All:
There are no normalized attention paid to noise floor and low SPL, like
wildlife sounds. The best measurement is dbA, or CCIR, but many
manufacturers try to mislead users by noting the "sensitivity" for whatever
it is. One does never know what level they refer to as "0" db. Each
manufacturer claim they have the right opinion.
A thumb rule:
A common 5 mm electret has a noise level around 28-30 db(A). Quite audible
and no good.
A common 10 mm electret has a noise level of around 24-26 db(A). Acceptable
when used in a parabol.
A big electret, some 20 mm membrane diameter, makes about 16-18 db(A) noise.
This is acceptable under very good conditions, and fully acceptable when
used in a parabol.
Sennheiser ME-6x series, like Audio Technica and all the others are at some
16-17 db(A)
The Telinga capsules - specially boosted for lowest "budget noise" - has a
noise of about 13-14 dbA
Sennheiser MKH-series has a noise around 10-11 db(A).
Very few and hand built mic's have a noise around 7-8 db(A)
There is a Neuman 1 inch capsule (which I have not tested) with a noise
around 8 db(A)
db(A) noise is measured like this:
You expose the mic for a sound pressure of 1 Pascal. At this sound pressure,
you set the db-meter at 94 db. Then you disconnect the sound pressure device
and measure the remaining noise from the mic through a db(A) filter.
CCIR is made the same way, but through a CCIR filter.
In real life, some mic noise is masked by normal "wildlife" ambience. Some
other mic noise comes on top of it, and is disturbing. This depends on the
frequency spectrum of the noise.
Klas.
At 00:12 2002-07-31 -0000, you wrote:
>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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