Compression acts like an automatic volume control, turning down the volume when
the signal gets too loud. It reduces the gain (amplification) when the input
signal exceeds a preset level (called a threshold). The greater the input
level the less the gain. Compression keeps the level of vocals or instruments
more constant, so they are easier to hear throughout the mix. It also prevents
loud notes that may saturate the recording.
Sometimes the lead vocal track might be too loud or too quiet relative to the
instruments. You can control this by running the vocal track through a
compressor. It will keep the loudness of the vocal more constant, making it
easier to hear throughout the mix. Use the compressor as an Insert Effect on
the vocal input or you can bounce with that effect to another track. Set it
for the desired amount of compression (typically 3:1 ratio, -5 to -10 dB
threshold). Its also common to compress the kick drum and bass.
RATIO: (Sometimes called Slope) This is the ratio of the change in input level
to the change in output level For example, a 2:1 ratio means that for every 2dB
change in input level, the output changes by 1 db. Typical ratio settings are
2:1 to 4:1.
THRESHOLD: This is the input level above which compression occurs. Set the
threshold high (about -5 VU) to compress only the loudest notes; set it low
(-10 or -20 VU) to compress a broader range of notes. A setting of -10 to -5
GAIN REDUCTION: This is the number of dB that the gain is reduced by the
compressor. It varies with the input level. Set the ratio and threshold
controls so that the gain is reduced on loud notes by an amount that sounds
right. The amount of gain reduction shows up on a meter.
ATTACK TIME: This is how fast the compressor reduces the gain when its hit by
a musical attack. Typical attack times range form 0.25 to 10 milliseconds
(msec). A long attack time sounds punchy; a short attack time reduces punch by
softening the attack.
RELEASE TIME: This is how fast the gain returns to normal after a loud passage
ends. This is the time it takes the compressor to reach 63 percent of its
normal gain. The release time can be set from approximately 50 msec to several
seconds. For bass instruments, the release time must be longer than about 0.4
seconds to prevent harmonic distortion.