Given the problems that arise from microphones having to be turned down in order to prevent excess noise from being introduced into the track, and signals having to also be loud enough to be heard and recorded clearly, the problem of keeping the volume at sufficient levels arises. The overall measurement of changes from the lowest to the highest of decibel range is referred to as the dynamics and this can be controlled by using specific effects.
The first is the compressor. A compressor turns down the volume every time it gets too loud. This is an automated function which, once set by the engineer, can remain unattended while it does its work. The primary settings which you would consider while calibrating a compressor are the threshold, which is the limit of how loud the track can get before the compressor engages, and the ratio, which is a measurement of how much it will get turned down when it does. Some will give you other controls such as: attack and release which can establish how quickly the action takes place, or there may be a knee control which automates the speed gradient so that the compressor can start out at one speed and gradually change to another as the signal level becomes even higher. Whatever controls that are used, they are usually followed by a separate gain stage which allows the volume to be brought back to the same place it was before the compressor was used. This is very important, because it reveals the purpose behind a compressor. After returning the volume to nominal, the louder parts of the track will be where they were originally, but the softer parts will now be louder. The compressor is a very useful and often necessary tool in tracking, especially when recording vocals, because it helps keep the average volume of the track. Most channel strips include a compressor and allow the dynamics to be controlled on the way into the DAW, (digital audio workstation).
The second dynamics processor is a limiter. A limiter is basically the same thing as a compressor but it gets used for a slightly different purpose. Where a compressor is used to maintain an overall desirable volume the limiter’s focus is on preventing the volume from reaching too high. There are in fact two classes of limiters. The standard analog style limiter, which has also been reproduced in software as well; (Variety of Sound’s ThrillseekerVBL is an example) is good for keeping the volume below a threshold in a transparent fashion. This means that the changes in volume, though they may be pretty extreme in comparison to a compressor, with a ratio of 10:1 vs. 4:1 or something like that, are not likely to be heard unless the threshold is turned way down. Very good limiters can be pushed really hard and still maintain the integrity of the signal. Remember what we said happens when a signal runs too hot. The waves get clipped! This is not the case in these particular limiters because they use compression after the threshold and the result is a pumping up and down of volume instead. The other class of limiter is a brick wall limiter. It’s effect on the signal is very fast and strict when it comes to not allowing the signal to pass above the threshold mark. If pushed very hard, however, the signal will eventually become too loud for the limiting to be effective and the signal will get through. But before it does, it will get clipped very badly. The audio runs out of enough headroom for the amplitude of the wave and then it gets cut off. The sound of a brick wall limiter being pushed like this is very bad and easily noticeable. Therefore, this type of limiter is used only in places where a slight amount of limiting is needed, and where the maximum volume needs to be accurate. The most common place would be on the master bus, but it can also be used before the signal gets routed to another source that reacts badly to overload.
The third type of dynamics processor is an expander. The expander has precisely the opposite function as the compressor. While a compressor turns everything down after it rises above the threshold, an expander turns everything down which is below the threshold. This means that if a signal isn’t loud enough, it gets turned down. The most common form of expander and the only one relevant to this discussion is a noise gate. Putting a noise gate on a microphone helps to ensure that unwanted sounds are not heard along with the source sound. The controls on an expander are usually the same as those on a compressor.
NOTE: A NOISE SUPPRESSOR IS SIMILAR TO A NOISE GATE BUT IS EXTREMELY COMPLEX IN ITS DESIGN WITH AN ABILITY TO USE A FREQUENCY ANALYSIS AS ITS GUIDE WHEN DETERMINING WHAT CONTENT WILL BE AFFECTED. SOME DYNAMICS PROCESSORS ALLOW SIDECHAINING WHICH GIVES THE ENGINEER THE ABILITY TO MAKE THEM RESPOND TO SPECIFIC THINGS TOO.