Questions on Levels
On 11/21/2010 10:16 AM, Scott Dorsey wrote:
Eric wrote:
On Sun, 21 Nov 2010 08:46:10 -0500, Mike
wrote:
As commonly used today, dB without any modifiers is usually
understood to be sound pressure level referenced to a
specific pressure in Pascals.
I think this is exemplary of one issue. This may be true in your
area of work, that dB without a modifier has to do with sound
pressure, but in communications dB without a modifier is generally
representative of a unitless scale factor in a system. e.g., an
amplifier that increases the signal power by a factor or ten has 10dB
of gain. If 0dBW goes in, 10dBW comes out, if 0dBm goes in, 10dBm
comes out.
I believe that Mike is incorrect in this. Some people DO use "dB" to
mean "dBSPL." However, those people are wrong.
--scott
In decades of working with sound, I have always heard coloquially "dB"
but it was always understood to mean "dB" relative to 1 micropascal for
underwater applications (up to 1970 it had been the microbar so we had
to add 100dB to absolute levels thereafter) and relative to 20
micropascals for airborne sound.
You can Google enough references to the need for knowing the particular
reference system you're using....
When you're in a system or physical context then it's shorthand to say
"dB" for absolute levels - but everyone who has thought about it even
just a little bit understands what they really mean. It was one of the
*first* things I learned out of school in the real world of acoustics.
Then, as one switches from underwater to air and vice versa, we
understand that the SPL absolute reference changes as above.
I don't think that Mike is wrong - he did say "referenced to a specific
pressure". In communications had been pretty typical to talk about "dB"
in reference to *particular* voltage levels.
That said, I won't argue against it being a unitless measure in a system
- as it is, after all, all about ratios.
It depends on your context. The amplifier example is a good one. But,
in that case we're talking in the context of out/in ratio. In system
examples we often talk about *absolute* levels and need a reference
level to do so. In other cases we do talk about out/in ratios: e.g.
transmission loss and the absolute level issue isn't included.
Here's a system example - it could be sonar or space communications or
.......:
We start with a transmitter with output of "150 dB". Well, that means
relative to something - it's a statement of absolute level.
Then, we run the transmitter output through a channel that attenuates
the signal by 100 dB. This statement of "dB" is purely a ratio with no
reference level involved as it is the ratio of in/out .
Then, we receive the signal and we're interested in the absolute level
being received because we have a transducer conversion to deal with and
environmental noise to overcome and system noise to overcome.
The absolute level received in this case is:
150 - 100 = 50 dB relative to our original reference.
Notice that here we mix references to fixed absolute levels with
references to pure ratios in order to get what we need. So, both uses
are appropriate.
The logs just make it easier to compute and to comprehend when one is
used to it. This is no different than saying:
200upa/10^5 = 0.002upa
or
7w/m^2/10^10 = 0.007uW/m^2
which both use a ratio equivalent to 100dB.
Fred
|