This month we'll continue with the second part of the compressor article (FOH, Theory and Practice, Sept. 2010, page 38). The first part was geared mainly toward what traditional compressors are used for, and the fundamentals behind the different basic functions.
In a nutshell, a compressor is a tool. It is used to manipulate the dynamic range of a sound source (hopefully for the better). Some compressors have more functions than just the basics, and that is what this month's focus is all about – side chain and multi-band compressors. These two types of compressors take manipulating sound to a whole new level.
Side Chain
Chances are, you've heard a side chain compressor at work before. Take any restaurant with background music and a paging system. While waiting for your table, the background music plays. When the hostess announces your name, the music fades automatically and only the voice is heard. So how does this work, and what does this have to do with mixing music?
Some of the most common compressors found on gig sites have a side chain function. The whole premise behind this type of device is to affect the dynamics of one sound source by the level of a different sound source. It's almost like a trigger. Take restaurant music as an example. The music goes in and out of the compressor with the voice sending into the "side chain input." With the side chain engaged, the music will compress according to what the side chain (voice) input level is.
This type of compressor can be very useful in many different ways. One example is its ability to bring out the vocal in a mix. To achieve this with a side chain, insert a compressor on a group with just the band. Then send the vocal into the side chain input. With the compressor set appropriately, every time the singer is singing, the compressor will bring down the level of the band, thus bringing out the singer in the mix. This is just one example of how a side chain compressor can be used. There are many different applications where side chains can be helpful.
Multi-band
The next – and my favorite – is the multi-band compressor. This, too, is a type of compressor that you have most likely used and perhaps not even realized it. Multi-band compressors have the typical controls such as threshold, ratio, attack and release. But they compress sections of frequencies independently from one another. It's like having a 3- or 4-way speaker cabinet with a digital crossover. The Lo, Lo-mid, Mid and High sections are all independently controlled, but all heard together at the end of the signal chain (the speaker cabinet). This is a very similar concept to what a multi-band compressor does. Each band of this compressor type has an adjustable frequency range as well as level control.
A very common type of multi-band compressor seen is a de-esser. The purpose of a de-esser is to get rid of the unpleasing "sssss" sound from certain vocalists. It will usually have a threshold and frequency selection. What this does is takes a narrow band of frequencies centered around the selected frequency and compresses only those frequencies, thus decreasing the amount of "ssss" heard without decreasing the entire signal as a traditional compressor would. This is a multi-band compressor in a very basic form. Many multi-band comps have an upwards of four separate bands.
These types of compressors are extremely useful in live sound applications. Although they are not seen as much in the analog world, these devices come in analog as well as digital "plug-in" format. I have been in many situations where one of these compressors save the day. Often, it has to do with a certain frequency range on an sound source that's sticking out to the point where I just can't get the EQ bell narrow enough to take it out on the channel strip. Take an upright bass, for example. They can be a nightmare on a poor sound system. Or a nasally vocalist – every time she or he sings a high note, you have to turn your head because you just can't take all that 3k.
Trying to EQ these kinds of things out of an instrument can create more problems than it solves. The issue then becomes a case where, once the problem frequency is pulled out, it's also pulled out at a different dynamic range where the frequency might not be so much of a problem. For example, if that singer is ripping your head off with those high notes, and you pull out the 3k, then when she or he is singing the lower notes, you might want that 3k back. That makes for a long night. With a multi-band compressor, you can hone in on the problem frequency range, set it appropriately to compress when it becomes overbearing and life is beautiful.
The possibilities are just about endless when selecting which compressor to use on a selected source. In the end, it becomes personal preference and opinion on what sounds better than the other. Side chain and multi-band compressors are fantastic ways to manipulate a sound source in a much more in-depth way than a standard compressor. The only downside to using these is time. The more knobs, the longer it will take to dial it in. But on a system or artist that is familiar, it doesn't always take as long and can be a much more "fine tuned" result compared to just a traditional compressor.
