As I explained, I have a db scale on my multimeter. I must say
I am still puzzled as to what to do with it. Let me ask it this way:
My multimeter has no DB mode on the rotary switch, only volts,
amps, and ohms (ac/dc). Could it be that it is simply an unused
feature, in other words, the designers of this multi meter didn't
need a DB scale, but this is what they had so they used it. Or is
this normal. Next: Suppose I wanted to measure sound level
from a circuit with an electret and an amplifier. Do I just connect
the output of the amplifier to a voltmeter and that's it, or do I need
to calibrate, if so, how.
Well since you'd be measuring AC volts, then you need to set it to
an AC volt range. And then you use it as an alternate scale to
the AC volt scale.
Keep in mind that dB is relative. "0" is defined as a certain voltage
into a certain impedance, and then the scales up and down from there
are relative to that reference point.
In those cheap meters, "0" may not be to any particular standard.
If you put the standard voltage into that meter (and the circuit is
the standard impedance), it may not read "0". Not just an inaccuracy,
but because the meter isn't set up for the standard.
But that doesn't matter, since there are plenty of cases where you'd
want a dB meter where the reference is not the standard. You'd have
some voltage, absolute or arbitrary, and that would be "0". Then as
you adjust things, you could look at the dB scale in reference to
that absolute point. Double the voltage being measured, and you'll see
a 3dB increase; half the input voltage, and it decreases by 3dB.
Think of a tape deck with a dB meter. When it reads "0" the voltage
measured may not be the standard voltage. But something is adjusted
so when the meter reads "0", it is a certain level that doesn't overload
the circuit, but which also gives a good signal to noise level. If
the signal level goes below "0", you know it is getting weak and maybe
down into the noise. If it reads too high, and such meters usually are
marked to indicate "too much", then you risk distortion. The absolute
voltages do not matter to the end user, only that the input signal keeps
the meter within a certain range of levels.
Michael