Recording - July 2013 - 30

More distant placement can also be a good idea
when the speaker cabinet being miked up has more
than one speaker in it—this is especially true with the
classic 4x12" speaker configurations of Marshalls
and other high-gain amps. With these cabinets, the
sound heard in the room is a combination of the contribution of all four (or, in a stack, eight) speakers, and
the phase cancellations that occur as the wavefronts
from each speaker meet a foot or two in front of the
cabinet produce a much richer sound—one which
you won’t capture fully with a mic an inch or so away
from only one speaker (or even by combining close
mics on each speaker, for that matter). These kinds of
cabinets are often used with high-gain amps for musical genres where part of the guitar sound is how the
sheer volume loads the room, and picking up a little
more of that room reaction also helps achieve a truer
representation of the style.
At the other end of the spectrum (literally!),
sometimes a mic is placed at the rear of a speaker cabinet. This is obviously not done with closedback cabinets like the 4x12, but with smaller
open-back cabs, typically with one or two 10" or
12" speakers. The sound to the rear is very lowend heavy (low frequencies radiate omnidirectionally—the higher frequencies beam out the
front), but it can be a interesting variation on the
sound of that amp.
There are two caveats to this technique: One,
the sound may be too dark on its own, but combined with a mic in the front, it could add a pleasant fullness to the overall recorded tone. And two,
since the wave coming off the rear of the speaker
is reversed relative to the wave a front-placed mic
will see, combining both mics will result in waves
that are in reverse polarity—peaks and nulls will
partly cancel each other, and the resultant tone
may be very thin and phasey. The solution: reverse
the polarity (often mislabeled “phase”) of the rear
mic, either on the mic preamp or at the console
channel strip, to put them back in phase for a
properly full tone. Incidentally, this is the same
approach taken when mics on the top (thwack)
and bottom (buzz) of a snare drum are combined.
For sound and visual examples of both distant (front)
placement and rear mic placement, see EG video 3.
1 plus 1 plus 1...
That brings us to an obvious point—if you have
all these mics set up to compare them, as is often
done before a session, why not record ‘em all,
and blend them together later for a really rich
sound? Well... why not? In fact, that’s exactly
what many engineers do, especially now with the
enhanced track counts of modern DAWs. But
even back in the days of limited track availability,
this was still often done whenever possible.
Very often, a combination of one or two close mics
is recorded (usually on separate tracks, to maintain
control over the blend later on, in mixing)—these are
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RECORDING July 2013

often different brands/models, whose different tonalities may complement each other.
And often a distant mic is also added on its own track, to be blended in for extra
ambience and to round out the peakiness of the close-miked sound—in EG video 3
this is demoed, with a polarity-reversed rear mic mixed in as well, for good measure.
Now, this practice of recording multiple mics on the same amp gets us into
an age-old studio argument about “time-aligning” the mics... okay, maybe it’s
not an “age-old” debate, but it’s certainly been going on as long as DAWs
have made it easy to do (or not)! When a mic positioned at, say, an inch away
from a speaker is combined with a mic placed, say, a foot from that same
speaker, the waveform peaks and dips do not line up. A positive peak from
the close mic may happen just as the negative dip from the previous cycle of
the distant mic occurs, causing partial cancellations at various frequencies.

The frequency response of the classic Shure SM57 dynamic mic.

A figure-8 (bidirectional) polar pattern, as available on the Shure KSM44A
multipattern condenser mic, will look something like this.
This effect is not inherently good or bad, but it’s a judgement call as to
whether, in a particular situation, it enhances the sound by adding that sense
of three-dimensional ambience we’re used to in live listening situations, or
degrades the sound by making it weaker and thinner, with less punch, than
either of the mic signals on their own. If the result sounds fine, then no tweaking may be needed; but, if the combined mic signals do sound weaker than
either alone, then many engineers may opt to time-align the two mics. This
involves either delaying or advancing the region (clip) of one of the two
recorded mic tracks, until the waves’ positive and negative peaks line up in
time, or using plug-ins to delay the clip that’s ahead (the close mic) until it
matches the phase of the later (distant) mic recording.
This is best done by eye, and when all the mic tracks are time-aligned (in phase),
the sound will be fuller and punchier, but possibly at the expense of some sense of
ambience or depth. It’s the engineer’s call—try it and listen to the difference (and
check out EG video Part 2 for a demo). By the way, this same argument goes on in
relation to combining the signals from the close mics on a drum kit and the more distant overhead mics on the same kit.

Recording - July 2013

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