Bass Compression -
Taking the Mystery out of Compressors
A (bass) compressor is a device, (pedal or rackmount) that decreases the dynamic range of a signal by lowering the gain above a certain threshold. Make sense? Didn't think so. It will after you read this article and listen to the audio samples at the end.
For the purposes of our discussion, we have an imaginary bass line with peaks at about -2dB. A lot of the notes sit at around -6dB while some are softer at -12dB.
First think of a device which simply passes the signal from your bass untouched. What goes in is exactly what comes out. That's simple enough.
Now imagine you have two knobs. One is called Threshold and the other Ratio. Suppose we are unhappy with our louder -2dB signals - they make the line sound a bit uneven. We want to tame them. So, we first set our threshold to -6dB. That tells the compressor we only want it to do its "thing" on signals above -6dB and pass through the rest of it untouched. But what is its thing?
It's thing is an automatic volume control (in very simple terms). We want it to turn down the volume on signals above -6dB. How much should the compressor turn it down? Well, we use the ratio knob to control that. If we set the ratio at 2:1, that tells the compressor that for every 2dB above the threshold (-6dB), it should only increase the volume by 1dB.
So what happens on our -2dB peaks? They are 4dB above the threshold. At a ratio of 2:1 the compressor will reduce the gain by 2dB, so our processed peaks will be at -4dB (-6dB threshold plus 2dB). Suppose we use a ratio of 4:1 (that's 1dB for every 4dB above the threshold). In that case our peaks will be at -5dB, just barely above the rest of the notes. If the math is puzzling you, fear not. There's a chart further down with more examples.
More Bass Compression Settings
A full-blown compressor will have two other knobs: Attack and Release. The attack knob tells the compressor how quickly to turn down the volume. A fast attack means that the whole peak is reduced by the ratio. A slow attack allows the first "umph" of the signal to get through before turning down the volume (as if the little guy inside has been drinking a lot and has slow reaction times).
The release tells the compressor how quickly to turn the volume back to normal after the signal drops below our threshold. If we have a long release, then some of those -6dB signals will also have their volume reduced a bit as well.
One additional feature you often see is the knee. What I just described is a hard knee because if you graphed it, it would look like a bent knee. A soft knee tells the compressor, "Hey, I don't want you to turn down the volume so abruptly. Instead, when the signal is at -12dB, set the ratio to 1.5:1 and gradually work your way up to 4:1 when we hit the threshold." Some compressors let you set the knee to a certain level. Others just have a switch where it does this automatically (dBx calls it Over-Easy).
The last knob you will see is the gain or make-up gain knob. Think of this as a volume control for the entire signal. If you add 6dB of gain, it will add 6dB of gain to the entire signal after it has been compressed. Why would you use it? Well, now that we've lowered our peaks, we can increase the gain by a few dB and make everything louder than we could before we compressed it. Without compression those peaks were just too loud.
That's it in a nutshell. Can you see why it's called a compressor? It compresses the signal from the top down. If you looked at a graph of the high points, the raw signal high-points would be much higher than those in the processed (compressed) signal.
Here's a table with some example bass compressor settings. The first row is our raw signal. Watch what happens to the peaks as we add more compression and lower the threshold. The blue line shows signal levels untouched. Notice how for the first two settings the signal below the threshold is the same, but above the threshold it doesn't increase in level as much as the unprocessed signal.
Example Bass Compressor Settings
Notice that in Setting 4 we added some make-up gain. Why did we do that? Look at Setting 3 (green). There's now a narrow dynamic range with the entire signal being lower in volume than before. By adding some make-up gain we put the signal right where we wanted: Not too loud but not too soft. What we've done now is make our loudest peaks sit at -6dB (and now they're not that much louder than most of the notes). Our quieter notes are now louder and not so far below the middle. Overall the bass will seem to punch through the mix a bit more.
It's important to note that this may or may not sound good. We also didn't discuss how the attack and release times affect the sound. I just want you to have a feel for what's happening to your signal.
Not every bass compressor or generic compressor will give you access to all of these controls. If you can afford it, get one that does. It's infinitely flexible. The Boss GT-10B multi-effects processor has several models of compressors built in including one with all sorts of control for you to tweak to your heart's content. If you can afford the $500, buy this box and replace all of your other bass pedals. You won't regret it.
Now it's time for the audio samples.
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