You may have noticed when you were playing around with compression in the last setting that a little bit of compression sounds good, but too much starts to sound noticeable. Sometimes, however, you may need a little more than light compression. The problem is that if you use a heavier setting, the effect starts to become too noticeable. One way around this is to stack your compression effects.
It's a really simple concept; apply a light compression across your entire file, and then apply another light compression. Lightly compressing the material twice is a much subtler effect than heavily compressing once. In fact, this approach is very common in recording applications. Audio engineers lightly compress material that is being recorded, to provide a little control over levels. Then, after the material has been edited, they apply additional compression to give the program that extra bit of presence.
Another approach, used in multi-track recording situations, is to use different compression effects on different tracks, and then to have a limiter placed across the master mix, to make sure the final mix never goes into distortion.
Multi-band compression
All the compression we've been talking about up to this point has been compression that analyzes the entire frequency range when determining what has exceeded the threshold. There are specialized compressors that break the frequency range into a number of different bands and then compress them all individually. Radio stations use ridiculously expensive multi-band compressors and set them up to try to get a unique, signature sound. (All they're really doing is competing to see who sounds loudest.)
One area where multi-band compression can be incredibly useful is as a corrective measure, in particular to deal with microphone pops and sibilants problems. In the EQ section, we discussed how you could deal with pops and "ess" sounds by turning down certain frequencies at the instant the problem syllable appears. A multi-band compressor does this for you automatically.
Figure 1 shows Sony Sound Forge's multi-band compressor, using a preset called "Reduce plosives and sibilants." It's using two bands, both of which have very fast attack and release times, because we want the compressor to react immediately. Band #1 is set to low shelf mode, meaning it monitors all frequencies below the shelf frequency, which in this case is 600Hz. Band #2 is set to notch mode, with a moderate Q of one octave, centered on 5KHz. When I preview audio, I can see the second band being compressed when the narrator pronounces "ess" sounds. Because my narrator hasn't popped the mic, the top band doesn't do much of anything. If you have a guest who has a problem with sibilants, you can use the de-ess preset (yes, that's the technical term for this) to get rid of the problem. Fabulous.
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