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Compressors are regularly used to tame errant dynamics or to increase the perceived volume of a track. As well as squashing the audio, the compressor can be used to bring out some elements more than others.

Letting the attack through

Remember when you were learning about compression? You probably wanted the attack at 0ms so that the whole signal was compressed equally. But it ended up in a squashed mess. The output sound had a different quality to what was put in. The guitar no longer sounded like a guitar, but like an old keyboard emulation of a keyboard. So we all gradually learned to allow the initial transients through with delays of 20ms upwards, depending on the instrument and the wanted effect. This allowed for a natural sound.

Reverse your thinking

Usually we're using a compressor to tame peaks or increase the perceived volume of the track by decreasing the dynamic range. This time we'll focus on increasing the dynamic range and still using a compressor.

Set the compressor to have a longer attack, e.g. 30ms to let the initial strike of a guitar string through. Set the compressor to moderately reduce the dynamic range of the rest of the signal. What you'll see if you compare the input to the output is that the initial peaks where the string is hit or plucked are louder in comparison to the rest of the signal.

Additional Options with Software

Pre-empting

The problem with hardware compressors is that they can only react once they receive the signal they receive. Software compressors have the advantage of being able to look-ahead at the signal and react before a hardware compressor could. That's useful for enabling the compressor to act on the initial transients.

Sidechain

You can also eq the input signal to trigger the side-chain. Some allow you to do this in the plug-in itself. For others you'll have to set up an aux/bus track and route the signal in parallel through that and eq the aux channel. This can be done in hardware as well if you eq the feed going into the sidechain.

Using Other Processors

Expanders

Expanders are built for job of increasing the dynamic range. Effectively, they're compressors with upward compression. And so should be more suited to increasing range. However, sometimes I prefer to use a basic compressor.

Enveloper

These react to the envelope of the sound. Think back to the basics of synthesis with every sound having Attack, Decay, Sustain and Release (ADSR). Envelopers provide a way to change the gain at the various stages. The one in Logic allows the user to change the gain at the Attack and the Release stages. So you could potentially just change the gain of the attack and leave the rest of the signal alone.

Gain

This doesn't increase dynamic range. I've included it because I had to increase the gain of the guitar track to match the results of the effects.

Results

I recorded a steel-string acoustic guitar using an x-y pattern of 2 small diaphragm microphones. These were routed to a bus (aux in Logic) and the effects were applied to the bus only. The main output for the microphone channels were removed, meaning that the signal was only going through the bus. I applied the following effects in turn: compressor, enveloper, expander and gain. For each one, I spent about 2 minutes changing the parameters on a part of the track that was of roughly average volume. Then I bounced the output to a separate audio file.

It's important to note that I only spent the time configuring the parameters for one part of the track. There were louder and quieter parts elsewhere in the track.

Rather than play the whole 5 minute track, I've extracted three parts; one quiet, one average, one loud.

The Screenshot

The screenshot shows the unprocessed as well as the results of having each of the compressor, gain, enveloper and expander applied in turn. The order from left to right is Average, Quiet and Loud.

Download a larger version of the image here

What you'll notice is that there's not that much different between most of them in the average and quiet. However, the compressor did a good job of using the dynamic range available. The gain effect just made everything louder, as it's meant to, but doesn't bring out the peaks. The expander and enveloper both coped well, but I prefer the compressor.

In the Loud section, we see the issues with applying parameters for one section only. In all reality, we'd probably be riding the faders or at least automating the fader/effect parameters. But for the moment, it gives us a good idea of how flexible the processors are. Most of them are showing clipped waveforms for the loud section, indicating too much make-up gain. The compressor waveform still has thin ends to the spikes indicating a more managed approach. And the sound does reflect that.

Quiet

Unprocessed

Gain

Expander

Compressor

Enveloper

Average

Unprocessed

Gain

Expander

Compressor

Enveloper

Loud

Unprocessed

Compressor

I've only published a few of these files in case anyone risks damaging their speakers. I haven't published the files with square ends, e.g. the "gain" and "expander" files.

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