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Understanding Compressors

April 25, 2012

Onward and upwards, we move into this weeks entry which is a great prequel to the ‘EQ and Filters’ post last week.  When you are processing sound, its usually best practice to EQ the sound first, then add compression after.  If you’re not sure what compression is, well then you have come to the right place.  This week’s blog is all about compressing sounds and how it is used in everyday audio and music productions.

What is Compression?

When trying to explain compression  in can get quite confusing (just like everything else in audio).  Once you start talking about ratios, thresholds and RMS types, the whole concept of compression soon turns into a lecture, but in lamest terms, compressing usually means to squash or reduce the volume of a sound by increasing lower frequencies and decreasing the louder frequencies thus giving us the perception of a louder volume setting.  Essentially, a compressor is used to tidy up messy wave forms.  The main thing to remember here is that although a compressor may seem like it is making a sound louder, it actually is not.  The only thing a compressor is doing is reducing the amount of space the sound holds which makes it seem louder then it actually is.

Imagine this, you are in a square cathedral with very high 18 foot ceiling.  There are no windows and no doors.  The cathedral is completely square and in the centre of it, there is a bongo drum.  When you hit that bongo drum with a light tap, the sound will be heard closest to the skin of the drum then if you were near the 18 foot mark of he ceilings.  Now imagine taking that same cathedral but building the ceilings only 8 feet high.  You play the bongo drum with the same force, and for some reason it sounds louder.  This is what a compressor does.  By adjusting the threshold on a compressor you are setting a ‘ceiling’ so that when any sounds surpass that threshold, a certain amount of gain reduction is applied, thus making your sound seem louder then it actually is.

A great example of where compressors are used are in television.  Have you ever watched a show where people are talking in a quite room, then suddenly the man sitting next to them gets upset and starts to yell?  If the audio of that show was uncompressed, then the speakers on your television would probably blow due to the increase in volume the man makes when he raised his voice.  Compressors allow sound to be audible when quiet, yet comfortable when too loud.

Compressor Ratios & Thresholds

Setting a compressors ratio determines how the effect will react when gain reduction is applied.  Compressors by default will be set at 1:1 ratio which means that for every dB that surpasses the threshold, 1 dB will be be sent to the output.  For this example its best to represent ratios as in and out.  Lets say you have your compressors ration set to 4:1, this means that for 4 dB going into the compressor, 1 dB will go out. In partnership with the ratio, we have the threshold setting.  The threshold represents the dynamic range of audio that will be effected by the set ration.  If you have a threshold setting of -5 dB, this means that anything below the threshold will be uneffected, but anything that goes over the threshold will be effected by the set ratio.  Are you still with me?

Ratio and threshold are really the only two parameters you need to adjust if you are new to compression.  There are no standard settings and each time you use compression you will need to apply different settings to achieve the sound you are looking for.  Play around with threshold and ratios with some of your favorite samples to see how it sounds.

Attack and Release Parameters

The next two adjustments after setting your threshold and ratio is the attack and release of the compressor.  Using the attack parameter adjusts how quickly the compressor will react to the set ratio.  Attacks are measured in ms (milliseconds) and the higher the number, the longer it will take for the compressor to respond to the set ratio.  Using faster attacks will give more punch to your sound, but try adjusting to longer settings for different effects.  Again, there is no perfect setting for compression so fine tuning and playing around is greatly encouraged.

The release parameter is used to tell the compressor that after this setting, please return the audio back to it’s original form.  For example, if there was no release knob on a compressor, the sound being processed would continue to be compressed.  A compressor does not automatically return a sound to normal.  You have to tell the compressor when to do this and the release knob is what you would use to complete this task.

Gain

The last basic adjustment on a compressor which can be used to increase or decrease the volume of the compressor is the gain.  Much like a volume knob on a stereo or DJ mixer, the gain does just that.  Makes sounds quieter or louder depending on whether or not it is needed after compression.  Normally, there is no need to increase the gain of a sound after compressing, but there are cases where very quiet sounds maybe need some more volume and vice versa for louder noises.

Conclusion

Depending on your production style, compressors may or may not be in your arsenal of tools.  When using audio samples, most sounds have already been processed by the record labels so there is no need for compression and some basic EQ tweaking would be find for sculpting your sound.  In some cases, audio samples may be delivered to you ‘raw’, which means that no processing has been added so the need for compression is much higher then those better sounding samples.  Listen to your samples, and make the decision to use compression yourself.  A lot of artists like to keep things simple so don’t feel you need to stack up loads of plug ins and effects to your sound.  Most of the time this will just make things worse.  If recording instruments, it is always best practice to use compression.  Always remember, EQ USUALLY comes before compression, but once again there are no rules.  Create, experiment and have fun!

I hope you appreciate my latest article and please feel free to comment or like through the social feed below.  Make sure to continue checking out DJ Mag Canada for updates on our growing culture here in our country of “eh” sayers and please do not hesitate to contact me if you have any questions.

Markos

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