Compressors usually have an adjustable threshold setting. The compression threshold is the volume level (gain) at which the compression effect is engaged. Any signal passing through the compressor which is louder than the threshold setting will be compressed. Any signal lower than the threshold will be unaffected.
If you can adjust the threshold setting on your compressor, adjust it to taste. It depends on if you only want really loud notes compressed or most everything compressed.
The compression ratio selects the amount of compression to apply on signals above the threshold. The ratio might be something like 3:1 (3 to 1). That means for every 3 dB you send to the compressor beyond the threshold only 1 dB comes out. Notice in the diagram how the slope of the dynamics changes past the compression threshold.
A 1:1 ratio would not compress anything.
An ∞:1 (infinity to 1) means for any signal above the threshold sent to the compressor only 1dB comes out. This is known as limiting. You may see units called compressor/limiters. Limiters are often used to prevent damage to speakers. (Many TV sets now include limiters to squash loud commercials.)
On some effects units the ratio is just marked as ‘compression’ or ‘amount’. This setting increases the ratio.
Most bass players use a 2:1 up to 5:1 compression ratio. You really have to listen closely to adjust it. It’s very subtle. At first you might wonder if it’s even on and working! Try starting at the highest amount of compression and you’ll hear the dynamics squashed. Then slowly lower the compression amount until it suits your taste. A little goes a long way.
Some compressors allow you to adjust the compressor’s attack. Compression attack refers to when the compression effect is engaged after the signal crosses the threshold. Attack is measured in milliseconds (ms). You might adjust it from 0 (engages immediately) to a few milliseconds (waits 2 to 10 ms).
The bulk of a volume spike comes at the beginning, or attack, of a note. If you wanted the sound of your plucking or pick attack to come through more, you could increase the attack time to let it pass through.
Compression release refers to how long the compressor is engaged after the signal exceeds the threshold. Release is measured in milliseconds (ms).
A short release time would mean only the attack would be compressed and the rest of the note would run its course like normal. A short release time makes for a real squashed sound.
A long release time will follow the note more naturally as it decays. You will feel more sustain from your notes with a longer release time.
Hard-Knee Compression and Soft-Knee Compression
On some audio compressors you may see the terms hard-knee and soft-knee. Hard-knee compression kicks into its full compression amount as soon as the threshold is crossed.
Soft-knee compression kicks in gradually as the threshold is crossed. Soft-knee is a smoother sound. It’s probably the setting I’d choose on bass if I had the option.
Compression Settings Summary
If you're new to compressors, it may take a fair amount of experimentation to find the setting that most complements your bass playing style and your equipment.
I would start off with a 3:1 ratio with 0 to 2 ms of attack and 100 ms of release. Try adjusting the threshold from there. Then tweak the other settings from that point. Don't over-compress. If you can really hear it or it makes a pumping/breathing sound, you need to experiment some more. Remember, compression is a subtle effect.