Re: Where does the audible 1kHz come from?

["F. Joseph Pompei" <pompei@xxxxxxxxxxxxx>]

Daniel:

The phenomenon you describe can be attributed to the nonlinearity in the
air itself, which creates audible distortion byproducts from ultrasound.
 The effect can be exploited to create very directional beams of
audible sound, in a device called a 'parametric array', after its roots
in underwater sonar.

I thoroughly developed this technique for audible sound reproduction
(using much higher frequency ultrasound) during my time at MIT, and call
the device (now a commercial product) the "Audio Spotlight".  You can
read more about it at http://www.audiospotlight.com .

Best regards,

Dr. F. Joseph Pompei


> Date:    Fri, 27 Sep 2002 13:33:50 +0100
> From:    Daniel Rowan <dr@ISVR.SOTON.AC.UK>
> Subject: Where does the audble 1 kHz come from?
>
> 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@isvr.soton.ac.uk
> *************************************************