Audio: compression (0:08)

Figure: compression

compression plays two rhythms. The first is uncompressed. The second, an exact duplicate of the first, is compressed using a -48dB threshold, -80dB floor, 6:1 compression ratio, attack time of 100 milliseconds and a release time of 1 second. Both sounds are normalised to -1dBFS. The compression figure shows the composite waveform. Uncompressed sound, shown in the left half of the waveform, has noticeable peaks and troughs in amplitude. Compressed sound, in the right half of the waveform, is squatter and thicker, indicating that amplitude does not vary much and that the sound is more powerful than the original.

Compression reduces the dynamic range of a sound.

Dynamic range is the difference between the highest amplitude and the lowest amplitude in a sound.

Compression always reduces the amplitude of a sound. It does this by squashing the dynamic range so that the difference between the highest and lowest amplitude gets smaller. Compression reduces light and shade in music, in the sense that the difference between loud and quiet passages becomes less noticeable.

The main parameters for compression are:

  • Threshold is the amplitude level above which compression is applied. It is usually in the range -60 to -5dB, with a typical value of -20dB. Lower threshold values mean that more of the sound will be compressed.
  • Floor is the amplitude level below which compression is not applied. It is usually in the range -90 to -60dB. A floor is often set on a speech sample to stop compression taking place during pauses.
  • Ratio is the amount of compression applied to sound above the threshold. The higher the ratio the more the loud sounds are reduced in amplitude. Gentle compression has a ratio around 2:1 and converts 2dB of uncompressed input to 1dB of compressed output. A ratio above 10:1 is severe compression in which the compressor acts as a limiter or noise gate. A hard limiter is a compressor with a value over 20:1, typically 60:1, and produces a harsh metallic sound.
  • Attack and release are the reaction times for the compressor to start and end compressing. Short attack times are useful for percussive sounds, and the release time is usually longer than attack time.

Humans have a wide dynamic range. The ear can distinguish between sounds that are 100dB apart in volume. It is sensitive to sounds that are very loud and sounds that are very quiet.

The Loudness War is about two effects, compression and normalisation. Compression reduces the dynamic range of a sound and makes it quieter. Normalisation increases the amplitude of every frequency present in a sound. Compression followed by normalisation always results in a sound that has more power than its original version. The effect of normalising a compressed sound is shown in compression, where the second version, compressed then normalised, is noticeably more powerful than the first, uncompressed version. Repeatedly compressing and normalising a sound makes its waveform more and more like that of white noise.

Compression is a type of distortion. Other distortion effects include overdrive, fuzz and clipping.