[Top] [All Lists]

Re: Making a parabola

Subject: Re: Making a parabola
From: "MARTIN" mcgarnett
Date: Thu Sep 20, 2012 1:34 am ((PDT))

Hi David,

Thanks for your interesting and detailed reply - it looks like parabolas ar=
e not the solution for my cormorant colony. The best results I've had so fa=
r are with a pair of Sennheiser ME 66s spaced about 20 cm apart and angled =
at about 60 degrees. Although it's a very quiet site, there is not enough s=
ignal for a really good recording. Would the 815T's be that much better and=
 if so what do you need to power them?

Best wishes,


--- In  "Avocet" <> wrote:
> > 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.
> Martin,
> 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
> kHz.
> > 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
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce

<Prev in Thread] Current Thread [Next in Thread>

The University of NSW School of Computer and Engineering takes no responsibility for the contents of this archive. It is purely a compilation of material sent by many people to the naturerecordists mailing list. It has not been checked for accuracy nor its content verified in any way. If you wish to get material removed from the archive or have other queries about the archive e-mail Andrew Taylor at this address: andrewt@cse.unsw.EDU.AU