bbystrek 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.
If they have such data it's unlikely you would be able to get it. There
are some standards for mic testing, and if you read through all that
stuff you may get some more insight into the numbers the manufacturers
do provide. Mic specs are part of the sales pitch for the mic, and
probably controlled as much by the advertising department as the design
engineers.
> 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.
On top of all this, the environment itself contributes. The higher the
frequency the less well it travels, in general.
When we talk about reach, we are giving a subjective judgment that is
really not something that's easily nailed to specific measured numbers.
It's really a relative term, we can say that mic A has better reach than
mic B. And what we are saying is that's our experience in real recording
conditions. I sometimes give a distance for the reach of a mic, but
that's really rough.
> 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.
I suppose at real high levels the electronics might change a little. But
in general the noise floor of a mic can be considered pretty much fixed.
In any case it's not going to be important if recording high noise
levels, it's important when dealing with low levels. That's why the
music folks kind of go duh when we start asking about mic noise. If you
are recording a 120dB rock band do you care that somewhere under all
that the mic is generating a quiet sound of 10-20dB? Those folks tend to
spend their time worrying about the max SPL rating of mics. And many
mics liked in that industry are pretty noisy.
Walt
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