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stereo parabola field test

Subject: stereo parabola field test
From: Lang Elliott <>
Date: Sun, 11 Apr 2004 14:40:03 -0400
Re: Stereo Parabola versus Monaural Parabola

This morning I did a fairly thorough comparison of the output of my stereo
parabolic setup and the output of my monaural parabolic setup.

My stereo setup was my Telinga Parabola outfitted with two Senheisser MKH 2=
0
omni mikes placed to each side of a plastic barrier just like the design
that Klas uses in his DAT Stereo mike. I provided pics of my design some
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 Moun=
t
outfitted with a single Senheisser MKH 20 omni mike.

My question was whether or not a stereo recording obtained from my stereo
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 ground
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 tha=
t
the ambience heard behind my test recordings would be the slight whooshy 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 instances 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 the
stereo setup plus some of the gentle breeze ambience that accompanied it. I
then created a monaural soundfile by mixing the two tracks together. Then 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 breeze
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-cable
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 the
ambience, one following the other with a fraction of a second of silence 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 used 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 recording
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 monaural
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 produc=
e
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 outpu=
t
to two channels for DAT recording is influencing the results. I know that
this could result in a slight increase in the noise floor created by the
mike, but it shouldn't affect the level of the ambience which accompanies
the bird song. Using my monaural setup, I typically record to two tracks
because it sounds better when monitoring (my recorder only allows for stere=
o
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


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