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Audio Compression

Do you want to make your beatbox recordings sound more professional?

Do you want them to sound louder and punchier?

Then read on...

What is compression?

To compress audio means to squash it in some way. For example, reducing the size of an audio file on a computer, or some other aspect of the audio such as the volume.

Technical tip: In audio engineering or sound technology, volume is called amplitude.

In this article, we are talking about compressing the loudness of audio.

Audio compression is about making the loud bits quieter and then turning the whole thing up. The tool used for this is called a compressor.

A compressor can be a software program or a piece of analogue or digital electronic equipment.

Here are some screenshots of different compressors:

Audacity for PC

Audacity for PC

Sound Studio for Mac

Sound Studio for Mac

Alesis NanoCompressor

Why use audio compression?

When we turn the loud bits down and turn the whole thing up, we increase the loudness of the sound. The quieter bits become louder.

Technical tip: Making the loud bits quieter and turning the overall volume up reduces the dynamic range. This means there is less difference in volume between the loudest and quietest sounds.

Here is a picture of an audio waveform before and after compression:





Compressors are used everywhere. Very few recordings are made without compression. Nearly every radio station and professional live show uses compression on individual microphones and on the whole thing!

Compression can be used to iron out the differences in volume levels on an acoustic guitar between picked and strummed parts, make vocals sound warmer and more intimate and make drums and beatboxing sound more punchy and clear.

Here are two audio examples. The first is without compression and the second is with compression. Can you hear the difference?

How does a compressor work?

A compressor works by using two amplifiers or volume controls. The first is used to turn the loud bits down and when this is being done, the second is used to turn the whole thing up.


Volume Control 1 (used to turn the loud bits down)


Volume Control 2 (used to turn the overall volume up)

As you can imagine, someone would have to work very hard and impossibly quickly to manually turn the loud bits down and turn the whole thing up throughout a song. This is why compressors are automated using software or hardware to control the two amplifiers.

The controls on a compressor.

A very simple compressor has at least three controls:

  • threshold
  • ratio
  • (make-up) gain

Most compressors also have two further controls:

  • attack
  • release

How the controls work on a compressor

The first amplifier is operated such that when the sound level goes over a certain point the volume is turned down. This point is called the threshold.

How much the compressor turns the volume down once the sound level has gone over the threshold depends on how strict it is about sound levels! This is called the ratio.

The higher the ratio value the more strict the compressor is such that if the ratio is set to maximum (infinity:1) then no sound is allowed over the threshold. This is called limiting.

Technical tip: The ratio numbers tell you the difference between the sound level going into the compressor (over the threshold level), and the output of the compressor. For example, a ratio of 2:1 means that for every 2 decibels of volume over the threshold, the compressor lets out 1 decibel of volume. 3:1 means for every 3 dB over, it lets out 1 dB. The ratio is setting how much you want to squash the audio that is over the threshold.

Finally, because all the loud bits have been turned down (compressed), the whole sound needs to be turned back up again and this is where the second volume control comes in. This is called the make-up gain.

Advanced controls

How quickly the sound level is turned down depends on the reaction time of the operator and is called the attack time.

Technical tip: The attack time is how quickly the sound is brought under control once it goes over the threshold. Transient sounds like drums may need a fast attack time.

Once the sound level drops below the threshold again, the operator needs to turn the volume level back up again or the whole sound will be quiet.

The reaction time of the operator in turning the sound back up again is called the release time.

Technical tip: The release time is how quickly the sound is returned to normal once the sound level has dropped back below the threshold.

Problems with compressors

Have a listen to the two recordings again:

- Beatboxing uncompressed

- Beatboxing compressed

Did you notice that you can here more hiss on the compressed recording?

Because you are turning the loud bits down and turning the whole thing up, compressors will raise the level of any background noise. This can also be seen on the audio wave forms for the two recordings:





To get around this problem, many people use something to reduce the noise before compressing the sound.

Technical tip: Many compressors come with noise supressors built in. For example, a noise gate, filter, or digital noise reduction tool where the noise is removed from the audio by analysing the frequencies of the noise and digitally subtracting it from the audio.

What's a valve compressor?

A valve compressor is an analogue compressor that uses valves in the amplifiers rather than transistors. Valves are older technology but make analogue audio signals sound 'warmer' - hence some guitarists prefer valve amplifiers and some recording engineers prefer valve pre-amplifiers for their microphones.

What's a multi-band compressor?

A multi-band compressor is where the audio is split into different frequency bands and each band is compressed using a separate compressor. For example, a 3-band compressor will compress bass, mid-range and high frequencies separately. This has the advantage of making each part of the sound punchy and gives the sound engineer more control.

Where can I get a compressor from?

Compressors are found in nearly all semi-professional and professional recording packages and digital recording consoles. You can buy a professional stereo hardware compressor, such as the Alesis 3630 for about 75 GBP (150 USD).

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