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What is a transient? Can it hurt me?

A transient is a sudden change in signal level. Can this harm a recording? What should an engineer do to accommodate transients?

A transient event is something that comes and goes very quickly. In audio, it means a sudden increase in signal level followed by an almost immediate decrease. In terms of milliseconds, we would be thinking about less than a hundred, or a tenth of a second. Anything longer wouldn't really be worthy of being called a transient, although it is a similar judgment call to when a stream becomes a river.

A transient in audio can also be a sudden increase in level, followed by a steady-state high level signal. So in this case the signal isn't coming and going, but the sudden change in level is the transient.

A transient can hurt your recording in a number of ways...

  • If your meters are not fast enough, the transient may not be measured correctly. In this case you could easily push the level high enough to get distortion. You won't see any red lights, but you will hear a change in the sound.
  • A recording that is full of transients, if metered correctly, will sound subjectively quiet. This is because you will reduce the level to accommodate the transients properly, but then the bulk of the signal will be well below the full level your system can accommodate.
  • Severe transients can blow speakers. Dropping a mic connected to a PA system is a good example of something to avoid.

It is good practice therefore to manage transients consciously. That doesn't mean getting rid of them, just reducing their effect where appropriate. The tool to use is the compressor or limiter. If you set a fast attack time, then the transient will be brought down in level. However, this will also change the character of the transient, and the 'front end' of a sound is important to the way the ear hears. So you have to be careful and achieve a fine balance. If you use a slow attack time, the transient will get through unaltered, so you haven't achieved anything.

If you have just one or two significant transients in your recording, then it will be better to reduce their level using the editing facilities of your recording software. You will be a far better judge of what to do than the automatic action of a compressor.

By David Mellor Monday July 31, 2006