For many recording musicians, the compressor presents an enigma. Most of us know that compressors and their more specialized cousins, limiters, are used to control the dynamic range (aka volume) of an audio track or mix. But exactly how do they work, and from a practical point of view, how can you use them to the greatest advantage? We’ll provide a brief overview, and then head into the studio and see how the use of a compressor can help the recording process.
While dynamic changes in volume help give music its shape and emotion, too great a change in volume, and sudden changes, may create challenges when recording and mixing your music. Using a compressor or limiter allows you to smooth out passages where the volume may be too loud.
What’s the difference between a compressor and a limiter?
A limiter is a compressor, but a compressor is not necessarily a limiter. Limiters are designed to prevent the sound level of an audio program from going beyond a certain, pre set point. They generally have very high compression ratios of 20:1 variable up to Infinity: 1. (A compression ratio of 20:1 means that for every 20dB that the input signal increases beyond a preset point, the limiter will only allow output gain of 1 dB.) Limiters are critical for in-ear monitoring systems, radio and TV broadcasting as well as vinyl disc cutting systems. Limiters are seldom needed in a typical home recording session. Compressors will normally use lower compression ratios, often in the 2:1, 4:1, 8:1 range, meaning that the amount of gain reduction applied is not as drastic as with a limiter.
Compressors, on the other hand, are frequently used tools that reduce or control the dynamic range of a recorded track. A good way to envision the dynamic range of a track is the difference in volume between the loudest notes to the quietest notes. Compressors are handy for controlling many of the typical tracks you may be recording such as bass guitar, vocal, acoustic guitars, keyboard, etc.
Not every track will benefit from or needs to be compressed. If the original performance has a fairly limited range from the loudest to softest note, then compressing it will make little or no difference. However, if we consider a lead vocal track where the singer starts off near a whisper for the introduction and by the song’s end is wailing over a pounding band, putting a compressor on that lead vocal track may help reduce the dynamic range of such a vocal track so that it can easily stay on top of the band and allow for an easier overall mix.
The controls on most compressors are fairly similar. The first and most crucial setting is the Threshold. This is the point at which the device will begin to reduce the level of the output signal.
How much the signal is reduced is determined by the ratio. At a 2:1 setting, for every 2 dB of increase to the input signal beyond the threshold, the output signal level will increase by 1 dB. At a 4:1 setting, for every 4 dB of increase of input level beyond the threshold, the output level will increase only 1 dB. Higher ratios result in more dramatic gain reduction. If you have a home studio, put a compressor in your audio chain, and click between the different compression ratios available while you compress a track and you’ll quickly be able to discern which ratio works best on various instruments.
Two other controls affect the sound of the compressor. These are the Attack and Release controls. The Attack control allows you to fine tune just how fast the compressor reacts to the input signal with gain reduction when the signal overshoots the Threshold point. Conversely, the Release control allows you to set how fast the compressor lets the original signal return to normal once it is no longer above the Threshold. If the Release is set too fast, it may result in a sound often referred to as “pumping” or “breathing,” which draws attention to the compressed track. It’s up to you to decide whether or not such a sound may be useful in your overall concept for a song. Once again, spending a few minutes in the studio playing with these controls is the fastest way to hear how they work.
One other important benefit of using a compressor is that when set properly, the net effect is to increase the apparent overall level of a track, making it seem louder, by compressing the loudest parts without creating any overload.
Into the Studio
I stopped by our campus studio where some musicians kindly offered to make a quick demo recording in Pro Tools and then process it with a plug-in compressor. We started by recording an electric guitar through a small amp with no EQ or dynamic processing. We played back the track and inserted the Pro Tools DIGI Rack Compressor/Limiter Dyn 3, a common plug-in with most Pro Tools systems. (Although we recorded without effects, some engineers prefer to record with a compressor engaged from the start, while others record without any compressors, preferring to make decisions about dynamics after the performance has been captured.)
At my request, our guitarist Nic played a soft picking pattern first and then shifted abruptly to a more aggressive (and louder) chordal texture. Without compression, the chordal texture dominated the demo guitar track.To demonstrate the before and after effect of the compressor, we actually decided to over-compress the track, so that when readers listen it to by clicking on the link below, you’ll be able to clearly hear the difference. For an actual session, this level of compression would likely be judged excessive, unless you were going for an effect. (Over-compressing a track for an effect is also a very creative tool, but outside the scope of this article. Listen to some of the Beatles recordings from Revolver or Sgt. Peppers to hear the heavily compressed sound of Ringo’s drum kit for an idea of how effective such a use can be.)
After tweaking a bit, the student engineers settled on a threshold of -25 dB, so even the softer part of the guitar track was being compressed slightly. We use a ratio of 6.6:1, which gave us a fairly consistent overall final track reducing the level of the chordal texture to a point where it was only slightly louder than the softer first section.
This particular DIGI plug-in offers a real time analyzer (RTA) screen, similar in look to an old school oscilloscope, allowing the user to see the point at which the signal is being compressed and by how much. The vertical orange line on the RTA screen represents the threshold point and the intersecting white line represents the compression ratio beyond the threshold point. So with this particular ratio of 6.6:1, the slope of the compressor looks similar to a human knee, hence, this type of setting is often referred to as knee compression.
After listening back to the extreme compression we used, we realized that although we now had a much smaller dynamic range, if we actually wanted to use this track, we would have to boost the level of the guitar a bit, but nonetheless, the wide dynamic range on the original recording was fixed.
How much compression you use is also determined in part by the complexity and number of tracks in your overall mix. The denser the mix, the less apparent the individual compressed track will be, as opposed to say, a solo guitar and vocal, where the compressor must be very carefully set to avoid distracting the listener.
Compressing Acoustic Piano
Next, we made a stereo recording of an acoustic grand piano, and asked Nic to play both a quiet and a loud passage. For this compressor demo, we used a lower ratio of 3:1, but to show the compressor at work, kept a threshold point much lower than one might normally select for a piano, setting it at -24 dB. This way the compressor would kick in a bit on the quieter passage, but really be working hard on the louder section.
The overall dynamic range of the piano track was roughly 25 db, from a soft intro around -30 db up to some peaks in the louder section that hit -6 db. With the compressor settings chosen, we should be able to reduce the overall dynamic range to one that is more usable.
On playback, we found that the overall dynamic range had been reduced by about a third, from 25 dB uncompressed to about 16 db, when compressed. As a result the overall track sounded more consistent and the softer intro was more apparent. The net result was a track that would fit more consistently and will neither disappear in the quietest passage, nor overpower the mix when the volume kicked up in the louder section, when added to a full band mix.
This particular compressor has another nice feature, which shows both the input and output levels as well as a third meter, which displays the amount of compression or gain reduction that is being applied. In the adjacent screen grab, the two green meters represent stereo input and output to the compressor, while the orange meter just to the right of the green meters tracks the overall amount of gain reduction being applied by the compressor in dB. As a general rule of thumb, it’s best to avoid heavy gain reduction on vocals, acoustic instruments, and percussion unless you are going for a specific effect. Conversely, electronic instruments such as bass, keyboard, synth, and electric guitar can handle a good bit of gain reduction and still sound like the actual instrument. As with so much in recording, it’s up to the ears of the recording engineer or musician to determine how much is the appropriate amount to use.
Start recording and get your compressor in the circuit to see how you can help smooth out your tracks to best effect.
Special thanks to musician Nic Chang, and student engineers, Joey Frantz, Noely Gonzalez and Josh Walkover, as well as studio manager Jeff Crawford for recording the sound files for this article.
Understanding Audio Compressors and Audio Compression
More on compressors in this extensive article by Barry Rudolph – he also gives a few of his personal settings near the end of the article.
Here’s another article about using compressors on vocal tracks as well as a mention about the vintage Fairchild compressors used by Geoff Emerick recording the Beatles.
Modern Recording: Chapter 14: Compression
Finally, here’s a short tutorial on compressors from the 7th Edition of Modern Recording by David M. Huber and Robert Runstein.