Hi all, there are two more examples worth of consideration and experimentin=
g:
- to record very weak sounds from insects it is often required to
place the mic as close as possible, but still a lot of noise come
from the surroundings. a technique is to use two identical mics, one
close to the insect and the second at a short distance. then by
subtracting the far channel from the close channel you suppress most
of the general noise and keep clean the sounds captured by the close mic
- while using a parabola with a mono mic in the focus, you can also
place an identical mic on the border of the dish and then subtract
the border sound from the focus sound.
Gianni
At 18.02 03/05/2007, you wrote:
>For anyone that wondered what was that guy taking about re: "phase
>invert noise cancellation". Here is an example:
>
>The sound file is a porcupine in a dead run to get to a tree. Yup,
>that means he was going about 2 miles per hour and still taking rest
>breaks. He is running through very dry brushland that is currently
>under an extreme fire hazard due to high winds with dry land. I use
>this an example not because it is the impressive example file but
>rather because it is broad freq sound and you can more easily hear
>what the dish does and what the noise cancellation does.
>
>Recorded using parabolic dish (this one 32") with two omni microphones
>spaced at 1.5" apart with no barrier.
>
>As a stereo file it sounds like this 800kb download.
><http://home.comcast.net/~richpeet/porkystereo.mp3>http://home.comcast.net=
/~richpeet/porkystereo.mp3
>
>Now I take one of the channels and invert the channel so that all the
>positive voltage peaks of the sound are now negative, and the negative
>voltage peaks are positive. I then sum the two stereo channels into a
>mono file mixing the two channels equally into one. I bring the gain
>up as needed to match the stereo file.
>
>The result is that any sound that was identical between the two
>microphones in the stereo file is cancelled out and goes away.
>
>As a noise cancelled mono file porky sounds like this 400kb download.
><http://home.comcast.net/~richpeet/porkymono.mp3>http://home.comcast.net/~=
richpeet/porkymono.mp3
>
>No other edits done than as discribed. Clearly not something you will
>always use but it is a tool with its uses.
>
>Rich Peet
>
>
>
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>Checked by AVG Free Edition.
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>02/05/2007 14.16
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Gianni Pavan
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Universita' degli Studi di Pavia, Via Taramelli 24, 27100 PAVIA, ITALIA
Phone +39-0382-987874 Fax +39-02-700-32921
Email
Web http://www.unipv.it/cibra
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CIBRA organizes the XXI IBAC Congress, Pavia, 15-18 September 2007
http://www.unipv.it/cibra/xxi_ibac.html
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