Walter,
Do microphone manufacturers ever already have (or perhaps be willing
to provide as "typical" data) test data for mics at low SPLs? I
guess what's really important would be frequency response and
sensitivity which would be different than the published numbers at
much higher music and voice levels.
I suspect frequency response characteristics under low amplitude is
what leads people to talk about a mic's "reach" of far sounds
relative to near sounds. It would see to me that other than
directional aspects, a different mic placed into the same
environment, would react simply to the distance squared. It seems
the definitive measure would be SNR at an appropriate reference level
somewhere between the near and far sound pressure level. Perhaps
frequency response further adds to the mystery as "reach" probably is
prone to frequency response affects when we operate near the lower
end of the sensitivity range.
Another mic question - does the noise floor change characteristics
(contribution) from quiet levels, with no input, all the way up to
maximum SPL rating? I'm sort of guessing anything could happen under
dynamic situations as the various sections of the amplifier circuitry
move through their operating ranges. I guess an interesting test
might be to ramp a pure tone, say 1kHz, from barely audible up to
some large value. Then take a look at the spectrum relative to the
noise floor at various points along the ramp. I might just try to
set this up. I have a sort of old Tektronix AM700 audio
generator/analyzer, but I'm not sure how much noise contribution
would come from the audio amplifier and speaker. Probably might
still be able to provide valid side by side comparisons.
Brian Bystrek
--- In Walter Knapp <> wrote:
> bbystrek wrote:
> >
> > Does anyone have any experience with the Audio Technica AT822
stereo
> > condenser mic? How does it stack up against other mics in terms
of
> > background noise? It's the only mic I own and I don't really have
> > much perspective. I was pleased with the results recording frogs
and
> > various water sounds this past spring. It seems a little tricker
to
> > capture lower level sounds like a meadow of insects. Close miking
> > helps improve signal relative to noise, but this can also throw
> > things out of perspective where I'm really trying to capture a
> > broader image.
>
> Your experience is similar to what I've found my Sony MS 957's can
do.
> They have a fairly decent noise floor, but the reach is not that
great.
> I figure them at 50' or less for good stuff. And for quiet calls,
much
> closer. Their noise floor does not allow much amplification. I use
the
> MS 957's mostly for bad weather situations as they can be protected
> fairly easily compared to my more usual parabolic.
>
> Picking up a good ambient field with mics of short reach is a
problem.
> Bernie recommends a approach of recording several locations and
mixing
> to do it, and this seems like one of the better ways if you can get
to
> all you need to. I deal with swamps a lot in my frogcall work, so
> getting close to all parts can be a real expedition even if it is
> doable. I like the field I get from the Telinga DAT Stereo parabolic
> setup and that can be done from one location.
>
> > Datasheet:
http://www.audiotechnica.com/prodpro/profiles/AT822.html
> >
> > Open Circuit Sensitivity: -45 dB (5.6 mV) re 1V at 1 Pa
> > Signal to Noise Ratio: 70 dB, 1 kHz at 1 Pa
> >
> > Reading various manufacturers datasheets and app notes, it appears
> > that quiet recordings at low SPLs are related to both Sensitivity
and
> > SNR. How does one compare two mics, when neither value is
equal? Is
> > it always subjective?
>
> Signal to noise ratio is often against 96dBA, so will give you a
> absolute noise floor for the mic. For instance a good estimate of
the
> AT822 would be that it's inherent noise is equivalent to 96-70 or
26dB.
> That's only a estimate remember, as mics vary quite a bit under
> different conditions. So, the mic will drown out any environment
that's
> at 26dB or below, real quiet places. But it will be noticeable well
> above that as it will restrict the dynamic range from your
environment.
> Say you are out where the area is 46dB, not a uncommon level in
natural
> areas, then the usable signal will have a dynamic range of 20dB,
and you
> will certainly hear the mics noise in the quieter parts. And any
> amplification raises that noise floor in absolute sound level,
which can
> quickly overwhelm anything unless the mic is very low noise. Anyway
> that's a simplistic explanation that does not take into account mic
sensitivity.
>
> Obviously sensitivity does help in that it can raise that ratio
> sometimes. But for quiet nature recording, that noise floor is the
one
> that's more noticeable. Note that the actual dynamic range we deal
with
> in nature recording is usually not all that high. Even with loud
frogs
> fairly close I'll rarely measure above 90dB peaks with my noise
meter,
> then when you account for the noise floor of the mic that might be
a max
> dynamic range of 70 or 80 dB at best. Most real situations are half
that
> or even less.
>
> Manufacturers and their specifications usually assume that you are
> recording relatively loud noises. Performance on quiet noises may
or may
> not relate. And in addition, each manufacturer seems to test mics
> differently and choose different specs, so it's hard to get more
than a
> general idea from mic specs if trying to compare. I usually try to
> estimate how noisy a mic is from the specs and only pay mild
attention
> to the sensitivity. If there's a frequency response curve I like to
look
> at that too.
>
> > Another reason I'm asking about noise - I'm thinking of getting
into
> > one of the more robust stereo imaging techniques like M-S or
> > boundry.
>
> To do these well does require a fair investment in mics. Worth a
bunch
> of study. The group archives have a lot on this sort of thing,
> particularly discussions of the SASS setup and how to modify it.
I'm in
> the middle of building a modified SASS.
>
> Not many have the figure 8 mics necessary for M-S. I've the mics and
> expect to begin recording some in M-S by next year. Once I get
> suspensions and windscreens built.
>
> Walt
>
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