Dynamics units are to either plug-ins or outboard gear (sometimes even inline in a desk) that are used to control the levels of sounds, or more specifically the range of levels a track can have. There are four main categories of dynamics units, but they are all closely related, these include: Compressors, Limiters, Expanders and Gates. All of these affect what is referred to as the dynamic range. The Dynamic range can be defined as the range of volume any sound has from it’s quietest part to it’s loudest part. Being able to control the dynamic range of a signal offers the engineer a powerful tool when balancing a mix.

A good starting point to explain dynamics units is compression. As the name implies compression compresses or shrinks the size a signal is allowed to operate in. The main advantage of being able to to this is that a signal can be made more “consistent.” By more consistent we mean that because the signal now has a restricted dynamic range the overall signal level will be more predictable. For example if a vocal seems get louder and softer during a song (and that is not the artistic intent) compression could be used to make the softer parts louder and the louder parts softer all at the same time. Once applied the overall level of the signal (the track fader or gain control on compressor) can often be raised to suit the mix.

If you haven’t realized it yet you will soon, audio engineering is all about compromise because “there is no such thing as a free lunch.” The compromise with compression is that by compressing the signal you are also raising the noise floor. Notice in the image below that by making the quietest parts louder we have also made the noise that was quiet, now louder. That is if the noise floor was originally between 0 and 2 dB, it is now between 10 and 12 dB. This means that if you compress very heavily you will start to hear more of the noise in the recording.

Now that we have discovered the balancing act that we must play with compression, let’s take a look at what we can actually control to keep our footing. Compressors will almost always have the following controls. Threshold, Ratio, Attack time, Release Time, Input level, and Output level (Gain). We will follow the signal flow through the compressor to explain what each control accomplishes.

Input level controls the level of the signal as it enters the compressions stages of the compressor. If the control is left at 0dB the signal hitting the compressor should be the same as what is coming off of the recorded media or previous effect. If you want to raise or lower the signal level prior to compression this is where you would do it.

Threshold is one of the controls you will use to control how much compression is to be applied. This control allows you to decide when (that what level) the compression will kick in. Setting a very low threshold will result in heavy compression, while a higher threshold will result in lighter compression. The threshold basically sets the point that the compressor says oops the sound is getting too loud, time to apply some compression to keep things under control.

Ratio, is the other control that determines how much compression is to be applied. If the ratio is 2 to 1 (2:1) that means that signal above the threshold is halved. That is if the threshold is set at 60 dB and the signal that is input peaks at 80 dB the amount of signal above the threshold is 20 dB. With a ratio of 2:1 the peak will be reduced by 10 dB (20 divided by 2) and the resulting signal will peak at 70dB rather than 80dB. A ratio of 5:1 would yield a 16 dB decrease or a peak at 64dB rather than 80dB (20 divided by 5 = 4dB, 20-16=4dB). If the ratio is greater than 50:1 the compressor is considered a limiter because at this point the compressor is basically “clipping” off the peaks and making them level with the threshold.

Attack time refers to the amount of time it takes the compressor to react to signals that breach the threshold. I could also be thought of as how sensitive the compressor is. The image below illustrates that a longer attack time allows more of the full transient (a sudden change in dynamic or level) through before clamping down on the signal. This is typically good for drums and percussive elements as the attack or transient of these sounds are essential to their character. A shorter attack time allows more control over the overall dynamic range this can be good for things like rhythm guitars and fingered bass.

Release time refers to the time that it takes the compressor to release control of the dynamic range back to the original signal. Say for instance the guitarist plays some very heavy loud stabs followed very quickly by some soft palm muted strums. The waveform may look something like the image below. Notice the effects of the different release times. A slow release time still compresses the quieter parts and does not allow them to be as loud as they would naturally be. With the medium attack time you can see that the quieter parts gradually increase in level to their natural level. Finally a fast attack time allows the quiet parts to return to it’s natural level very quickly. The slow and medium attack times in this case are examples of what is commonly referred to as a pumping or breathing compressor (sometimes this is a desired effect, but commonly it is a sign of over compression).

Output level or (make up) gain allows you to control the output signal from the compressor back into your channel. Often after compression the overall level of the signal is decreased. Most times this is the intent, if you used compression to tame an extra loud part of signal you can now raise the overall signal in the mix. Sometimes there is an auto gain setting that attempts to match the output signal to the loudest part of the input signal. This never seems provide a level I’m happy with so I prefer to set the output level manually. Remember the issues of the noise floor when setting the output gain, if there is too much of the noise floor in the signal after the make-up gain is applied, you may have to go back and re-adjust the ratio and threshold to make a compromise.