Questions on Levels
On Sun, 21 Nov 2010 15:18:08 -0800 (PST), PStamler
wrote:
On Nov 21, 12:16*pm, (Scott Dorsey) wrote:
I believe that Mike is incorrect in this. *Some people DO use "dB" to
mean "dBSPL." *However, those people are wrong.
Scott, we had this debate here a couple of years ago, and what it came
down to was a discussion between what would in the world of
lexicography be called prescriptivists and descriptivists. A
prescriptivist writes a dictionary to tell people what words mean and
how they should be used. A descriptivist writes a dictionary to tell
what people mean by words and how they use them. It's a philosophical
and practical division.
So a descriptivist would say that one meaning for dB is as a shorthand
for dBSPL, which is how a lot of audio engineers use it. A
prescriptivist would say, as you did, that's wrong, because it doesn't
correspond with the officially-defined meaning of dB.
More to the point, someone who uses "dB" to mean the voltage gain of
something, ignoring the power aspects, is violating the official
definition. But that usage is near-universal among audio circuit
designers, who talk about opanp circuits with "20dB of gain", and mean
a voltage gain of 10x, with no reference to impedance or power. They
may even refer to dB of gain in a transformer, which can never have
any power gain, being a passive device.
Whether you accept that this is the usage of the population, or
condemn it as wrong, is a choice, just as a dictionary-maker must
choose whether to be prescriptivist or descriptivist. The fact is that
the speech of the community has taken a turn which deviates
significantly from the official standards.
Parenthetically, we invented dBu as a standard when people stopped
using dBm; perhaps it's time to invent dBG for voltage gain
situations.
There is a sense in which calling a voltage gain of 10 a gain of 20 dB
does refer to power. In a circuit in which nothing is changed but that
gain (including output loading and input signal level), if that gain
is reduced to 0 dB, the output power level will be reduced by a factor
of 100.
Similarly, suppose a converter is fed a signal that runs it at a level
of -6 dBFS. Halving the input power (f.e., by decreasing a voltage
input by a factor of 1.414) will shift the converter to -9 dBFS.
Quadrupling the input power by doubling the input level will move the
converter to 0 dBFS. The output powers will show the same dB changes
(assuming linearity and no tricks).
I believe these examples show the power nature of dB measurements.
--
John
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