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RE: sound wavelength and parabola size

Subject: RE: sound wavelength and parabola size
From: "Ranft, Richard" <>
Date: Wed, 19 Feb 2003 11:05:16 -0000
In response to: 

> > 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.

Walter Knapp wrote:

> Then perhaps you will explain how my Telinga (20") reflects 
> sound down 
> all the way to 50 hz? The Telinga is directional and has better than 
> unity gain all the way down.

I don't think your 20" parabola reflects sounds down to 50Hz; they mostly
diffract around it.  However, the microphone in the parabola will still pick
up such sounds - and from all directions in the case of the omnidirectional
Telinga mike. There may also be some directionality at 50Hz due to the
'shadow' effect of the parabola, but not much. Parabolas of 20" are rather
poor for recording low frequency sounds.

> It would be really nice if parabolas act as low cut filters 
> with a sharp 
> cutoff, but they don't. No matter what physics thinks 
> happens. I have to 
> apply a cutoff filter separately. Just below the lowest 
> frequency frogs.
> What happens is the gain falls off at low frequencies, it's 
> not cut off 
> sharply.

This is because the mike also receives sounds directly from the source at
all frequencies.  The parabola on its own still has  a low cut off, and it
sounds unnatural on some low-pitched sounds.  I use a MKH 816 gun mike to
record a large owl rather than my Telinga or other parabola.  Sure, the owl
sounds come over in the parabola recording, but not as well as with the
Sennheiser. As you say, you have to get closer with a gun mike, but for me
that's all part of the challenge of 'hunting' such sounds.

Best wishes,
Richard




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