The Music Espionage

Noise Gates


Basic operation:
A gate’s basic function is to remove unwanted noise between the sections of wanted material. A Noise Gate will keep a signal turned off until it reaches a certain threshold, after this allowing the sound through. So the Gate can be set to completely turn-off the sound when it drops below the threshold or simply lower it by a certain amount. At the most basic, a gate acts as an automatic switch, separating the louder sounds from the quieter ones. You can set a threshold, so above that sound is kept, and below that it is rejected. Is fed into the noise gate’s channel input and the gate will remain closed, effectively silencing the sound until it rises above the threshold and opens the gate.

It is important remember that Gates DO NOT remove noise from the signal. When the level is above the threshold both the main signal and any noise that you are trying to reduce with both be heard.

 

So when do you use one?
In live sound applications gates are used primarily on drums to ensure each individual drum’s microphone picks up sound from that drum only. In recording applications, using a gate while actually recording can be dangerous as you could accidentally gate out part of the performance. Therefore in studios, gates are more likely used at in post-production, in the mix down.

A typical example of a gate’s more conventional role is to clean up an individual track of a multitrack recording. If a recording of a guitar has amplifier hiss plus low-level bleeding from other instruments, the gate could be used to shut down the track when the guitar is not playing so the unwanted sound will not clutter up the mix.

On drums, gates can be employed to reduce leakage from other elements of the kit. For example the bleeding from cymbals or Toms may be cluttering a kick track. Setting the threshold would reduce this background noise and only allow the gate to open when transient rises above this level.

The example below of noise-gating, shows how a instrument’s channel can be cleaned through the careful use of gating. By applying this effect and also some carefully selected EQ a great deal of the unwanted background noise can be removed. However the trick is not make these effects to noticeable and so they sit nicely in the backdrop.

Parameters:
So what are the main controllers you will be using when adjusting and setting your gate?

Threshold:
This is the main element of the Gate. With this fader you are setting at what point you would like the gate to open and let those amazing sound shine through. So the lower the level it means more of the signal will travel through. If you have this set to high, only the loudest of transients will jump through, often giving a strange popping effect.

If a gate could only be on or off, with no transition in between, decaying sounds would be cut off abruptly as soon as they fell below the threshold level. Because of this, most gates have the facility to make the opening and closing of the gate more gradual. How quickly the gate opens and closes and how long it remains open is set by its envelope controls, these deal with the shape of the sound over time. Is it s sudden quick sound, or does it time to reach its loudest before having a long decay time until its not heard?

Attack:
The attack control sets how quickly the gate opens after the threshold has been reached. In the majority of cases a fast attack time will be the norm, but the attack control can also be used as a creative tool; by setting a slow time sounds will swell in slowly for a ‘bowed’ effect. Also if a gate is set to open very quickly and the signal being processed has a slow attack, the abrupt opening of the gate, when the input signal exceeds the threshold level, can cause an audible click. At its fastest, an attack of microseconds is typical, whereas sounds with a slower attack may respond better to gating with an attack time of 10mS or more.

Release:
This controls the signal after it has dropped below the threshold. This is often a very important parameter to make sure the gate’s effect is not too noticeable and obvious within the mix. Many natural sounds start fairly abruptly and the majority have a well- defined decay characteristic, so matching the release time of the gate to that of the sound being processed is extremely important if you wish the fade into silence to be made to sound smooth and natural, not just abruptly jump.

Hold & Hysteresis:
Between both the Attack and Release is often a parameter called Hold. Basically, the hold control keeps the gate open for a preset time after the signal has passed the threshold. If a gate is set with a fast attack time and a fast release time, any signal level which hovers around the threshold, just jumping above and below the threshold, can cause problems known as ‘chattering’, where the gate opens and closes rapidly several times in succession. One way in which manufacturers have addressed this problem is by adding a hold-time control. This allows the user to specify the minimum time for which the gate will remain open once the signal level has exceeded the threshold. When the signal falls below the threshold, the gate is held open for the duration of the hold time before starting to close.

The second method used to avoid chattering around the threshold level is called hysteresis. This sets the difference (in decibels) between the threshold values that open and close the gate. It prevents the gate from rapidly opening and closing when the input signal is close to the threshold. This means that, whatever the setting for the threshold level (which opens the gate), the signal must fall a few dBs before the gate will be allowed to close again. As long as you also set the release and hold times properly, hysteresis can help make gates behave much more smoothly and predictably.

Range:
Not all Gate completely ‘shut-down’ the signal when it drops below the threshold, but rather attenuate the signal level by a user-defined amount. Such gates require a range control, which specifies the number of decibels by which the signal level is reduced when the gate is fully ‘closed’. The main advantage of reducing the range setting is that this can produce a more natural effect where the degree of unwanted background sound (such as spill from other instruments) that its complete disappearance would be very noticeable. By setting the gate’s range control to attenuate by a only few dBs the ambience can be allowed through during pauses, but at a reduced level, and ultimately creating more realistic qualities.

Filtering:
In some situations, you may find that the level of the signal you want to keep and the level of the noise signal are close, making it difficult to separate them. For example, when you are recording a drum kit and using the Noise Gate to isolate the sound of the snare drum, the spill/bleeding from the hi-hat may also open the gate. To alter this many commercial gates come with side-chain equalisation built in, usually in the form of high-pass and/or low-pass filters.

This means that the input signal going into the gate is spilt, this is if you do not send a separate signal to the gate through side-chaining, for example a Radio DJ will have the music track side-chained to his microphone, so when he talks the volume is dropped on the music channel. So when you are not using another track to trigger the gate the main audio signal passes through the gain-control element, allowing the signal level to be altered by it, while the side-chain carries a control signal to which the gain-control element responds.

For example the Noise Gate included with Logic Pro software allows you to control both high and low areas of the frequency spectrum through two faders. It is important to remember that these EQ parameters do not affect the frequencies of the signal, but allow you to narrow down the amount of frequencies that are affecting the gate so you can state what frequencies ranges will allow it to open or close.

But why spilt signals in the audio path and side-chain? One of the most common reasons is when you need to alter the frequency content of the side-chain in order to get the gate responding exactly how you want it to. For example, lets say that you are attempting to gate a snare drum track to reduce spill from the rest of the drum kit, but find that the kick drum is also opening the gate even when you’ve set the threshold so that everything else is excluded. By EQ’ing the side-chain signal to roll off the low frequencies, the gate is less likely to respond to the kick drum and is therefore more likely to open only for the snare.

Similarly, if hi-hat spill is causing false triggering on your snare track, you can take some top end out of the side-chain to reduce the problem. What is important to realise in both these cases, though, is that the EQ only affects the side-chain signal, not the timbre of the gated sound.

Side-chaining:
Just like compressors and other forms of audio dynamic control, a noise gate will allow you side-chain, this letting the gate be set through the input of another signal and affecting the output of the instrument/track the gate has been placed on. In this case the filtering EQ parameters discussed before, will be adjusting the side-chained signal sent into the gate.

By allowing the Noise Gate to listen to a different element of your music, however affecting the instrument/track you have placed the gate on. This is because this input allows you to control the gating action of one signal from the level envelope of another. It will allow you to create some very interesting and imaginative effects. Chopping up sections of audio in a rhythmic manner is an effect frequently used by bands like the Prodigy on some of their guitar parts and has become very common on many trance genre tracks.

This is a very simple effect to achieve: the signal being chopped is passed through the gate in the usual way but the gate is externally triggered (meaning the gate is listening to another signal) from a rhythmic sound fed in. Then adjusting the envelope parameters on the gate will allow for fascinating effects to be created, for example a tremolo very is common. This producing a wavering effect through rapid reiteration of the note.