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Re: Making a parabola

Subject: Re: Making a parabola
From: "Avocet" madl74
Date: Wed Sep 19, 2012 7:50 am ((PDT))
> I'd like a large enough parabola to successfully record cormorants
> nesting on a tall cliff not far from me here in north Wales. The
> cormorants have quite low frequency calls starting at about 400Hz.


The frequency response of a parabolic mic rig is very complex, but
there are some "bare bones" guidelines.

You need a number of wavelengths to get anything like a focus and
directional amplification. As a rough guide, the wavelength at 1Khz is
340mm or about two wavelengths for a dish diameter of 700mm. You won't
get much focussing at this frequency, or much gain.

Another factor is direct mic pickup mixing with reflected pickup on
the mic at the focus. This will cancel out when the 700/4 focal length
equals half a wavelength, theoretically 1Khz, but other factors can
move this null point. Using a cardioid mic reduces this effect, but
cardioids are designed for free air, not a messy reflection pattern.
The bottom line is that you will get a comb filter response rising in
sensitivity with frequency becoming very directional in the higher

> Any ideas as to how large a dish I'd need?

At 400Hz, several metres wide. :-)

A more sensible suggestion would be to use a long gun mic in a blimp.
I bought as couple of old beaten up but working MKH 815 T's for under
=A3100 recently, but T powered mics are good value for money as most
recordists are now using phantom power.
See eBay Item number: 300777364229
Search for Sennheiser MKH-815

Unfortunately, your cormorants are a bit too far away and I'm a bit
lame, or I'd be up there with my stuff.


David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce

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