How to properly use an audio compressor?

Question

I'm on my schools sound crew for our upcoming play, and we have a dbx 166xl compressor, but none of us understand how to use it. We have 8 wireless mics given out to individual characters, 3 hanging mics and some sound effects on cd, all of which are hooked up to our mixer. How and where would a compressor be used in this? Where would it be hooked up to?

Thanks!

Answer 1

a compressor is meant to prevent overdriving an amplifier or other audio circuit. It's purpose isn't really to make audio sound better but to prevent it from sounding bad. You don't have to use a compressor is you simply make sure the sound levels aren't too high for the circuit they're in. You'll see compressor a lot in radio station transmitters. Radio stations like their modulation to be high most of the time. They set the audio level to make sure they get the modulation they want and use a compressor of limiter to prevent over modulating and creating bad sound and possible harmonics in the output of the transmitter. Some mixers have a compressor built into them. A compressor is normally a line level device. It would go between a mixer and the input of an amplifier.

Answer 2

Wires has good advice there. However, a school play is a little different animal.



Your wireless and overhead mics should be OK without the intervention of the dbx compressor. One thing a compressor is used for in live applications is to 'even out' the audio signals. But you don't really need this on the actors as to capture all the differences in the volume (or emotion) of the actors' lines.



Your dbx will come in real handy if you have prerecorded sound effects and/or music in the production. The dbx can automatically even out the relative levels of the prerecorded material, and that's the 'bee's knees'.



Without knowing what type mixer you have I can tell you this much. Pice-O-Cake!



It looks a though they have (on the xl model) done you a huge favor. Without going into great detail you can simply do this. Take the unit for prerecorded material's output to the input of the dbx, and set the button near the input and output to -10. Out from the dbx 'output' into the mixer. Simple? Gets better.



Now, at the far left on the dbx, turn the 'Expander/Gate' to the 'Off' position. Next, and here's where it gets good, all the other knobs can be set to the '12 O'clock' position. Kidd you not. Then, on the chanel where you have the dbx plugged in, simply set the volume for the prerecorded material as you wish on the mixer, and the dbx takes over, making sure that the prerecorded material is always on an even level no matter what it is.



Oh, one more thing. If you're using both chanels of the dbx, push the 'Stereo Couple' button in. In this mode chanel 1 is the master for both 1 & 2 so you don't have to mess with the chanel 2 knobs. In other words, chanel 2 will follow chanel 1's controls.



Now, is that easy or what!

db

Answer 3

Both of the above are good responses.



The trouble is "properly" is subjective and varies by application. You'll need to learn what the features do. Then with some advice like that of the other answers you set it up for your application.



I've done that very task many times at my kid's school for their programs. Some have been relatively large productions with multiple hanging mics, handhelds, piano, CD, with outputs to a video camera which drives a closed circuit TV system and the speakers. I use the exact same model.



To set it up properly you'll also need the correct connectivity. I'm guessing by the number of microphones you've listed you have a reasonably large audio mixer, 16 channel or there about. I would set up the dbx as an insert. Your mixer probably has an insert loop. You'll need the appropriate wires. A pro sound or music store can help you acquire them if needed. They look like a Y cable but are wired in a special way. What happens is the audio leaves the mixer, goes through the dbx and back in, before the main mix output. That way you can set up your mixer levels and dbx to perform as they should and retain complete control of the main mix level separately. The rear level button for this configuration would be +4.



For a school performance you don't need the gate or expander. But you can use the gate to mute low level audio to avoid noise if you like.

Dave B's method takes advantage of the automatic mode. It works, and pretty well, but I'd set it up for manual control.

Main mix level for this set up doesn't matter if you use the dbx as an insert as above.

From left to right...

expander/gate:

set threshold all the way to off.

SC enable off

set ratio mid way

set contour off

compressor:

threshold - save this for later

over easy ON

I would set ratio to about 5:1

attack mostly fast, around "8 o'clock"

auto off

release - "2 o'clock"

output gain 0

bypass off (when on the compressor does nothing)

peak stop +20 (for now)

stereo couple on is probably best.

with stereo couple on the channel 2 settings do nothing. Or you can set the channel 2 settings the same and turn stereo couple off. Get channel one set up first.



Now get some audio going at 'normal' level. Get someone speaking or play an audio CD at about the level you would run the microphones. Watch the three lights at the threshold knob. Adjust threshold so that the middle yellow light blinks only on the peaks like a drum beat. Your gain reduction meter will move slightly. Levels below this point will have no compression. Above this point the compressor becomes active.

Now turn up the source louder to where the gain reduction level is active in the mid point around 15. Slowly reduce peakstop until the 'above' light just blinks on the peaks (again on the beat)

that's it

Adjust output gain if needed to give a good output level with your main mix at it's nominal position.

After that use your eyes and ears to tweak.



I highly recommend a feedback controller too, like the Behringer DSP1124. That takes a while to learn how to use but works great. Convince the school to get one. Nothing spoils these performances like feedback and there always seems to be one performer that just isn't loud enough and you'll end up with feedback because you have them cranked up so high.