Indicator LEDs: A “compressor active” LED is a critical feature on a compressor, particularly if you don’t have much experience working with compression.
Since you can’t always hear when compression is happening, particularly with a low ratio, it really helps to have an LED that lights immediately when the compressor’s threshold has been exceeded.
It provides great visual feedback as to how the compressor is operating with regard to the dynamic range of the performance you’re recording.
A compressor without this LED is much harder to use effectively, requiring more guesswork and listening skill. Some compressors have additional “gain reduction” LEDs or a gain reduction VU meter; these are nice to have, but they aren’t as important.
If you’re shopping for a compressor, by all means get one with at least a compressor-active indicator LED.
For the following exercise I’ll assume you’re using a compressor with ratio and threshold controls as well as a compressor-active LED.
Using A Compressor
Set up a microphone for vocals and run it into an input channel on your board.
Put the signal through your compressor by way of the channel insert jack, and put on a pair of headphones. If you have an integrated compressor/expander, set up the expander section as described above—get it to clean up your studio’s background noise, but don’t let it chop off any final consonants or slowly decaying vowel sounds.
If you find that the expander’s gate is fluttering open and closed (which can happen if the threshold is right around the background noise level), try raising the threshold a bit, increasing the decay time a bit, or both.
Now you can go to work setting up the compressor section.
Set the compressor’s ratio knob to about 3:1 and stand in front of the mic, about where you’d be when you’re singing. While you watch the indicator LED, make vowel sounds that start soft and increase in volume, and notice when the LED comes on. (If it doesn’t come on at all, turn down the compressor’s threshold knob and try again.)
A good starting position for the threshold is the point where your vocal starts to get loud—in musical terms, somewhere in the mezzo-forte range.
If your compressor has a gain-reduction meter or LEDs as well, watch what happens when you sing even louder above the threshold. (When this kind of meter says that 6 dB of gain reduction is occurring, it means at that particular moment, the output would be 6 dB hotter if the compressor weren’t there.)
Now try turning the compressor’s ratio knob up or down and repeat the exercise, and try to hear a difference. You may notice that with a higher ratio setting (like 6:1), as you sing louder and louder above the threshold, the sound of your voice in the headphones may seem to get quieter.
What’s actually happening is that the sound of your vocal cords being conducted through the bones of your skull is overtaking the headphone sound, because the latter is being compressed while the former isn’t.
That’s okay. If you want to hear what the compression really sounds like, record yourself.
Record your vocals getting louder and louder fi rst at 2:1, then 4:1, and then 8:1, and listen to the difference. The best compression—and this applies not only to vocals but pretty much to every instrument—should do its thing without calling any attention to itself.
If you recorded yourself in the above exercise, you may have noticed that the sound can start to get “squashed” near the top of the dynamic range, particularly with lower compression thresholds and higher ratios.
How much “squashing” you can get away with depends largely on the context of the track you’re recording. If it’s a loud, rocking song with loud, rocking vocals, you can get away with a more “squashed” sound—in fact, very heavy compression is a cool effect frequently used on vocals, drums, and other sounds.