Recording - September 2015 - 53

explanation I'll leave it to Mike.) You'll
note that while the left-hand side of the
ladder is calibrated in dBu, the right-hand
side shows the actual voltage equivalents
of the steps on the dBu ladder.
Here's what the dBu Ladder is good for.
If a signal comes into an audio circuit at,
say, -22 dBu and gets boosted by 30 dB
(we call the amount of boost the gain), it'll
come out at:
(-22 dBu) + (30 dB) = +8 dBu
It's a lot easier for the human brain to
think about adding and subtracting decibels
and dBu than it would be to contemplate
multiplication and division of actual voltages. The above example in voltage terms
would be a much less intuitive equation:
0.06156V x 31.623 = 1.9467V
Humans perceive levels (and other sensory things) in terms of multiplication and
division, but our brains handle addition
and subtraction a lot better. So decibels
were invented to make it easier for our
chimp brains to do the math. (They were
devised by engineers at the Bell
Telephone Co., and the "bel" in "decibel"
is no coincidence.)
Landmarks
I've marked some "landmarks" on the
ladder at levels commonly found in
audio-for example, +4 dBu, which is the
standard nominal level of professional
audio gear. (And what's nominal level?
We'll talk about that in an article to come.)
+4 dBu is also referred to as line level or
pro line level, indicating that it's the nominal level of a signal that gets sent down the
line to the next component in the chain.
You'll note that there are a couple of
defined "line levels" on the ladder (+4
dBu for pro gear, -7.8 dBu for semi-pro
or prosumer gear), but there isn't a
defined "mic level". Rather there's a wide
range of mic levels, ranging from about
+6 dBu to about -60 dBu.
Remember a few articles ago we talked
about how microphones varied in their
sensitivity, or how much signal they put
out when confronted by a given sound

intensity? There's a range of about 30 dB
in the relative sensitivities of real-world
studio microphones, from the hottest condenser mics to the softest ribbons.
Well, that assumes the same sound
level hits all the microphones. Real sound
levels of course vary all over the place,
from the cooing of a gentle vocalist to the
slam of an amp turned up to 11, or the
bang of a close-miked drum. That's why
mic levels can vary so greatly-as I said,
from about +6 dBu to about -60 dBu.
Is there a bottom to the scale? In practical terms, yes; most professional mics
have an impedance of about 150 ohms,
and a mic with that impedance generates
thermal noise or self-noise all by itself
(never mind why, just take my word for it);
that self-noise is about -130.8 dBu at
room temperature. (You could make the
mic quieter by dipping it in liquid helium.
Don't try this at home.)
If a mic put out a nominal level of -70
dBu, it would have a signal-to-noise ratio
that was equal to the difference between
that and its self-noise:
(-70 dBu) - (-130.8 dBu) = 60.8 dB
Now, 60.8 dB is a pretty crummy signalto-noise ratio; it's about what we used to
get from cassette recordings in the
Paleolithic Era. If your mic was producing
that sort of level on a particular source, it
might be time to reach into the suitcase
and pull out another mic.
And that brings us, by a route that was
certainly circuitous, back to preamps.
Boosting it up
The great variety of mic levels found in
real-world recording situations means
that, if we want to boost each mic's output
to standard pro line level (+4 dBu, remember?), the preamp that does the work
needs to have gain that's variable across
a wide range. If we assume that the levels
vary from -60 dBu at the low end of the
scale to +6 dBu at the high, that means
the gain of the preamp needs to vary from
+64 dB to -2 dB (when the gain is negative, it's logically called a "loss".)

RECORDING September 2015

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