Limiters

Limiters are often one of the most misused effects, quite commonly applied to individual sounds in mix to allow the volume of such sounds to be pushed to the limits (no pun intended), typically with very audible, unpleasant sounding results. A limiter is actually designed to prevent an audio signal from exceeding a preset level, and is designed to be used in situations such preventing a signal from clipping during a recording, or to prevent damage to equipment and power amplifiers within a PA system during a live performance. To give you a better understanding of limiters and their uses we have prepared the following guide.

A limiter is a member of the dynamics family of effects, along with compressors, maximisers, gates and expanders. Much like a compressor, a limiter is designed to turn down the loud parts of an audio signal, and also contains a similar internal setup, utilising an envelope follower to track the level of the incoming signal, which then controls a gain control stage that adjusts the level of the sound coming out of the limiter. Unlike a compressor, however, a limiter is designed to act only on the highest peaks.

To allow a limiter to accomplish it's job properly it needs a very quick reaction time, with an attack time typically within the range of one microsecond to one millisecond. Some software / plug-in based limiters also utilise a "look ahead" system that allows the limiter to "see" forward in time so that it can react more smoothly to the peaks it is taming, without as much distortion and as much of an audible effect as you might get with a typical hardware based limiter. Limiters also typically utilise a very high compression ratio, with typical ratios being in the range from ten to one, to infinite to one.

The controls on a typical limiter include:

• Threshold: Sometimes also known as an input level or gain control. This control adjusts the point at which the limiter starts to act on the sound going through it. For a peak limiting application the threshold is typically set fairly high (or low in the case of an "input level" control), so that it is just taming the highest peaks.
• Ceiling: This control allows you to set the absolute maximum volume level that limiter allows the audio signal to reach. This control is more commonly found in software / plug-in based limiters.
• Attack: The attack control is not found on all limiters, though some limiters do allow you to adjust the attack time, typically with much quicker times than is available on a compressor. Some limiters also feature the option to automatically control the attack time depending on the incoming audio signal.
• Release: The release control is similar to the control found on a typical compressor, although some limiters also provide the ability to automatically control the release time based in the incoming audio signal.
• Look-ahead: This control is only found on software / plug-in based limiters and allows the limiter to "see" ahead in time, giving a much smoother and less audible reaction to the incoming peaks. The use of look-ahead does add some latency to the audio signal as it means that the limiter needs to buffer audio in between the envelope follower and the gain control.

Limiters should not be used in situations where you want the sound to become louder, as this is a task that is much better accomplished with cleaner results when using a compressor, although if you need a more extreme volume increase than a compressor can provide then a maximiser should be used.

Although limiters are designed to be used to prevent clipping of an audio signal, they can also be used as an effect, allowing a heavily audible pumping sound to be applied to an audio signal by setting the threshold level to a much lower level than would be used when you are trying to prevent clipping.