At 7:55 AM +0000 3/31/08, Raimund Specht wrote:
>Hi Rob,
>
>It is probably quite difficult for a layman to fully understand these
>things.
Yes, it is and I appreciate you taking your time
to provide such a thorough explanation. The
ramifications are important even if one does not
understand the different measurement methods
involved.
I, for one, will stick with using dB(A)
self-noise numbers when drafting lists of mics to
study for certain applications. Sensitivity seems
to become much more of a factor with noisier,
lower-gain mic preamps. The rule of thumb of
avoiding those under 10 mV/Pa for nature
recording holds up pretty well.
Rane Notes 148 Table 3 can provide some crucial
insights about the role of pre noise. Ironically,
it might be most useful if one is trying to find
the specs for mics to use with a recorder that is
not ideally suited for nature recording.
Otherwise, after using Table 3 to "see" how
limited mic options become with noisier pres, the
simple lesson is to invest in a recorder with a
low noise mic pre. Your pdf is another very good
graphic representation:
http://www.avisoft.com/test/noisefloors.pdf
As long as people tend to buy recorders before
starting to think about micing and mcs, I guess
we'll need to keep pursuing convincing models of
explanation.
Thanks again,
Rob D.
>
>
>Rob D. wrote:
>
>> It seems the intended use of Table 3
>><http://rane.com/note148.html>http://rane.com/note148.html
>>is
>> for determining whether the noise output of a preamp is at least 10dB
>> lower than the noise output of the mics. It creates dBu, A-weighted.
>> equivalents for the mics and your A-weighted, EIN measurements on
>>
>><http://www.avisoft.com/recordertests.htm>http://www.avisoft.com/recorder=
tests.htm
>>plug in with no additional
>> adjustment.
>
>Yes, this table is intended for comparing the absolute noise voltage
>levels of the microphones and preamplifiers.
>
>> My question is whether it is appropriate to use Rane Table 3 to
>> compare fully adjusted noise output between mics? When I try to do
>> this, it seems I should interpret higher negative values as lower
>> effective noise output, like preamp numbers. For example, a Rode
>> NT1-A (5.5 dB(A) and 25mV/Pa) computes to -117.5 dBu and a MKH-20 (10
>> dB(A) and 25mV/Pa) computes to -113.5 dBu (all numbers are
>> A-weighted). Assuming all noise is equal for the moment, and 1 dB is
>> the smallest unit of difference that one can hear, the table suggests
>> the MKH-20 has about 4 audible increases or "steps" in noise above
>> the noise produced by the Rode NT1-A. For this example, the Table
>> seems to be in agreement with my experience with higher negative
>> numbers resulting is less, cumulative noise output.
>
>Yes, this is true if you are looking at the absolute noise VOLTAGE
>levels. Because these two microphones have the same sensitivity of
>25mV/Pa, the absolute noise floor difference (expressed in dBu or =B5V)
>equals to the difference of the equivalent SPL noise levels (expressed
>in dBA). So it is important to know that the noise voltages on this
>table don't tell anything about the noise floor that you finally hear
>in your recording.
>
>> Table 3 computed mic comparisons, however, that come out contrary to
>> experience.
>
>Note that this does not work for microphones that have different
>sensitivities. This is the reason why one must use the dB(A)
>specifications to compare the equivalent sound pressure noise levels
>of different mics.
>
>> Are you using Table 3 to compare noise output between mics?
>No.
>
>> Are you
>> saying that the Table 3 grid numbers are not really noise output
>> equivalents between mics and pres-- that mics compute to "noise
>> voltage floor" numbers where lower negative numbers effectively mean
>> less noise output?
>
>Yes, but a lower negative dBu number does not always mean that the
>specific mic is less noisy! If this mic also had a lower sensitivity,
>you had to turn up the gain on your recorder to compensate for the
>lower sensitivity and this would also amplify the audible noise floor.
>
>[I'm going on your example, "The 20 mV/Pa model
> > provides a noise voltage floor of -112 dBu. The 50 mV/Pa model
>> produces a higher absolute noise voltage of -104 dBu. The 50 mV/Pa
>> model could just be interpreted as a 20 mV/Pa microphone with an
>> additional ("zero noise") 8 dB preamplifier."] I can see how -104 dBu
>> is -8dB hotter than -112 dBu, but I thought that Table 3 attempts to
>> show equivalent noise output, apples and apples, for all mics and all
>> pres.
>
>Yes, the table compares the absolute noise VOLTAGES (and not the
>equivalent sound pressure noise levels). This is necessary because the
>preamplifier noise can only be described by the absolute noise voltages.
>
>Regards,
>Raimund
>
>
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