“When the Levee Breaks.” “In the Air Tonight.” Californication. One of these things is not like the other. Compression can make or break a song, or an entire album, and yet remains a mystery to most listeners, puzzling even the savviest of musicians.

Dynamic range reduction, or compression, is present at every stage of the recording process from individual instrument rigs, tracking, mixing, mastering, all the way to radio broadcast signals. It’s the shot-heard-around-the-world in the Loudness War. So what is it? And why do we only talk about compression in the context of awesome drum sounds and awful sounding records? Compression rules! It’s given us so much – the distorted guitar, that Nashville Tele twang, funky basslines, and breathy in-your-face vocals, but people treat it like a band-aid in the mix sometimes, and I won’t stand for it. So, before I go any further, let me get this off my chest: compression is a tool, not an effect – unless you want it to be. And, because this is Aaron’s Bass Hole, after all, we’re going to hear examples of compression on the bass guitar from the Warden.

What is Compression?

Compression is dynamic range reduction. What this means is that when an audio signal is sent to a compressor, the loudest parts of the signal are automatically turned down, while the quieter parts remain the same. This reduces the difference in relative volume between the loud stuff and the quiet stuff. The entire dynamically reduced signal is then amplified so that the quiet parts become more audible in the mix, but the loud parts aren’t so loud that they overpower other elements of the song.

Compared to their rackmounted pro-audio peers, most stompbox instrument compressors have simplified controls, two or three knobs, tops. One of these is usually labeled “Sensitivity,” “Sustain,” or “Compression,” something like that; and the other is marked “Volume” or “Output.” The “Sensitivity” control adjusts the amount of compression in one of two ways: 1). by adjusting the threshold, which is the amplitude (or volume) at which the compressor starts reducing the input signal, or 2). if the threshold is fixed, changing the input gain of the compressor, which affects how much signal is sent to the threshold and has a similar effect. The “Volume” control, as you might expect, sets the output level for the compressed signal.

Now let’s take a look at the controls present in most rack unit compressors. I’m going to turn it over to recording guru David Miles Huber and his excellent textbook Modern Recording Techniques:

• Input gain – This control is used to determine how much signal will be sent to the compressor’s input stage.

• Threshold – This setting determines the level at which the compressor will begin to proportionately reduce the incoming signal . . .

• Output Gain – This control is used to determine how much signal will be sent to the device’s output. It’s used to boost the reduced dynamic signal into a range where it can . . . be better heard in a mix.

• Ratio – This control determines . . . the amount of input signal (in decibels) that’s needed to cause a 1-dB increase at the compressor’s output . . . For example . . . a ratio of 2:1 will produce a 1-dB increase in output for every 2-dB increase [in input].

• Attack – This setting . . . determines how fast or slowly the device will turn down signals that exceed the threshold.

• Release – Similar to the attack setting, release . . . is used to determine how slowly or quickly the device will restore a signal to its original dynamic level once it has fallen below the threshold point . . . (461-462)

And we’re back. The oversimplification trend in stompbox compressors is changing, and luckily for us, the Warden has all the controls you might expect in a studio-grade pro-audio compressor. Cool, right?

So Why Compress?

There are about as many reasons to compress a signal as there are compressors on the market. The question you need to ask yourself before patching in a compressor is, “Why?” Here are just a few reasons:

• To even out your playing dynamics – many stringed instrument players like compression because it’s useful in matching the level between single note phrases (which can get lost in a dense mix) and chords. Bassists might use compression to smooth over the difference between pick and fingerstyle playing, or between fingerstyle playing and slap-and-pop techniques, or two handed tapping. Before you compress, make sure that you’re making appropriate adjustments to your technique, because after all, tone is in the hands. Also keep in mind that if you can’t hear yourself in the mix, you might be playing the wrong part. On the flip side, compression can be a great way to let those tricky ghost notes and other stylistic flourishes shine through.
  

• To alter the attack and release envelope of your instrument - The Warden’s “Attack” and “Release” controls are powerful. A quick “Attack” setting can drastically alter your pick attack (or transient) by squashing the front end of the notes you play, which on the bass guitar, results in a thumpy, Jaco-esque, almost fretless sound. Likewise, adjusting the “Release” control can give you anything from smooth enhanced sustain at slower settings, to breathy, pulsating “pumping” sounds at faster speeds. To begin, I recommend setting the “Attack” to a moderately slow speed to let your initial pick attack poke through, then adjusting the “Release” to taste, which will depend on the tempo and style of music you play.
  

• To bop those pesky transients on their heads – Is that 3rd position G sticking out too much? Tasteful compression can level off some of those tonal inconsistencies caused by the quirks of your particular instrument. Follow your ears.
  

• Because it sounds good – While compressors do smooth out musical passages on a note-to-note basis, they also work on the constituent harmonics (or overtones) present in an individual note, which is why they're useful as overall tone enhancers. Instruments get their unique timbre from the presence of a fundamental pitch, which is how we can distinguish C from C#, for instance, and also from the distribution of harmonic overtones, which exist in multiples of the fundamental frequency. This is how we’re able to tell the difference between, say, a trumpet and a saxophone. Or between Kerry King and Jeff Hanneman. Compressors increase the level of harmonics relative to the fundamental and that’s a small part of the reason why the classic Tele country twang leaps from the speaker.

What’s it sound like?

In Example 1, I use the Warden to make the grace notes stand up and say hello in what might be the most ubiquitous bass riff in the modern world. I’m using a moderate “Sustain” setting, around 2 o’clock. My “Attack” time is slow enough to let the pick attack through and my “Release” time is set quick enough to get out of the way so the only mush you hear is my own sloppy playing. Ha! I begin with the Warden off, then click it on halfway through the example.

For Example 2, I roll back the “Sustain” and “Ratio” controls and speed up the “Release” to add a little bit of pumping swagger and smooth out some heavy-handed alternate picking in this down and dirty punk jam. In both examples I'm playing a Rickenbacker 4001 through a Tronographic Rusty Box and a Tech 21 SansAmp RBI direct into my DAW.