This article is excerpted from Karl Coryat’s Guerrilla Home Recording - 2nd Edition.
Even with today’s inexpensive recording systems, it’s possible to achieve a dynamic range of over 90 dB—in other words, the loudest sounds you record can be over 90 decibels louder than the background noise.
Managing all of the “area” between these two extremes, for each and every sound you record, is a skill that’s critical to making a good-sounding recording.
Fortunately, there are expanders, compressors, and limiters (collectively called dynamics processors) that help in this task. We’ll start here with expanders, and in a subsequent article, move along to the others.
Using An Expander
Of the three types of dynamics processors, compressors are probably the most familiar—but let’s begin with expanders, because they’re a bit easier to understand. A lot of people record without using an expander in the signal chain, and I think that’s a shame, because effective use of an expander can do an awful lot to clean up the tracks that you record.
Expansion refers to the process of increasing a signal’s dynamic range—making it bigger. Isn’t 90 dB a large enough dynamic range, you ask? Certainly—but that number refers to a system’s potential dynamic range, not necessarily the range you’ll get if you plug in a mic and start recording.
An expander is important in optimizing the actual dynamic range you get out of a system.
An expander operates at the low end of the dynamic range, where signals are at their quietest, or perhaps nonexistent. In other words, when audio is coming through the signal chain, the expander may be doing nothing at all.
But when that audio stops coming through, the expander goes to work by lowering the signal further, expanding the background noise fl oor downward so that there’s a larger dynamic range overall (see Fig. 1).
Fig. 1 Without expansion (left), the noise level in the signal chain can far exceed a stage’s background noise. With expansion (right), the noise is brought down to the noise floor, without affecting the signal itself. (click to enlarge)
Not surprisingly, this is called downward expansion. There is such thing as upward expansion, but you don’t really need to know about it.
To understand this better, consider what happens when you plug a microphone into a mixing board and crank up the gain. If you talk or sing into the mic, you’ll hear yourself coming through the headphones loudly. (Careful—you might also get a feedback shriek if it’s too loud.)
But if you stop singing, odds are you won’t hear silence—especially in a bedroom or den Guerrilla studio. You’ll hear the heating or air-conditioning system, planes going overhead, street traffi c, or your kid brother’s video game down the hall. This is all stuff that doesn’t belong on your recording!
Sure, domestic sounds are charming—if it’s 1970 and you’re Paul McCartney recording your first solo album. But we Guerrilla recordists are going after a slick, clean sound, and part of “clean” means not having anything on your tracks that you don’t want there.
Here’s where expansion comes in. You may have encountered a device called a noise gate, which is a crude form of an expander. In a noise gate, once the signal falls below a certain threshold, an electronic gate closes and no sound is allowed to pass through (noise or otherwise).
However, when the signal begins to rise above that same threshold again, the gate opens up, allowing the signal to pass through once more.
Naturally, this also allows unwanted noise to pass through along with the signal, but the idea is that noise is less troublesome when signal is present to mask it. Like faint starlight in the night sky, noise is most noticeable when it’s by itself. Mix in a little signal (or sunlight in this analogy) and you’re less likely to notice the faint background stuff.
An expander works on the same principle as a noise gate, but an expander is a bit more subtle: it’s not as obvious to the ear when it’s doing its thing.
Here are the parameters that you’re likely to find on an expander, or the expander component of a compressor/expander:
Threshold: This control sets the level at which the expansion effect begins to set in. Imagine a cymbal crash that begins at 0dB (the top of the dynamic range) and slowly decays to –_dB.
Fig. 2 As the sound of a crash cymbal decays, it eventually falls below the noise floor and becomes inaudible. (click to enlarge)
At a certain point in its decay, the sound of the cymbal will get so quiet that you’ll hear background noise mixed in with the cymbal, and at a still-later point you’ll hear only background noise, as the noise masks what’s left of the crash (see Fig. 2).
If you were miking this cymbal by itself (perhaps to sample it for a collection of drum sounds), you might want an expander to kick in toward the tail end of the decay in order to take the background noise out of the sonic picture (see Fig. 3).
The threshold control determines when this happens.
Fig. 3 When you run the crash-cymbal sound through an expander, the threshold level determines how much of the cymbal’s decay makes it through before the expander closes down the noise. (click to enlarge)
If you were to set the expander’s threshold to –30 dB, the expander would begin to shut down the signal when the cymbal decayed 30 dB below its initial peak. In this case you could get away with a lot more noise happening in or outside your studio without worrying about these sounds making it onto your cymbal sample.
But if you wanted to make a long, realistic sample of the cymbal and capture a lot of its decay, you’d probably want to set the control lower—perhaps –60 dB—and record it at a time when your studio is at its quietest, such as late at night. (Bummer for your sleeping housemates!)
Since the expander is set to a low threshold, the signal chain will be more susceptible to noise coming into the mic or created by the mic preamp.