Dear Pratap
The relationship between sound wavelength and parabola diameter is probably
more important than parabola focal length, in that parabolas simply won't
reflect sounds whose wavelength is greater than the parabola diameter. In
effect, the parabola has a severe bass cut below a frequency determined by
its diameter.
wavelength =3D speed of sound divided by frequency
so if the speed of sound in air is 332 m/s, to record frequencies above 500
Hz you will need a parabola of diameter (332/500=3D) 0.664 metres (=3Dabout=
26
inches). Many owls and larger pigeons have frequencies below 500 Hz. In
practice, however, the larger the parabola the more unwieldy it is to use i=
n
the field, which is why many recordists settle on a 20-inch diameter dish
with a bass cut off below 650 Hz. The parabola is better for recording the
higher-pitched sounds especially those that are far away, and is more
directional, but I use a gun mike when recording larger birds or mammals
because its frequency response extends into the lower frequencies. Many
times when recording in remote places I find I have taken the 'wrong' mike
out on a recording session and the more appropriate one I left back at camp=
!
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Richard Ranft =
Wildlife Section,=20=20=20=20=20
The British Library Sound Archive, =
96 Euston Road, =
London NWI 2DB, UK.
email:
voice: +44 (0)20-7412-7402/3
fax: +44 (0)20-7412-7441
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> -----Original Message-----
> From: pratap singh
> Sent: 16 February 2003 08:21
> To:
> Subject: [Nature Recordists] sound wavelengh and parabola size
>
>
>
> I request knowledgeable members to explain to me or direct me to
> relevant website for understanding the physics of focussing
> soundwaves using parabola and in particular the relationship
> between sound wavelength and parabola focal lenth for good
> focussing.
>
> I use an old Telinga parabolic microphone for birdsound recording.
> The results are very good for high frequency sound but for low
> frequency owls, pigeons etc. I do not get desired intensity. What
> can be the options for me within a budget of $400 or so.
>
> Pratap Singh
>
>
>
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