Welcome back to the series on “How to Mix Live Music”. Today we are looking at compressors: how and when to use them. The effect of a compressor might be subtle, but it is a very important and useful processor in pop and rock music. Once you’ve mastered how to use it correctly, your mix can be greatly improved. Though traditionally they were never found on a mixing console: they were always in separate units in a rack, but now almost every digital mixer includes them on every channel. And even the MGP mixers have a simple compressor on many of its input channels. Compressors can be used in a variety of ways: either to gently smooth the dynamics of a sound, making it more pleasant to listen to, and easier to blend with other instruments. Or to make a sound more punchy and full-bodied, to help it stand out from the other instruments in the mix. The first smoothing technique is often used for vocals and lead melody instruments, while the later punchy technique is used for rhythm instruments such as drums and bass guitars. Let’s begin by looking at all the parameters available on a TF mixer’s compressor, and consider how they actually shape the sound. First, and most importantly is the Threshold. This determines the lowest level at which compression is applied. If you set it too low, the sound will get squashed too much, and will sound life-less. Set it so the quiet sounds from the source are not reaching the threshold, but the loud sounds are over the threshold: see this indicator by the IN meter on the left. Next, adjust the Ratio. This determines how much compression is applied. If you select 3:1, which is a setting often used for lead vocals, it means that when the input level increases by 3 dB, the output level will only increase by 1 dB. It is unusual to set a compression ratio greater than 8:1 for live music. This determines how stable the dynamics of the sound will become. A low ratio will sound more natural, while a high ratio will give more consistency. Attack and Release times are both parameters which affect the behavior of a compressor: how quickly it is applied to a loud sound, and how quickly it stops being applied after the sound reaches below the threshold again. These two parameters are tricky for less experienced users: every type of voice or instrument requires a different range of settings to obtain a good sound. A fast attack time of 5 ms or less will catch and compress most transient sounds, and will be suitable for managing vocals and melody instruments. However, it would reduce the natural punch and attack of drums, bass and guitar strumming. In those cases, an attack time of around 20 to 30 ms would be more suitable: let the start of the sound through without compression, then control the level of the remaining sound. The Release time will need to be varied depending on how fast the instrument is played. A smooth vocal or instrument could have a release time between 100 and 300 ms. This might be too slow for quickly played drums and guitars. As a general rule, keep the release time at least 2x the attack time. So for drums and bass, somewhere between 40 and 80 ms should work well. Out Gain is a useful parameter if a lot of compression is applied, causing the overall level of the channel to reduce. Boost the Out Gain to bring the level back up. For smooth vocals and melody instruments I would not expect to need more than 5 or 6 dB added here. More could be applied for extreme drum and guitar sounds. Basically, use Out Gain to match the compressor’s OUT level to the IN level. Finally, the Knee selection chooses whether compression is applied instantly when the threshold level is reached, or if it is applied more gradually. Use "Soft" for a more natural sound, "Hard" for a more industrial effect. Listen to these three files, and look at the wave forms. The blue one is a bass guitar, without compression. Both the green and purple ones have a ratio of 6:1 and Out Gain of +5 dB. Just the Attack and Release times are different. The green file uses an attack time of 25 ms, and release of 90 ms: notice how the start of each sound is emphasized while the remaining decay of the sound is smoothed out. It sounds more rhythmic and exciting. The purple file uses an attack time of 1 ms, and release time of 1,000 ms: its waveform is much smoother, but at the expense of character. It has become dull. If all that parameter tweaking is too much for you at the moment. Don’t worry: you can adjust the sounds easily by using the simple 1-knob COMP mode in the TF console, or the 1-knob COMP on the MGP mixer. The first 50% of adjustment keeps the compression fairly subtle, suitable for applying to vocals and lead instruments. After that it becomes more severe, and can be used to add weight to more rhythmic input sources like kick and snare drums, bass and rhythm guitars. It can be used to provide a more consistent and powerful sound. Or for instruments like guitars, it can help to emphasize the finger picking while holding back the louder strumming. But once it has been applied, keep an eye on the Stereo Master level: the Out Gain increase of the compressors means the input channel fader levels might need reducing a little. When mixing live, be careful not to add too much compression. Just because its available on every channel doesn’t mean it should be used! Use it to make sounds either more exciting, or more comfortable. But preserve the live, dynamic, energetic feel of the real musicians and their instruments. Next time we’re going to be applying noise gates to a few input channels. These can help to clean up the sound. After that, we’ll take a look at output compression. Join me again then!
