Compressors/Limiters/Gates
Compressors, limiters and gates still cause confusion, not only in the way they work but what they can be used for. All 3 can normally be found in a 1u rack mount unit size, so not too big to accommodate in most audio systems. Limiters and compressors often built into many amplifiers or the amplifiers in powered speakers, all these have inbuilt limiters to help protect speakers drivers. Some mixers have compressors built in to help process vocals.
Potential Uses
- Managing vocal levels
- To add sustain to a guitar
- Protect processors from overload
- Limiter for a bass guitar
- Limiter to protect speakers
- To compress and limit during recording
Compressors See Diagram A
Compressors work by looking at the incoming signal and adjusting its gain. It can prevent quiet sounds form disappearing and stop loud sounds distorting. There are normally 4 basic control settings for a compressor, Threshold, Ratio, Attack and release.
Threshold: This is used to set the point of when the compressor starts acting on a signal.
Ratio: Controls the compression applied to a signal. With the ratio set to 1:1 the output signal is the same as the input. With a ratio of 6:1, if the input signal increases by 6dB the output signal only increase by 1db.
Attack: Is used to set the time the compressor reacts to a signal above the set threshold
Release: Is used to set the time for the compressor to return to its normal gain after the signal fulls under the set threshold.
A well set compressor can not be heard, if you hear a pumping sound you may need to adjust attack and release times. Its best to have longer attack and release times for slow music and faster attack and release times for up tempo tracks.
There are no set rules to how to set your compressor, most is just trial and error to achieve the correct effect you are looking for, but here is a good starting point.
Vocals: 2:1, 3:1
Bass: 4:1, 6:1
Electric Guitar: 3:1, 4:1
Acoustic Guitar: 2:1, 3:1
Strings and Horns: 3:1, 4:1
Percussion: 2:1 to 6:1
Increasing the presence of drums and percussion: 4:1 to 6:1 fast attack medium release
Making one instrument stand out: 2:1 to 4:1 fast attack fast release
Limiters See Diagram B
Limiters do as the word suggests, the signal is limited to a certain level. Limiters can be used for recording but they are mostly used for protecting mixer and effects inputs from being over driven or power amplifiers from being driven into distortion. Normally Limiters have there own control separate form the compressor, but can be part of the ratio control of a compressor and are labeled by a horizontal 8 (meaning infinity). Once set, the output level never goes over the desired level. Limiters are a good way to restrict amplifier output, protecting voice coils in speakers from being burnt out and damaged. This normally occurs when a distorted signal is sent to a speaker driver, producing a DC content and then overheating the voice coil. Limiters are a great way to protect expensive drivers and sound systems in general, especially in situations where systems are not being used by a sound engineer.
Noise Gates See Diagram C
Noise gates help to reduce unwanted noise when no signal is present. When you are compressing dynamic range is reduced, so loud sounds are quieter and quieter sounds are louder. So by compressing the loud sounds by 6dB you are also raising the quiet sounds by 6dB. So you could be raising the sound of unwanted hum and hiss. This is not always noticed when the music is playing but if the music level drops these disturbing sounds can be heard. A gate sets the input threshold determining the point when input signal is allowed through. The best way to set this is by not playing any music through the system and adjusting the gate threshold control until the gate is closed. The unwanted noise will then disappear. Check you have set this right by playing some music and making sure the gate opens when a very low signal is received. This may take some experimenting as you do not want the gate always closing when a music signal drops.