Where does the audble 1 kHz come from? (Daniel Rowan )

Subject: Where does the audble 1 kHz come from?
From:    Daniel Rowan  <dr(at)ISVR.SOTON.AC.UK>
Date:    Fri, 27 Sep 2002 13:33:50 +0100

Hi list A trivial question for you. A colleague here recently described (from a text book) a phenomenon that we neither anticipated nor can explain and I wondered if anyone had the answer... Apparently if you play a 22 kHz and 23 kHz tone over two loudspeakers in the sound field to normally hearing listeners they report hearing a 1 kHz tone, the detectibility of which depends on the location. Is this so and why? Our immediate thoughts were: (A) Its beats - but to hear the beats (1) the spectral components must be audible (and the beat frequency isn't in the spectrum) which is unlikely at such high frequencies; (2) if they are audible they must go unresolved in the auditory system - possible; (3) the modulation frequency must be represented in the auditory system which is unlikely to be the case given the low-pass nature of normals' TMTFs. We think that even if these were the case (and we seriously doubt it from consideration 1 and 3), listeners would not hear a sound with a clear pitch sensation let alone a pitch of 1 kHz with only 2 spectral components. (B) Distortion product i.e. a 1 kHz spectral component comes from somewhere. As the two signals are delivered from two independent channels in the instrumentation, this can't be distortion from the equipment. We can't imagine that the DP arises in the air / room acoustics. So that leaves the cochlear. We have doubted this because (1) very little energy would be transmitted through the middle ear into the cochlea; (2) even if a f2-f1 DP was generated in the cochlea, I can't image that this would be sufficiently intense to be audible and; (3) I thought that any cochlear DPs other than 2f1-f2 were exteremly hard if not impossible to hear. However, I know little about the perception of cochlear DPs. Any thoughts? (Note, we have not attemtped to try and replicate the experiment.) Daniel ************************************************* Daniel Rowan MSc Research Student Institute of Sound and Vibration Research University of Southampton University Road Southampton SO17 1BJ England, UK Tel: 023 80594968 Email: dr(at)isvr.soton.ac.uk *************************************************

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DAn Ellis <dpwe@ee.columbia.edu>
Electrical Engineering Dept., Columbia University