site has many sound files to listen to to help you decide.