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Re: Making a parabola

Subject: Re: Making a parabola
From: "Klas Strandberg" klasstrandberg
Date: Thu Sep 20, 2012 8:07 am ((PDT))
Hello H=E5kan!

I think you are perfectly right.
Here is a study:

Often, the use of a parabol shows better results than expected. Only
the experienced user of parabols can predict these results, as
general technical data like freq. response and directivity are misleading.
Cormorants and other sea birds: When recording birds close to water
and cliffs, the result is very unpredictable, regardless of mic
system used. There are lots of reflections from everywhere and phase
distortion is sometimes a huge problem. Using a parabol, choosing
recording position is most important. A almost impossible situation
is when the parabol is facing a background of water, while it works
fine when the parabol is facing a cliff. Even if phase distortion is
audible, the forward amplification of a parabol can mask it good enough.

A way to think:

Here, the rate of overtones must be considered. Cormorants, for
example has a high rate of overtones and the call will sound "much
closer" to the parabol, even though the ground tone is below the
freq. where the dish starts to amplify.
Some other birds do not have such a high rate of overtones, like
bitterns, some doves and some owls. In such cases, omnis or shotguns
must be used, or very big parabols.


At 11:41 2012-09-20, you wrote:
>Hi Martin!
>I just have to ask one thing.
>Did you intend to buy a parabola just for recording one single
>specie of bird?
>If you are going to record more bird species I still think a
>parabola is the best choice. 95% of the time it's a great tool for
>recording bird species. The cormorant also have other sounds that
>are higher than 400hz so those will get a little higher gain, but
>that's often not a problem because lower frequencies travel further
>so it will probably end up nice. If the sounds are very weak you
>need a low noise microphone and a recorder with good specs that
>doesn't add noise.
>I don't know what you believe, but a few people I have talked to
>that wanted to record bird species, read around on internet and then
>thinks that parabolas can't record below 1000hz AT ALL... but they
>do... yes, you don't get the same gain like with the higher
>frequecies, but it still works.
>The same goes for the frequency range of a microphone.
>Some people think that if the specs is 100-18000hz it can't record
>anything below or above that range...
>It's like one of my microphones that have specs that goes to
>17000hz, but I still record bats at 60000hz with it... yes, there is
>probably better microphones for recording bats, but it works.
>The best thing of course would be if you could compare gunmics with
>a parabola side by side and then make your choice.
>/H=C3=A5kan O.
>--- In  "MARTIN" <> wro=
> >
> >
> >
> >
> >
> >
> > Hi David,
> >
> > Thanks for your interesting and detailed reply - it looks like
> parabolas are not the solution for my cormorant colony. The best
> results I've had so far 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 signal 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" <brini@> 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 i=
> > > 340mm or about two wavelengths for a dish diameter of 700mm. You won'=
> > > 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 lengt=
> > > 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
> > > =EF=BF=BD100 recently, but T powered mics are good value for money as=
> > > 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
> > >
> >
>"While a picture is worth a thousand words, a
>sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
>Yahoo! Groups Links
Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01

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