Audio signals have very wide peak-to-average signal-level ratios, sometimes referred to as dynamic range, which is the difference between the loudest level and the softest level. This can create issues in both live and recording environments when gain staging a signal because when enough gain is applied to capture the softest level adequately, a peak signal can cause overload in the audio chain, resulting in signal distortion. Dynamics processing is the process of altering the dynamic range of an audio source to make it easier to place in the overall mix. Common types of dynamics processors include:
- Compressor. A compressor is a type of amplifier in which gain is dependent on the signal level passing through it. You can set the maximum level a compressor allows to pass through, thereby causing automatic gain reduction above some predetermined signal level, also known as the "threshold." Punch, apparent loudness, and presence are just three of the many terms used to describe the effects of compression.
- Limiters. Similar to a compressor, a limiter is an amplifier that limits the upper dynamic range of a signal to a specific threshold. Unlike a compressor, which works gradually to reduce the signal, the limiter prevents virtually any increase in gain at the upper end of the dynamic range.
- Expanders. Expanders increase the dynamic range of a signal after the signal crosses a threshold. There are two basic types of expansion: dynamic and downward.
- Noise Gates. A noise gate helps to reduce unwanted sounds by only allowing the signal to be heard once it has exceeded a certain amplitude.
PreSonus has put together this brief tutorial to answer both commonly asked questions and explain dynamics terminology
The Problem with Dynamic Range
Dynamic range can be defined as the ratio between the loudest possible audio level and the lowest possible level. For example, if a processor states that the maximum input level before distortion is +24 dBu, and the output noise floor is -92 dBu, then the processor has a total dynamic range of 116 dB (24 + 92 = 116 dB).
Some styles of music are much more dynamic than others. For example, the average dynamic range of an orchestral performance can range from -50 dBu to +10 dBu, on average. This equates to a 60 dB dynamic range. Although 60 dB may not appear to be a large dynamic range, do the math, and you’ll discover that +10 dBu is 1,000 times louder than -50 dBu!
Rock music, on the other hand, has a much smaller dynamic range: typically -10 dBu to +10 dBu, or 20 dB. This makes mixing the various signals of a rock performance a much more tedious task because the level differences between the instruments are much more subtle, so just using faders to balance the mix isn't going to solve the problem alone.
Compressors are commonly used for many audio applications and function by lowering the uppermost dynamic range of a signal at a set threshold. Once the signal exceeds that threshold, the compressor will engage and reduce it, by a set amount. Compressors also allow the engineer to determine how fast the gain reduction will engage and how slow it will release.
Compression reduces the amount by which a signal’s output level can increase relative to the input level. It is useful for lowering the dynamic range of an instrument or vocal, making it easier to avoid distorting the output. It also assists in the mixing process by reducing the amount of level changes needed for a particular instrument.
How severely the compressor reduces the signal, is determined by two parameters: ratio and threshold. A ratio of 2:1 or less is considered mild compression, reducing the output by a factor of two for signals that exceed the compression threshold. Ratios above 10:1 are considered hard limiting. Assuming a nominal input-signal level, as the compression threshold is lowered, more of the input signal is compressed.
So, how does compression help?
Let’s take the example of a vocal performance, which usually has a wide dynamic range. Transients as usually the loudest portions of any instrument, but these can be far outside the average level of the vocal signal. This means that the level changes continuously and dramatically. This problem is compounded by many vocalists, who move around in front of the microphone while performing, making the output level vary up and down unnaturally.
One solution to this problem is known as “riding the fader.” As its name indicates, this means that the audio engineer constantly keeps their finger on the channel fader, monitoring the level to ensure that it is never too quiet or too loud. While this solution may work if you have a very attentive audio engineer who's concerned with only one or two channels, it is not a good solution in a complex mix. This is where a compressor comes into the mix.
A compressor automatically controls gain without altering the subtleties of the performance, correcting this problem by reducing the louder passages enough to be compatible with the overall performance.
Let’s say that you are mixing a rock performance with an average dynamic range of 20 dB (-10 dBu to +10 dBu). You wish to add an uncompressed vocal to the mix. The average dynamic range of an uncompressed vocal is around 40 dB, ranging from as soft as -30 dBu up to as loud as +10 dBu. The passages that are +0 dBu and higher will be heard over the mix pretty clearly. However, the passages that are at -20 dBu and below will never be heard over the roar of the rest of the mix. A compressor can be used in this situation to reduce (compress) the dynamic range of the vocal so that it is approximately 10 dB (from 0 dBu to +10 dBu). At this dynamic range, the quieter aspects of the performance are still well above the lower level of the mix, and louder phrases will not overpower the mix, allowing the vocal to “sit in the track.”
The same points can be made about any instrument in the mix. Each instrument has its place, and a good compressor can assist the engineer in creating the overall blend. For example, a kick drum can get lost in a wall of electric guitars. No matter how much the level is increased, the kick drum stays lost in the “mud.” A touch of compression can tighten up that kick-drum sound, allowing it to punch through without cranking the level way up. A solo guitar can seem to be masked by the rhythm guitars. Compression can make your lead soar above the track without shoving the fader through the roof. Bass guitar can be difficult to record. A consistent level with good attack can be achieved with proper compression, giving the bass the punch it needs to drive the bottom of the mix.
Because of this, most instruments need some form of compression, often very subtle, to be properly heard in a mix. Of course, if an instrument sits perfectly in a mix without compression, then don’t mess with it. But most of the time you’ll need it.
Overcompression: When Too Much is Just Too Much
A well-designed and properly adjusted compressor should not be audible —unless, of course, it is done intentionally for effect, but that’s another article.
Too much compression can destroy the acoustic dynamic response of a performance. Furthermore, some people use a lot of compression to make their mix as loud as possible, emulating what they hear on the radio, but the top professionals usually get that effect by careful compression during mastering, not during tracking and mixing.
If you use a lot of compression in your tracks and mix, you might reduce the dynamic range to the point that you haven’t left the mastering engineer enough flexibility. So use compression often but don’t go over the top unless you are doing it as an effect. You still want your mix to "breathe." If it feels like it ramps up to one level and stays there without moving, you've probably squashed your signals too much.
Sidechaining the Compressor
Sidechaining a compressor allows you to reduce the level of one input source to make room for another. This is especially useful in live broadcast or applications where music and commentary are happening simultaneously. In recording applications, this is generally accomplished with careful level automation. Compressor sidechaining allows you to achieve a similar result without having to ride the faders. Because of this, both StudioLive® Series III mixers and Studio One® provide sidechaining.
Below are two of the most common uses:
- Dialogue Ducking. This is probably the most well-known compressor sidechaining application. Consider a common House-of-Worship situation where the pastor is leading a meditation while the piano or pre-recorded music is being played. Apply a compressor on the stereo music track with a fairly low threshold, high ratio, a fast attack and a long release time, using the pastor’s microphone channel as the key source for the compressor. Now the compressor will react to level fluctuations from the pastor’s microphone, allowing the music to naturally rise and fall in counterpoint to the speech pattern.
- Bringing Out the Kick Drum. In some instances, a synth bass line, or a busy bass guitar part interferes with the kick drum presence and punch in the mix. For this application, you will apply a compressor to the bass channel, using the kick drum channel as the key source, allowing your kick drum to punch through the bass line.
- Attack. Sets the speed at which the compressor acts on the input signal. A slow attack time allows the beginning component of a signal (commonly referred to as the initial transient) to pass through, uncompressed, whereas a fast attack time triggers compression immediately when a signal exceeds the threshold.
- Auto mode. Fixes the attack and release times to a preprogrammed curve. Every StudioLive digital mixer offers this feature, helping to simplify compression for novice and exasperated professional users alike.
- Compression ratio. The ratio sets the compression slope, which is a function of the output level versus the input level. For example, if you have the ratio set to 2:1, any signal levels above the threshold setting will be compressed at a ratio of 2:1. This means that for every 2 dB of level increase above the threshold, the compressor’s output will only increase 1 dB.
- Hard/soft knee. With hard-knee compression, the gain reduction applied to the signal occurs as soon as the signal exceeds the level set by the threshold. With soft-knee compression, the onset of gain reduction occurs gradually after the signal has exceeded the threshold, producing a more musical response (to some folks).
- Look-ahead compression. Some digital compressors can analyze what it is about to process (“look ahead”) and can place the attack time right at the onset of—or even before—the sound, resulting in zero attack time. This is great for catching unwanted transients, but it must be used with care so as not to remove desirable transients, such as the attack of a snare drum. Because of the amount of processing power necessary for this analysis, look-ahead compressors are most commonly found as plug-ins such as the PreSonus Channel Strip, Compressor, Multiband Dynamics, and Tricomp plug-ins for Studio One.
- Make-up Gain. When compressing a signal, gain reduction usually results in an overall attenuation of level. The gain control allows you to restore this loss in level and readjust the volume to the precompression level (if desired).
- Release. Sets the length of time the compressor takes to return the gain reduction back to zero (no gain reduction) after crossing below the compression threshold. Very short release times can produce a choppy or “jittery” sound, especially when compressing instruments that have a lot of low-frequency components, such as a bass guitar. Very long release times can result in an overcompressed, or “squashed,” sound. All ranges of release can be useful, however, and you should experiment to become familiar with different sonic possibilities
- Threshold. When the signal’s amplitude (level) exceeds the threshold setting, the compressor engages. As the threshold is lowered, compression begins at a lower amplitude (volume) and further limits the dynamic range of the signal.
Compression Settings: Starting Points
The following are the compression presets that were used in the popular but discontinued PreSonus BlueMax. We have included them as a jumping-off point to get you started.
Soft. This is an easy compression with a low ratio setting for ballads, allowing a wider dynamic range. It’s good for live use. This setting helps the vocal "sit in the track.”
|-8.2 dB||1.8:1||0.002 ms||38 ms|
Medium. This setting has more limiting than the Soft compression setting, producing a narrower dynamic range. It moves the vocal more up front in the mix.
|-3.3 dB||2.8:1||0.002 ms||38 ms|
Screamer. This setting is for loud vocals. It is a fairly hard compression setting for a vocalist who is on and off the microphone a lot. It puts the voice “in your face.”
|-1.1 dB||3.8:1||0.002 ms||38 ms|
Snare/Kick. This setting allows the first transient through and compresses the rest of the signal, giving a hard “snap” up front and a longer release.
|-2.1 dB||3.5:1||78 ms||300 ms|
Left/Right (Stereo) Overheads. The low ratio and threshold in this setting gives a “fat” contour to even out the sound from overhead drum mics. Low end is increased, and the overall sound is more present and less ambient. You get more “boom” and less “room.”
|-13.7 dB||1.3:1||27 ms||128 ms|
Electric Bass. The fast attack and slow release in this setting will tighten up the electric bass and give you control for a more consistent level.
|-4.4 dB||2.6:1||45.7 ms||189 ms|
Acoustic Guitar. This setting accentuates the attack of the acoustic guitar and helps maintain an even signal level, keeping the acoustic guitar from disappearing in the track.
|-6.3 dB||3.4:1||1262 ms||400 ms|
Electric Guitar. This is a setting for “crunch” electric rhythm guitar. A slow attack helps to get the electric rhythm guitar “up close and personal” and gives punch to your crunch
|-0.1 dB||2.4:1||26 ms||1262 ms|
Piano.This is a special setting for an even level across the keyboard. It is designed to help even up the top and bottom of an acoustic piano. In other words, it helps the left hand to be heard along with the right hand.
|-10.8 dB||1.9:1||108 ms||112 ms|
Synth. The fast attack and release on this setting can be used for synthesizer horn stabs or for bass lines played on a synthesizer.
|-11.9 dB||1.8:1||0.002 ms||85 ms|
Orchestral. Use this setting for string pads and other types of synthesized orchestra parts. It will decrease the overall dynamic range for easier placement in the mix.
|3.3 dB||2.5:1||1.8 ms||50 ms|
Stereo Limiter. Just as the name implies, this is a hard limiter, or “brickwall,” setting—ideal for controlling the level to a two-track mixdown deck or stereo output.
|5.5 dB||7.1:1||0.001 ms||98 ms|
Contour. This setting fattens up the main mix.
|-13.4 dB||1.2:1||0.002 ms||182 ms|
Squeeze. This is dynamic compression for solo work, especially electric guitar. It gives you that glassy “Tele/Strat” sound. It is a true classic.
|-4.6 dB||2.4:1||7.2 ms||262 ms|
Pump. This is a setting for making the compressor “pump” in a desirable way. This effect is good for snare drums to increase the length of the transient by bringing the signal up after the initial spike.
|0 dB||1.9:1||1 ms||0.001 ms|
At the simplest level, a limiter is a compressor that is set to prevent any increase in the level of a signal above the threshold. For example, if you have the threshold knob set at 0 dB, and the ratio turned fully clockwise, the compressor becomes a limiter at 0 dB, so that the output signal cannot exceed 0 dB regardless of the level of the input signal. Typically, compression ratios of 10:1 are considered to be limiting.
A true analog peak limiter is not just a compressor with a high ratio. A compressor’s detector circuit is usually designed to detect RMS, or average, levels, so transient peaks will usually overshoot a compressor’s threshold level. A true peak limiter employs a detector circuit that responds to peak energy levels and thus reacts faster.
Noise gating is the process of removing unwanted sounds from a signal by attenuating all signals below a set threshold. As described, the gate works independently of the audio signal after being "triggered" by the signal crossing the gate threshold. The gate will remain open as long as the signal is above the threshold. How fast the gate opens to let the "good" signal through is determined by the attack time. How long the gate stays open after the signal has gone below the threshold is determined by the hold time. How fast the gate closes is determined by the release. How much the gate attenuates the unwanted signal while closed is determined by the range.
Noise gates were originally designed to help eliminate extraneous noise and unwanted artifacts from a recording, such as hiss, rumble, or transients from other instruments in the room. Since hiss and noise are not as loud as the instrument being recorded, a properly set gate will only allow the intended sound to pass through; the volume of everything else is lowered. Not only will this strip away unwanted artifacts, it will also add definition and clarity to the desired sound. This is a very popular application for noise gates, especially with percussion instruments, as it will add punch or “tighten” the percussive sound and make it more pronounced.
Consider the compressed-vocal example we gave earlier; you now have a 20 dB dynamic range for the vocal channel. Problems arise when noise or instruments (air conditioner, loud drummer, etc.) in the background of the vocal mic become more audible after the lower end of the dynamic range is raised. You might attempt to mute the vocal between phrases in an attempt to remove the unwanted sounds; however, this would probably end disastrously. A better method is to use a noise gate. The noise gate threshold could be set at the bottom of the dynamic range of the vocal, say -10 dBu, such that the gate would shut out the unwanted signals between the phrases.
Noise gates are also commonly used on on drumkits. If you have ever mixed live sound, you know the problems cymbals can create by bleeding through the tom mics. As soon as you add some highs to get some snap out of the tom, the cymbals come crashing through, sending the horn drivers on your loudspeakers into a small orbit. Gating those tom mics so that the cymbals no longer ring through them will give you an enormous boost in cleaning up the overall mix.
Sidechaining and Key Filtering with a Gate
Some Noise Gates, like those on the StudioLive digital mixers, allow you to set the frequency at which the gate will open. Setting a specific frequency, in addition to a specific decibel level, provides more sonic shaping. A properly set key filter on a gate can greatly improve the overall sound quality of a mix.
For example, if you are inserting a gate on a snare-drum mic, you may get enough bleed from the kick drum to open the gate. This is where a key filter can come in handy. By setting the key filter to the frequency at which the snare drum is resonating, you can set the gate to only open only when the snare is struck. This will filter out all the extraneous noise and give your snare more presence to punch through the mix with more impact.
The key filter on StudioLive mixers provides further control by allowing you to triggered it by the selected channel or bus’s signal or by sidechaining it to another channel and using its signal as the source. This allows you to select a different channel as the trigger source for your StudioLive Gate’s Key Filter. Sidechaining has many uses, two of the most common are:
- Tighten up a Rhythm Section. You can use a sidechained key filter to tighten up a rhythm section by sidechaining the kick drum channel to the bass channel and setting the gate to open at the frequency of the kick drum. This, combined with a fast attack and release, will make your rhythm section more cohesive. Increase the release time to loosen the feel. Please note, that while sidechaining the kick drum to the bass channel can tighten up a good rhythm section and make them sound even better, it will not correct timing issues and will actually exaggerate them if your bass player and drummer aren’t in the pocket.
- Rhythmic Effect. Another great use for a sidechain is as an effect in EDM production. Try sidechaining a drum loop to a sustained source, like pads or strings. By doing this, every time a drum hit triggers the key filter, your sustained source will be heard. Between hits, this source will be silenced. Playing with the attack and release will transform this effect from a rhythmic pulse all the way to a chopped-up stutter.
Noise Gate Components
- Attack. The gate attack time sets the rate at which the gate opens. A fast attack rate is crucial for percussive instruments, whereas signals such as vocals and bass guitar require a slower attack. Too fast of an attack can, on these slow-rising signals, cause an artifact in the signal, which is heard as a click. All gates have the ability to click when opening but a properly set gate will never click.
- Hold. Hold time is used to keep the gate open for a fixed period after the signal drops below the gate threshold. This can be very useful for effects such as gated snare, where the gate remains open after the snare hit for the duration of the hold time, then abruptly closes.
- Key Filter. The Key Filter is a filter with a variable frequency that allows you to remove problem frequencies from the gate’s trigger signal. Let’s say you want to gate the snare drum so that the gate opens every time the drummer hits the snare, letting the snare through, and then closes again. That can be used as an effect or simply so that the noise leaking into the snare mic won’t muddy up the overall sound. To make the gate open, you use a copy of the dry signal from the snare mic to trigger the gate, allowing the snare sound to pass. The problem is, other instruments – notably cymbals – often leak into the snare mic, and they could accidentally trigger the gate. The key filter enables you to filter out the high frequencies in the cymbals so that only the snare signal remains to trigger the gate.
- Key Listen. Key Listen lets you audition the gate’s trigger signal so you can adjust the Key Filter’s frequency control until the problem frequencies are eliminated.
- Range. The gate range is the amount of gain reduction that the gate produces. Therefore, if the range is set at 0 dB, there will be no change in the signal as it crosses the threshold. If the range is set to -60 dB, the signal will be gated (reduced) by 60 dB.
- Release. The gate-release time determines the rate at which the gate closes. Release times should typically be set so that the natural decay of the instrument or vocal being gated is not affected. Shorter release times help to clean up the noise in a signal but may cause “chattering” in percussive instruments. Longer release times usually eliminate “chattering” and should be set by listening carefully for the most natural release of the signal.
- Threshold. The gate threshold sets the level at which the gate opens. Essentially, all signals above the threshold setting are passed through unaffected, whereas signals below the threshold setting are reduced in level by the amount set by the range control. If the threshold is set fully counterclockwise, the gate is turned off (always open), allowing all signals to pass through unaffected.
The major difference between expansion and noise gating is that expansion is dependent on the signal level after the level crosses the threshold, whereas a true noise gate works independently of a signal’s level beyond the threshold.
In practice, expanders and noise gates are used almost identically. The main difference is that an expander is smoother and more gradual, so that it is easier to set the attack and release times correctly.
Expanders can be divided into two categories: Dynamic and Downward.
- Downward Expansion. Downward expansion is the most common expansion used in live sound and recording. In contrast to compression, which decreases the level of a signal after it rises above the compression threshold, expansion decreases the level of a signal after the signal goes below the expansion threshold. The amount of level reduction is determined by the expansion ratio. For example, a 2:1 expansion ratio reduces the level of a signal by a factor of two. (e.g., if a level drops 5 dB below the expansion threshold, the expander will reduce it to 10 dB below the threshold.)
Commonly used for noise reduction, expansion is very effective as a simple noise gate. The major difference between expansion and noise gating is that expansion is dependent on the signal level after the level crosses the threshold, whereas a true noise gate works independently of a signal’s level beyond the threshold.
This type of expansion reduces the level of a signal when the signal falls below a set threshold level. This is most common used for noise reduction.
- Dynamic expansion. This is basically the opposite of compression. In fact, broadcasters use dynamic expansion to “undo” compression before transmitting the audio signal. This is commonly referred to as companding or COMPression followed by expANDING.
Because an expander is very similar to a noise gate, you will find that it has most, if not all of the same controls, functioning much the same as they do on a noise gate. The exception is the Range control, this is a control you will find on a Noise Gate only.
Expansion ratio. The expansion ratio sets the amount of reduction applied to a signal once the signal has dropped below the expansion threshold. For example, a 2:1 expansion ratio attenuates a signal 2 dB for every 1 dB it drops below the threshold. Ratios of 4:1 and higher act much like a noise gate but without the ability to tailor the attack, hold, and release times.