Of all of the audio processing effects, dynamics are some of the most important and most baffling. Part of this confusion is that it is often hard to hear dynamics effects unless they are used incorrectly. Dynamics effects adjust the volume of the sound automatically based on a set of variables that you can adjust. Because the goal of dynamics effects is to compensate for variance in volume, a normal person often won’t notice the impact of the effect, because it sounds “right.” If there were no dynamics effects (or if the effects are setup incorrectly), you will notice the volume variance, but if they are setup appropriately, everything sounds smooth (that is, “normal”).
For the layperson, “dynamics” are changing elements. Some dynamics are easy to understand, such as volume or panning.
Dynamics effects don’t directly alter the tone of the sound like an equaliser would, so their impact can sometimes be more difficult to discern. However, when used effectively, dynamics effects can bring out the best elements of a sound while reducing those elements you would rather not be heard. This makes them some of the most important audio effects a sound engineer will use, and they are vital to ensure things sound their best.
Because there are several different types of audio variances you might encounter, there are actually a number of different dynamics effects with different names and functionalities. That said, the basic theory behind the way the technology works is generally the same. Dynamics effects adjust volume based upon a volume limit (called a “threshold”) that the user sets. When the sound crosses the threshold, the effect adjusts the volume based upon the type of effect and its settings.
The Elements of Dynamics Effects
Let’s start by explaining the basic elements of dynamics effects. To make it easier to understand, I will use the example of a compressor, a dynamics effect that attenuates loud sounds down when they cross a threshold. This effect is the most common and often the model by which other effect settings are designed. Depending on the effect, these settings may go by different names and have slightly different functions. In addition, different effects may use only some of these or add other variables to them. However, generally speaking, dynamics effects use these basic elements:
This is the core setting behind any dynamics effect. It is essentially the decibel level at which the dynamics effect engages. Some effects (like compressors) attempt to regulate sounds that are above this loudness threshold, while other effects regulate sounds that go below it. Either way, the threshold is the point the volume must reach in order for the effect to begin altering the dynamics.
Dynamics effects alter the volume of sound (that is, attenuate it) once it crosses the threshold. The amount it attenuates the sound is called the ratio. The higher the ratio, the more the effect will attenuate the sound. The ratio is typically written as n:1, where n is the amount of volume change the effect will attenuate to 1 dB. In a compressor, this means for every n dB the sound goes above the threshold, the effect will attenuate it down, so it becomes a 1 dB change instead. If you have a ratio of 2:1, for every two decibels the sound goes above the threshold, the effect reduces it 1dB above the threshold. If the sound goes 6 dB over the threshold, that same 2:1 ratio will attenuate the sound to 3 dB above the threshold.
The Attack setting determines how quickly the effect will respond to a change in dynamics once the sound crosses the threshold. Measured in milliseconds, the attack determines how responsive or forgiving the dynamics effect will be by determining whether the effect should immediately “attack” every time the sound crosses the threshold, or whether it should wait to make sure it isn’t a momentary spike or dip in volume.
Again, let’s use a compressor as an example. An audio source that makes loud sounds very quickly, such as a snare drum, often requires a compressor with a faster attack to keep those loud spikes from overwhelming the overall mix. Other audio sources that sustain volume over a longer period often require a slower attack. Instruments like guitars, for example, often have a short punch of loudness at the start, and a faster attack will flatten that punch out too much and make things sound dull and lifeless.
The release setting is essentially the reverse of the attack, and it is also measured in milliseconds. When the volume returns to the “normal” state (that is, when it “uncrosses” the threshold), the release determines how long the effect will wait before it disengages. For a compressor, this determines when the effect should assume that the sound is going to remain below the threshold, and thus, it should stop attenuating the volume. When adjusting this setting on a compressor, it’s important to understand how often the sound typically dips below the threshold. If you set the release too slow, you might miss important, quieter elements of the sound, because the compressor isn’t released quickly enough. If you set the release too fast, you can end up with sound that has an uneven pumping effect, as volume is turned up and down too quickly.
Types of Dynamics Effects
Now that you have a basic understanding of the different elements you might find in a dynamics effect, let’s look at some of the different dynamics effects you might encounter and see how they work.
The operation we’ve been describing so far has been based on the concept of a compressor, the dynamics effect that prevents the volume of a source from getting too loud. Compressors are probably the most common dynamics effect you will encounter, as almost all live audio sources require them. With the ability to adjust threshold, ratio, attack and release, compressor effects can be adapted to manage nearly any source. Another setting you might encounter on a compressor is a knee adjustment, which turns the threshold into a range that softens the attack. Compressors also often offer a post-gain (or a make-up gain), which adds gain to the signal that is coming out of the compressor to make up the gain that was taken out by the compressor. This allows more of the sound to be heard at a consistent, louder level.
A limiter is essentially a compressor with a ratio of infinity-to-one (∞:1). The limiter has a hard ceiling, and any sound that goes above that ceiling gets turned down, no matter how loud it is. This effect is designed to catch peaks of sound to prevent overdriving or damage to speakers. The most common application of a limiter is on the master output of a mixed signal (called a master limiter). They are also often used on playback devices. It is important to be careful with limiters, because if misapplied, they can quickly take all the life out of the sound by eliminating dynamics altogether.
You could think of a gate as a limiter in reverse, though that isn’t quite correct. A gate reduces the level of signals that fall below the threshold. In other words, the goal of a gate is to turn quiet signals down, and they are typically applied to sources that have buzzing or some other signal noise. A common application is for electric guitars using distortion. Guitar amplifiers generally have some amount of noise floor that is noticeable if not properly managed. When the instrument is playing, the sound is buried in the mix, but when the instrument stops playing, the buzzing cuts through. Sound engineers will set the threshold above the noise floor to manage the sound. One unique setting you will find in a gate is the range. This adjusts how deeply the gate will reduce below-threshold signals. Even with the range adjustment, the gate still has a ratio of infinity-to-one, meaning it still has a flat reduction of sound (just as the limiter has a hard ceiling). However, instead of turning down the sound to negative infinity (silent), you can set the gate to turn the sound to some quieter level.
Where a gate is the sort of reverse to a limiter, an expander is the sort of reverse to a compressor, as it reduces the level of signals falling below the threshold by a certain ratio. The most common use for an expander is on sources with a high signal-to-noise ratio. Because the sound reduction happens over time rather than all at once (as with a gate), using an expander on these sources can make the noise reduction less noticeable.
A ducker is designed to automatically duck (or turn down) the sound on a source when a different source gets louder. It is found in applications where there isn’t an operator to mix the sound. The effect works similarly to a compressor, but rather than placing the threshold on the source itself, it instead places a threshold on some other source. We’ll use the example of having a microphone and background music. If we place the ducking effect on the background music, it will listen to the microphone input. When the microphone gets louder (that is, the sound on the mic crosses the threshold), the background music will turn down. This allows the speaker to be heard. Like a gate, the ducker has a range setting. It also has a range control that works similarly to an attack, adjusting the time it takes for the ducker to fade the music down when the mic signal levels goes above threshold.
A leveller is a dynamics effect found mostly on digital signal processors, like the BSS Soundweb London Series. This effect works by using a Target Output, the setting that determines the level the leveller tries to maintain. The effect attempts to keep the nominal level of the signal close to the desired target output by gently changing the gain. The more positive the value, the higher the output level will be. The effect also has a threshold, which determines the lowest input level that the leveller will attempt to correct. Correct setting of this control is critical to the leveller’s ability to discriminate unwanted noise from the wanted signal. Lower values allow the leveller to pull-up lower level signals, but will also make it more prone to activate on noise signals. This effect prevents sound variance, and like the ducker, it is common in applications where there isn’t an operator to adjust the signal.
As you can see, there is a wide range of different dynamics effects that vary in their operation, but still use many of the same principles. By learning the basics of how these are similar—and how they are different—you can be more confident as you use them in different applications.
View the range of processors and equalisers here.
First published in the HARMAN Insights Blog.
Written by S. Kyle Davis. Kyle has been involved in AV in one way or another for 20 years, working as an audio engineer in various capacities, as well as a university AV tech, camera operator, and live video producer. Kyle is an integrated AV solutions expert, and is an avid fan of technology in all its forms.