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Re: stereo parabola field test

Subject: Re: stereo parabola field test
From: Klas Strandberg <>
Date: Tue, 13 Apr 2004 02:05:44 +0200
Lang, Raimund, this is why I use 4 electrets in parallell (per channel)
with the stereo Datmic. 2 mics lower the noise by 3 db, 4 mics by 6 db. As=

all 4 mics are so close to oneanother, you won't get audible phase losses.

Klas.

  At 23:55 2004-04-11, you wrote:
>Lang,
>
>I think that your measurement results are indeed very precise and nicely
>confirm a basic law in signal processing, that says that independent noise
>signals add geometrically (sum =3D sqrt(a*a+b*b). If the two signals have=

>the same
>amplitude (e.g. 1 V), then the sum of two signals is 1.41 V (the square ro=
ot
>of 2.0 V) and not 2.0 V as one would perhaps expect. In other words, the
>resulting noise level drops by the factor 0.7 ( =3D 1.41 / 2.0), which exa=
ctly
>corresponds to -3 dB. The fact that you really got that 3dB difference is=

>also a
>proof for the good acoustic isolation between the two mikes. The two mikes
>pick up noise from different sources that are not correlated to each other=
.
>
>The same principle applies to the inherent noise sources within the record=
er
>(the two preamplifiers and AD converters). Therefore, splitting a monoaura=
l
>signal to a stereo input will also decrease the self-noise level (when the
>two channels are finally mixed together). However, I guess, that this bene=
fit
>can be almost neglected because the self-noise level of a state-of-the-art
>recorder is usually small compared to that of the microphone.
>
>Raimund
>
>
> > Re: Stereo Parabola versus Monaural Parabola
> >
> > This morning I did a fairly thorough comparison of the output of my ste=
reo
> > parabolic setup and the output of my monaural parabolic setup.
> >
> > My stereo setup was my Telinga Parabola outfitted with two Senheisser M=
KH
> > 20
> > omni mikes placed to each side of a plastic barrier just like the desig=
n
> > that Klas uses in his DAT Stereo mike. I provided pics of my design som=
e
> > months back. For those new to our group, the pics may be found here:
> >
> > http://www.naturesound.com/telinga/telinga.html
> >
> > My monuaral setup was a Telinga Parabola with a Telinga Universal Mike
> > Mount
> > outfitted with a single Senheisser MKH 20 omni mike.
> >
> > My question was whether or not a stereo recording obtained from my ster=
eo
> > parabola has a signal-to-noise ratio that is comparable to that of a
> > monaural recording from my monaural parabola.
> >
> > So here's what I did:
> >
> > To produce a standard signal for my test, I chose my best recording of =
a
> > winter wren and put a single song on a CD. I used noise reduction to
> > reduce
> > the background of this recording to almost nothing. I went to a quiet
> > woodland setting near Ithaca, placed my portable CD recorder on the gro=
und
> > and fed the output to a small speaker, using the repeat function of the
> > player so that the song would play over and over again, with a pause of
> > about five seconds in between. There was a very gentle breeze, insuring
> > that
> > the ambience heard behind my test recordings would be the slight whoosh=
y
> > or
> > hissy sound of the gentle breeze, not the mike noise floor, and not the
> > ambience from the CD recording itself.
> >
> > Using the two different setups (stereo parabola and monaural parabola),=
 I
> > recorded the winter wren song and adjusted my recorder in both instance=
s
> > so
> > that the peak record volume was -3b.
> >
> > After making recordings using both setups, I returned to my studio and
> > inputted the test recordings into my computer.
> >
> > Using PEAK audio editing software, I first chose a song recorded with t=
he
> > stereo setup plus some of the gentle breeze ambience that accompanied i=
t.
> > I
> > then created a monaural soundfile by mixing the two tracks together. Th=
en
> > I
> > normalized this recording so that the song peaked exactly at -3db. Then=
 I
> > deleted the song itself, leaving about five seconds of the gentle breez=
e
> > ambience.
> >
> > I then processed a monaural recording using exactly the same procedure.=
 Be
> > aware that the monaural recording was made to two tracks using a Y-cabl=
e
> > that splits the output of the monaural Telinga into two inputs for the =
DAT
> > recorder. So first I mixed the two tracks to create a monaural track in
> > PEAK. Then I normalized to -3b. Next I removed the song itself, leaving
> > about five seconds of ambience.
> >
> > Then I made a new soundfile in PEAK, which included both examples of th=
e
> > ambience, one following the other with a fraction of a second of silenc=
e
> > in
> > between.
> >
> > Finally, I compared the loudness of these two samples of ambience with =
the
> > assumption that this would give me an accurate measure of the
> > signal-to-noise capability of the two parabola setups when they are use=
d
> > to
> > create monaural recordings ("noise" here refers to the level of the
> > ambience, not mike noise).
> >
> > The results surprised me. The ambience from the processed stereo record=
ing
> > was exactly 3 db quieter than the ambience from the monaural recording!=
! I
> > tried several examples and got exactly the same result each time.
> >
> > This is good news because it means that recording in stereo using Klas'=
s
> > design yields a two-track recording that can be used to produce a monau=
ral
> > recording that is every bit as good as what I can get using a standard
> > monaural parabolic setup. In fact, the result was 3 db better in terms =
of
> > signal-to-noise.
> >
> > I had actually expected the opposite: that recording in stereo would
> > produce
> > a nice-sounding result, but at the expense of some signal-to-noise in
> > comparison to a monaural recording (as long as both are made using the
> > same
> > parabolic reflector, the same microphone type, and Klas's stereo and
> > monaural types of design).
> >
> > What do you tech-heads think of this result? Does this make any sense?
> > Don't
> > you agree this is good news?
> >
> > One question I have is whether or not splitting the monaural parabola
> > output
> > to two channels for DAT recording is influencing the results. I know th=
at
> > this could result in a slight increase in the noise floor created by th=
e
> > mike, but it shouldn't affect the level of the ambience which accompani=
es
> > the bird song. Using my monaural setup, I typically record to two track=
s
> > because it sounds better when monitoring (my recorder only allows for
> > stereo
> > monitoring), and because it provides two tracks just in case there's a
> > glitch in one. Anyway, when the two tracks are mixed back to one track =
in
> > the studio, the results should be the same as when only one track is
> > recorded from the parabola.
> >
> > Any thoughts about my test and my conclusions?
> >
> > Lang
> >
> >
> > "Microphones are not ears,
> > Loudspeakers are not birds,
> > A listening room is not nature."
> > Klas Strandberg
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
>
>--
>NEU : GMX Internet.FreeDSL
>Ab sofort DSL-Tarif ohne Grundgeb=FChr: http://www.gmx.net/info
>
>
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
>

Telinga Microphones, Botarbo,
S-748 96 Tobo, Sweden.
Phone & fax int + 295 310 01
email: 
        



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