Probably the most recognizable aspect of music is dynamics, the changes in amplitude that we hear as changes in loudness, subjective, perceived changes, and in volume, measured, objective changes. Manipulating dynamics is one of the most versatile and important tools the producer uses in post production to shape the sound both of individual instruments and the mix as a whole.
Dynamic processors change certain parameters of dynamics, under certain rules, to alter volume, at any gain stage point in the signal path, for any recorded track or combination of tracks. Perhaps the simplest dynamic processor is the producer himself. The producer can change the volume of the sound on a track through application of amplitude automation. He can even physically change levels on the fly, such as riding faders to balance the dynamics of a track. The human dynamic processor uses a two stage process to change dynamics in real time. First, they analyze the dynamics of the track and determine where the volume should be raised or lowered. Second, they manipulate the faders based on that analysis. This two step process carries over to dynamic processing done by outboard gear and software plugins.
All hardware and software dynamic processors have a side chain or key section, which does the analysis of the input signal and a volume fader section, which changes the volume over time. This is because all dynamic processors are acting as some type of volume control. And because they all work on the same thing, they all use the same parameters and the same means of changing those parameters. The four essential parameters of dynamic processing are, threshold, ratio, attack, and release.
Threshold represents the amplitude level at which the processor begins working. Changing the threshold can influence the sound a great deal, as the different settings vary the results of using the same processor. Ratio is the amount of processing applied to the signal once it is triggered by the volume reaching and surpassing the threshold. It is expressed as the ratio of input to output. A ratio represents how much the input changes as it passes the threshold. For example a 4:1 ratio means that for every four decibels of input that is processed, the signal will change 1 decibel. Attack is how fast the processor begins to work. Changing this parameter influences how the beginning of a signal sounds. Instruments can often begin with a transient, or a rapid change in amplitude. A snare drum is a good example of an instrument that makes a lot of transients. Changing the attack changes how much of the transient we can hear and influences how smooth or punchy the sound is. Release is the opposite of attack. It changes how long the end of the processed sound lasts before it is “released”. This will influence how choppy or slowly the processed sound finishes.
The four major types of dynamic processors are, compressors, expanders, noise gates and limiters. All four of these processors act on the same four parameters, changing those parameters to some degree, and operating under different rules. A compressor reduces the dynamic range by either reducing the loud sounds or by raising the soft sounds. The rule for compression is as the input gets louder the output gets softer. This effect shrinks the dynamic range so that the loud parts are softer and the soft parts louder. This allows the producer to raise the gain of the entire track so that it is louder in the mix without distorting. There are many different applications of compressors to different instruments, to entire tracks and for different effects. The manipulations of the four parameters are interactive, thus creating an almost limitless number of possible effects. This vast opportunity for change makes the compressor widely used but difficult to master. Even the best producers continue to learn more about compression over time.
Expanders are the opposite of compressors in that they increase the dynamic range, either by making the loud parts louder or the soft parts softer. Expansion is not used as much as compression but can be very useful in a situation such as mixing a heavily compressed recorded track to give back some of the differences in the volume of the original track. For example, an orchestra recorded with compression to reduce the chance for distortion could have some of the dynamic range returned in the mixdown through expansion. The rule for expansion is as the input gets louder the output gets louder.
Limiters and noise gates use compression in special ways and function in somewhat opposite ways. Noise gates allow only the sounds above a certain volume to pass through the “gate” by cutting off all sound under the threshold. Gates are useful in removing unwanted sounds from the mix, such as foot tapping, finger movements , squeaks from chairs etc. Limiters allow only sounds under the threshold to be heard. They are essentially compressors that operate with ratios over 10:1. They are traditionally used to prevent loud sounds from distorting but in modern usage they act as loudness maximizers, as heavy limiting can allow the signal gain to be increased a great deal, making the whole track apparently louder, while assuring the producer the signal will not distort.
Dynamic processors have a multitude of uses in the studio in both the tracking and post production stages. Knowing the basics of how each processor works and what it does gives you a good head start to the fine art of applying dynamic processing to change the emotional and spatial presentation for the listener. You will be improving your processing skills forever, as you gain experience as a producer. Enjoy.