What Mike Feldman, Syd Curtis and John Neville have written about the
mantra "halve the distance" for improving nature sound recordings is
worthy of further reinforcement, qualification and explanation. Mike's
summary follows for your convenience:
>
>Syd Curtis wrote:
>
>> ... "Half the distance and double the signal."
>>
>> Getting close is in fact even more effective than that. Signal strength
is
>> inversely proportional to the square of the distance. So that half the
>> distance means four times the signal strength.
>
>Inverse square holds for free field (open 3-D acoustic space in all
directions)
>but I wonder if there shouldn't be a correction for half-space, where the
>ground reflects half (or no more than half) of the acoustic energy back
>into the non-planet part of our atmosphere. The inverse-square law breaks
>down inside inclosed (reverbrant) spaces where the (non-)decay of reflect
>energy can dominate the soundfield. And inside an acoustic waveguide
(pipe)
>there may be very little attenuation with distance.
>
>This is just a techincal nit, and inverse-square is 1st approximation for
>outdoors acoustics. But in some of the recently posted recordings
>I've heard some pretty interesting reflections. Gotta dig out the
acoustics
>text to check the intensity law in 1/2-space, where most of us record.
>
>-- Mike
All that is good information, insight and advice. Note, importantly, the
implied situation of a single point source of the sound to be recorded.
This situation is what is often called "species recording" rather than
"survey recording" or "ambience recording." In species recording the
objective is usually to obtain the highest fidelity recording of the single
sound-producing organism as is possible within the constraints of one's
equipment and the acoustical environment. A directional microphone
system (shotgun or parabolic system) is usually the sensor of choice,
and full-bandwidth lossless recording is usually the storage type of
choice for species recordings. Now let's pick up the thread and take
it further.
The acoustic signal strength can be increased by getting closer to
the source. In a 3D, free-field environment with a point source,
halving the distance increases the signal power by 6dB, i.e. four
times. Unless one is recording over a lake or a parking lot (where
surface reflections can increase the power even further), this 6dB
"rule" is quite accurate for most natural sound recording venues.
OK, we've gained 6dB of signal, but that gain is much better than
simply cranking in 6dB of additional pre-amp gain. Let's explain
this next important step. In most natural sound recording venues
there is a plethora of ambient noise coming from myriad sources
all around. Typically the amount of that noise emanating from the
immediate vicinity of the desired signal is negligible compared to
the noise coming from much further in the background. Thus, in
halving the distance to the desired signal source, one is usually
not getting significantly closer to the sources of background noise;
hence, the noise power remains essentially unchanged. This is
an important realization for the species recordist, as it allows one
to improve the signal-to-noise ratio (SNR) of the recording by
6dB while at the same time gaining 6dB of signal strength. Note
that cranking up the pre-amp gain cannot improve the SNR.
Halve the distance again and get another 6dB improvement, etc.
This technique is very useful in species recording, and is best
applied when taking additional factors into consideration. For
example, I always take what I often refer to as an "acoustical
inventory" of my recording venue before attempting to record.
By acoustical inventory I mean critically listening, sometimes
with the aid of your directional microphone, in every direction
and mentally noting where various background sounds are
being generated. Then use this information to orient your ap-
proach to your subject so as to minimize co-linear background
noise. Also, assess the terrain and vegetation between you and
the subject, and choose a route for approach that affords maximum
stealth yet assures a good "shot" for your microphone. And
while doing this be constantly aware of your subject and do not
approach so close as to influence its behavior - or if you do, be
sure to note that in the metadata accompanying your recording.
Good recording,
Randy
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Randolph S. Little <>
111 Berkeley Circle, Basking Ridge, NJ 07920-2009
Phone: (908)221-9173 Fax: (908)630-0871
URL: <http://rsl.home.att.net>
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