Recording - March 2013 - 43

set to 3:1 (three to one). This means that if the
input amplitude is 1 dB below the threshold,
the output amplitude will be pushed down to
3 dB below the threshold. In this case the
input amplitude is 6 dB below threshold, so
the expander applies 12 dB of gain reduction which results in an output amplitude of
–48 dB.
To set the expander, you find a spot where this rack
tom is being hit as softly as it gets hit and adjust the
threshold just until the expander opens up. The Attack
control lets you adjust how fast the expander opens. I set
this as fast as possible for drums. The Hold parameter
tells the expander to stay open for a specified time, and
the Release control lets you adjust how long it takes for
the expander to attenuate to the maximum amount set
by the Range parameter. By adjusting Hold and Release
times, you can control what the decay of the drum
sounds like.
A gate is the same as an expander but there is no
ratio; the gate just attenuates fully until the input signal
crosses the threshold again. If you have a choice
between the two, an expander gives you more control.
Sometimes expanders are somewhat incorrectly referred
to as “gates”. Also be aware that in Apple’s Logic
DAW, the included “expander” plug-in is actually an
upward expander that makes things louder when they
exceed the threshold. I won’t explain that plug-in here,
but a Google search will point you in the right direction.
In the meantime, Logic users can use the Noise Gate
plug-in, which has no ratio control.
Side-chaining
You will also notice two filters in the Side-chain box at
the top right of the plug-in. The side-chain takes a bit of
explaining; check out the little diagram in Figure 5 and
follow along. At the input of this plug-in the audio signal

is split two ways. One split goes into the expander itself, and the other
split goes to the detection circuit, which analyzes the audio and tells the
expander what to do. Right before the detection circuit are the two sidechain filters. These allow you to filter out some of the leakage so that the
detection circuit only “hears” the drum you are trying to expand.
If you turn off the expander on one of my tom tracks, you will hear
lots of leakage from the snare. This makes it hard to expand this track
because the expander keeps wanting to open up when the snare gets
hit. By clicking the little speaker icon at the top right of the side-chain
box (see Figure 6), you can listen to the signal that the side chain hears.
In this example, I have turned on both filters, using a lowpass filter to
remove most of the higher-frequency snare leakage from the side chain.
I have also activated the bandpass filter in the LF section. I swept the frequency control around until the low fundamental frequency of the tom
jumped out. Now the side chain hears less snare leakage and even
more of the tom itself. This makes it easier for the expander to cut out
the bad without chopping off the good. Once you adjust your side-chain
eq, remember to turn the little speaker icon back off! This leaves the filters active in the side-chain but not in your real mix. At that point you
can readjust the threshold on the expander. If you bypass the expanders
on any of the example ProTools sessions, you will hear a dramatic difference in the tightness of the sound as all that muck goes away.
The kick tracks presented their own expander challenge. One of the
mics was inside the kick and the other a foot away. We probably should
have draped a heavy blanket over the kick and its mics to avoid leakage from the other drums, but again, in the fog of war we neglected
this. So after routing the two kick mics to a mono aux and
6 adjusting their fader levels, I had so much snare leakage
on the kick aux that it was difficult to expand cleanly... so
I whipped another trick out of my bag.
In Figure 7, you can see the expander plug-in on the kick
aux. Notice the little skeleton key icon in the side-chain
box? If you activate that button you can send some other
signal into the expander detection circuitry to control this
expander, instead of the audio input to the plug-in. As you
can see, I put an aux send on the first kick track, which is
the mic inside the drum. I set it to pre fader with the little P
button so that the same level from this track will always be sent onto a
bus which I named “Kick Key” in the I/O menu.
At the top left corner of the expander plug-in I have chosen this bus
as the key input to the plug-in. Now the side-chain only “hears” the mic
that was inside the kick, and on that track there is almost no snare leakage. So the expander is expanding the mix of the two kick mics, but
only using the mic signal with no garbage in it to decide what to do!

We’ll come back to this trick later in a very interesting way... next
month, when we wrap up with some more cool dynamics tricks (including
a visit to the land of parallel compression), a bit more about eq, a quick
listen to that secret discombobulation mic I promised you, and a full list of
audio that we’ll make available on our website. See you then!
Michael Schulze (schulze@recordingmag.com) directs the Music
Recording and Production Bachelor’s degree program at the Lamont
School of Music, University of Denver (www.du.edu/lamont/audio).
RECORDING March 2013

http://www.du.edu/lamont/audio

Recording - March 2013

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