Posted by: "Adam Liberman"
> 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.
Whether or not you like the term, gain is often what you are going to
find in published specs. One has to understand all the things that go
into that terminology to work with it, but that's not hard.
If you want to really confuse folks start talking entirely in voltages.
And then get into impedance.
> 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.
I've found the assumptions above to be less than accurate if one gets
into recording ambiance. There you are not so much recording the calls,
but the silence in between the calls. You are a very, very long ways
from the standard test conditions.
I'd think you could not accurately test a mic's self noise without a
soundproof chamber. Certainly not the very low self noise mics preferred
for nature recording.
I fully understand the derivation of the standard tests, I consider them
incomplete for my uses. I'm certain from my own experience that a set of
tests based on much lower level test signals would bring out differences
from the standard tests. That sort of data would be very useful.
> 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.
Indeed the raw technical stuff is limited in usefulness. It can become a
hobby in itself, as it clearly has with some. But for me I'm only
interested in how it can help me to get better recordings in the field
and judge it's usefulness on that basis.
I've, for a long time, recommended that people get (or maybe borrow) a
good sound meter and check out their recording sites to learn what sound
levels they are working with. I have two sound meters, and often have
them along.
Part of the problem in sound meters is that they too have mics with self
noise, noise from other components, etc. You can find moderately priced
used sound meters that can get down to 20dBA or so, below that it goes
very expensive. So you have to generally estimate from your readings
what the quietest moments are.
Figure your lowest sound level site ambiance at 0-10dBA, maybe lower.
That's a really quiet site and not common even if you consider only
quiet sites. 10-25dBA would be a more common measure for a site most
would call quiet. This is the quietest ambiance moments in between any
calls. Calls themselves can be all over the map. I've measured
individual frogs from 30dBA to 125dBA. That's careful stalking to get
very close, not necessarily normal recording distance. Not one single
species over that whole range. Typical frogcall levels at closer
recording distances would be something like 50-70dBA. Combined calls of
thousands of frogs can exceed the threshold of pain for hearing.
If you are looking to come up with ideal top end quiet ambiance
recording gear then figure 10dBA or less for the quietest part of the
ambiance. That's a real challenge for gear. What it means in practice is
that the self noise of the mic will be audible in the final recording.
The trick is in hiding that noise, MKH mics are very good at this as
their self noise is very smooth and can blend in to become nearly
unnoticeable. Some other mics fail the test of the smoothness of their
self noise.
Walt
|