Just recently, I began to notice some strange noises coming from our subwoofers. To my surprise the speaker’s cone was completely detached from the voice coils on both of them. I asked myself, how in the world did this happen? I am always making sure everything is set to prevent this from happening. One thing I didn’t inspect properly is that our youth group also uses our same system but uses a different scene on our mixer. Sure enough, no compression, no limiting, and they ran the sound at decibels that could almost make your ears bleed.
What is compression?
Compression is the process of gradually lessening the dynamic range of an audio signal between the loudest and the quietest parts of an audio signal. Compression occurs when the volume of an audio signal exceeds a pre-determined level. An example, would be when the worship leader decides to belt out a few notes louder than usual instead of having to jump for the fader the compressor does the work for you. Compression is one of the most common processes in audio work, but it’s also one of the least understood and misused.
The four most basic terms used in conjunction with compressors are threshold, ratio, attack, and release.
Threshold: In regards to audio compression, the threshold is a level that is set at which the audio signal will be compressed. The lower the threshold the more of the signal will be compressed. Anything below the threshold will not be compressed. Anything above the threshold will be compressed at the set ratio.
Ratio: The set ratio determines the rate at which the audio signal above the threshold will be reduced or compressed. Common ratios are 3:1, 4:1, 6:1, 16:1, and are spoken as three to one, four to one, etc. A ratio of 3:1 one means that the audio signal will have to cross the threshold by 3 decibels for the signal level to increase by 1 decibel. The higher the ratio the more compression is applied.
Attack: The attack determines the time that the compressor takes to “react” and apply compression to the signal from the point it crosses the threshold. Attack time is usually given in milliseconds, usually ranging from 1-100 milliseconds.
Release: The release setting determines how soon after the signal dips below the threshold that the compressor stops. Release time is usually given in milliseconds to seconds ranging as short as 25 milliseconds up to 4 or more seconds.
Two other terms that are also commonly used when talking about compressors are “ hard knee” and “soft knee.” Knee settings determine how the compressor reacts to the audio signals once the threshold is passed. A soft knee compression kicks in softer and more gently as the audio signal passes the threshold. A hard knee hits the signal right away.
Also, most compressors have a make-up gain knob which allows you to boost the compressed signal.
Common Uses For a Compressor
The most common thing I use a compressor on are the drums. Using a compressor on the cymbals or tom drums will give them more of a sustaining tail sound. They can be used on vocals to get rid of sibilance. They also can be used on the bass guitar to bring in a warmth to the low end.
What is limiting?
Limiting is very similar to compression in that it reduces the dynamic range of an audio signal. However, while a compressor gradually reduces the dynamic range a limiter completely prevents a signal from going over a specified setting. Limiters usually have a high ratio setting of 20:1 and higher. Some limiters have other settings like a compressor such as attack and release times, which are usually set to be very fast sometimes at zero seconds.
There is so much more to using compressors and limiters, I hope this article began to clarify basic functions in a clear way that you can begin to put the information to use right away. Once you get a hang of the common terms and uses of compressors and limiters they will become more relevant.