Hi Rob,
It is probably quite difficult for a layman to fully understand these
things.
Rob D. wrote:
> It seems the intended use of Table 3 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 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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