--- In Walter Knapp <> wrote:
> I've always wanted to see tests of mics when recording ambient levels
> around 20dBA. That level is common in nature recording, though at times
> the ambient background can be much lower. I expect a lot of difference
> from working with the standard test level which is 74dBA higher.
> Standard test levels correspond well with music recording, not nature
> recording. It's about time to come up with a testing protocol for
nature
> recording that's as precisely applied and measured as standard tests.
>
> Walt
When I test mic preamp noise, I test it at all gain settings that the
preamp or recorder can be set to, at 10dB intervals. This covers any
possible situation. For a recorder, you can't really express it as
gain, as that term is irrelevant, but you can express it as the input
level that would generate a 0dBFS (full scale) recording, and
calculate the EIN as I explained in my previous message. Of course,
figuring, for example, a headroom figure of 20dB, the actual mic
signal would be 20dB lower. Many published mic preamp noise
measurements do specify the gain level that was used (otherwise the
measurement is meaningless), and normally it is at a high gain like
60dB or 70dB, which is meaningful, but to compare two preamps they
need to be measured at the same gain setting.
The acoustic test level of 74dB or 94dB SPL at 1 meter is a fixed
reference level for testing microphone output and frequency response,
as virtually all microphones can handle this, and it swamps out normal
background noise so you don't need to be in a silence chamber. As
microphone gain and response normally don't vary with acoustic level,
these test levels should be valid for any situation, from birds
chirping to loud music. These are sound levels are for measuring
microphone output and response, and don't relate to the electrical
levels used for microphone preamp noise testing.
What would be useful to know is the actual approximate acoustic level
for various nature recording situations, for example a) morning
chorus, b) bird (type) in tree at 200', c) small stream at 6', etc.
Maybe Bernie has some of this data? Then, taking this, plus the output
levels and noise levels of various commonly used mics, plus the
measurements on recorders, I could calculate the actual performance of
various packages in real situations (noise, signal to noise, mic vs
recorder noise contribution, etc), and actually produce visual graphs
that show performance in real situations instead of just a bunch of
technical figures.
- Adam
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