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Re: Questions for audio gurus

Subject: Re: Questions for audio gurus
From: Gianni Pavan <>
Date: Fri, 24 Feb 2006 09:38:57 +0100
I would now consider a different point of view. Recording at high
sample rates is important for studying sounds used by animals to
communicate and to echolocate. These signals can be infrasonic, sonic
or ultrasonic and thus the sampling rate must be chosen according to
the expected bandwidth of the signal, also considering the importance
of harmonics. In some studies it has been demonstrated, for example,
that the relative level of higher harmonics gives cues about the
orientation of a directional sound source (directionality increases
at higher frequencies).
A further motivation for extending the bandwidth is to have control
on the whole acoustic context. In studies about communication in
mices it has been discovered that other than audible sounds,
ultrasonic sounds up to 90 kHz are used for communication. Thus we
should not limit our attention to audible frequencies even if we
"hear" sounds from an animal. Also, if you're studying the behaviour
of insects that produce sounds, you could be interested in detecting
ultrasounds, for example emitted by predator, that may affect the
behaviour of your insect.

Of course we can't hear what we are recording in the ultrasonic range
- we normally need to slow down a recording to hear ultrasounds - but
we can have full control on what we are recording by using a
real-time spectrographic display. In this way, while recording or
when playing back the files,  we can "see", rather than "hear", any
ultrasonic component.

 From a purely technical point of view it is important to consider
that real-world anti-aliasing filters are not perfect and any signal
with frequencies, including those of harmonics, that cross the
Nyquist frequency may produce artifacts that may corrupt the quality
and integrity of the signal. Poor a-a filters at 22.05 or 24 kHz may
produce artifacts that fall down to audible frequencies or, in any
case, corrupt your spectrographic images. Thus, moving Nyquist up,
you also lower the possible impact of aliasing.

Any comment on these thoughts will be appreciated,

PS: I intentionally avoided to talk about our perception abilities
and about the recording/reproduction of musical instruments that may
have high order harmonics and transients with high frequency contents.

Gianni Pavan
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Universita' degli Studi di Pavia
Via Taramelli 24, 27100 PAVIA, ITALIA
Tel        +39-0382-987874
Fax        +39-02-700-32921

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Loudspeakers are not birds,
A listening room is not nature."
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