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,
Martin
--- 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
>
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