Why do we need dynamics control?

Limiters, compressors, expanders and gates are all devices which control the level of audio signals and therefore the dynamic range of a track or piece of music. They affect the loudness of a sound just as a fader or volume control does. In a dynamics device, however, the device according to parameters set up by the engineer controls the level automatically. In a traditional analogue device this would have been an electrical circuit element that would first sense the level of the signal and then adjust the output as required. Analogue compressors are still regularly used at the input stage, during recording, but in digital systems most dynamics control is now achieved through plug-ins which emulate the analogue world in terms of both performance and parameters. Below is a diagram to show how a traditional analogue dynamics unit would work.

dynamic control

As we noted earlier on, the useful dynamic range of the human ear is from OdB SPL (the threshold of hearing) to 115dB SPL (the threshold of pain). Digital sound equipment is now able to cover this dynamic range with 24bit recording, which has a range of 144dBs (theoretically anyway as the real limit of even the best Digital to Analogue converter is 122dB SPL of dynamic range). CD audio is a 16-bit system and has a dynamic range of 96dBs, which is more than adequate for most music and speech recording. Classical music is most renowned for having very quiet and very loud passages — a wide dynamic range. Even if reproduced in a Hi-Fi environment this would equate to about 65dBs difference between the loudest and quietest parts. Most pop and rock formats area designed to have a much smaller dynamic range since they need to be intelligible in a wide variety of situations – such as cars, factories, kitchens, railway stations etc – where ambient noise might be considerable. Under such conditions the signal must be compressed so that the quietest level is audible above the background noise without the loudness of reproduction being unacceptable for the given situation. Background music, for example, must be reproduced quietly yet should at all times be audible enough to be intelligible. The key to a successful commercial mix lies in an ability to engineer loudness and impact into a mix that is heard on a transistor radio. The limiter/compressor is a tool that makes this possible.

 

 

Common applications for automatic gain control

Protecting a system from being overloaded
In order to optimise the signal-to-noise ratio and maintain a high average record level without the fear of accidental overload, a limiter can be inserted to operate just prior to the onset of significant distortion

Increasing loudness
Changing the sound by making it either “denser (by reducing the dynamic range) or lighter (by increasing it).

Noise reducing
Reducing background noise, “spill”, cross-talk and improving the signal-to-noise ratio of analogue tape recorders and other noisy signal paths by the use of elaborate systems such as the Dolby ones.

Reducing sibilance
Extreme “s” and “t” in vocals and other similar problems with the use of de-essers.

Special effects
Voice-over and ducking (as used by disc-jockeys), dynamic equalisers such as the Opti-Mod for increasing loudness in radio broadcasts and a myriad of other inventive applications that we shall describe later.

Cover photo by Pedronchi

Compress Drums Quietly

Compressor is a very crucial part of a DAW and is a super helpful tool which enables you to compress any instrument. Drums could be compressed by mixing them and by turning down the speaker volume. Making the compression decisions based on mixing at low volume levels will make the mix efficient. So the main idea is while compressing the drums if the drum sound could be made punchy and pumping at low volume levels it is going to be super easy for them to sound interesting at that normal listening level.

Mastering EQ – make music sound louder, with LESS compression and limiting

Learn about Mastering EQ with Ian Shepherd. EQ is a great tool in a DAW which makes the music sound louder with less compression and limiting. While working on songs with heavy drums with loud snares there is a possibility of crushing the track sound and it is important to keep the songs at proper level. This problem is solved by Mastering EQ.

Compression and limiting — practical applications

dynamic compressor pultec

As we have seen the main job of a compressor is to reduce the dynamic range of the audio material it is applied to. Peaks are reducedand quiet sounds are more audible. The main effect is to give a more stable overall levelbut an added density or thickness to the sound and enhanced character or ‘warmth” can be welcome side effects. There can be trade-offs though. Over-compression can increase unwanted noise that is not part of the musical signal such as breaths, rustling paper and fret noise or it can increase room ambience. If you have recorded in a great sounding room this is not necessarily a problem but if you’re recording in a home environment you’ll probably want as little of the room sound as possible so as to add a nice convolution reverb later. Badly applied compression can also introduce audible ‘pumping’ which is not usually desirable unless you want it for a special effect.

On the plus side again though you can affect the amplitude envelope of a sound by adjusting the attack and release times of a compressor. This means you can enhance (or for that matter, reduce) the front end crack of a snare or the click part of a bass drum, or you could bring out the sustain part of notes on acoustic, or clean electric guitar. If your compressor is quite transparent and you want to hear more effect as a general rule use a faster attack, a shorter release and a higher ratio. If you’re after a smooth compression sound use a soft knee approach but if you need tighter control and a more obvious effect go for the hard-knee option.

 

Drums

The impact of drums can be changed dramatically through the use of carefully applied compression. By nature drums have a large transient peak followed by a quieter “body” sound. Compressing the transients will make the kit seem louder but it is easy to squash the life out of it too by eliminating the crack in the sound completely! A slowish attack time will let though this initial crack so that you retain some life in the sound (possibly peak Limiting will be needed to prevent overload). The decay of a drum will be altered by the release control: with a slow release you will get a bit less of the “body” part of each drum sound since the gain will still be at a reduced level. A fast release will seem to extend the sustain, the natural decay being in effect faded up. A possible use for a slow release is to subdue an over bright and rattly snare. On the other hand if you have a snare that needs to be fattened up a bit and perhaps you’d like to hear more of those rattly snares then a fast release will bring up those sounds (and the noise too unfortunately) after the initial compression of the snare’s attack. Compressing the drum overheads will tend to make the room character more noticeable. If your recording environment did not flatter the sound then it might be better to use less compression on the overheads and use a good room ambience on the close-miced dry kit sounds. If you have made a stereo audio subgroup for your drum kit it often makes sense to apply some overall compression (or even limiting) at this stage to help gel the sounds together and present the kit as a coherent instrument in its own right rather than as a set of individually played disparate elements.

Bass

This is one area where you can be quite aggressive with your compression, as bass tends to fluctuate in level by considerable amounts. It is important for your whole mix for the energy levels in the low frequencies to be as smoothly controlled as possible especially if you’re using a mix compressor and you want to avoid pumping artifacts on the mix bus. Bass compression will fatten up the tone and prevent more quietly played notes from getting lost. High ratios of up to 10:1 are not uncommon for bass depending on the playing style and quality of the performance. You can also adjust the attack and the release times, of course, to enhance or reduce front-end click on the bass as desirable.

Guitars

Acoustic and clean electric guitars also tend to benefit from compression. They often sound smoother and fatter and, if the release is fairly short with a high ratio, it will bring out the sustain, If you want to enhance the initial transient, just as for drums, you can use a slower attack time to let it through. For slighty distorted guitars, compression can help to increase sustain. With a heavily distorted sound there is unlikely to be much dynamic range anyway since the process of overdriving an amp or using a distortion pedal limits the output in itself. So there’s not a lot of point in using compression here. It is more likely to bring up the level of unwanted noise or hiss during any pauses.

Vocals

Compression is widely used on vocals in rock and pop mixes. Rock and pop backing tracks tend to be quite powerful with up front drums, guitars and bass. The voice is a comparatively subtle instrument prone to large changes in dynamics as a result of, the singer’s expressiveness. Therefore, in order for the voice to sit comfortably in this kind of mix, the engineer will generally use some compression to bring down the peaks, even out the sound and thereby raise the average signal level. This results in a tighter, punchier and more confident sound for the vocalist. Compression can also help with unwanted level changes due to poor microphone technique.

As always the settings you choose will depend on the vocal performance and musical style, as a rule of thumb try adjusting the threshold so that the quiet parts are not compressed. This should help to avoid completely flattening all the expression in a performance through over compression. Ratio’s of between 1.5:1 and 6:1 are usually sufficient with attack times of around 1ms to 10ms and a releases of around 80 to 150ms as starting points. These figures are just a rough guide — you can always try out some of the presets that come with your software compressor and adjust from there. Try to work out why the programmers have chosen particular settings for a particular vocal style. Remember your ears are always the best judge.

Remember too that different parts of the song may have elicited different vocal styles from the performer. So you may need to use different settings, or indeed different compressors, in the chorus as compared to the verse. This can be done through automation or by chopping up the audio file and putting the verse vocal on one track and the chorus vocal on another.

If you have a selection of compressors, either internal, outboard or both it’s a good idea to try them out on vocals, as this is where differences between models can come to light. For example the UREI1176 – which is widely available in software form – tends to be quite transparent — evening out the levels without making the process obvious even with quite heavy compression- whereas certain units, such as the Joe Meek ones, deliberately introduce ‘character’ to the sound. Neither one is best. Different material will require different approaches – it’s a matter of personal taste. Ideally it’s good to have both ‘character’ and ‘transparent’ styles of compressor in your armoury.

As we noted earlier backing vocals are often grouped together for level control and processing in a stereo audio subgroup. Very often compression, EQ, and possibly limiting are applied at this stage. If computer-processing power is an issue then grouping multi-tracked BVs and adding compression at this stage can be a valuable way to save resources. Even if you are