'Noise Gates' are now an inherent part of modern recording practice. We explain their function and provide some practical applications.
Recording engineer Dave Lockwood explains the functions of a typical Noise Gate, giving practical examples of its use in contemporary recording.
The Expander, or Noise Gate as it is more commonly known, is an electronic device that can be used to combat unwanted noise in a recording system by attenuating all signals below a certain level. This 'threshold' level as it's known, can be set by the user to the point at which a microphone signal, or mixdown channel, ceases to contain any useful information, so that as the sound dies away, the 'gate' closes, shutting off all unwanted ambience, spill, or electronic noise (Figure 1). When the signal level rises above the threshold again, the circuit reopens, and the sound is allowed to pass through unaffected.
Although the noise content is still actually present at all times when the gate is open, it will tend to remain unnoticed, or at least subjectively less annoying, when it is 'masked' by the presence of a high level of the wanted signal (Figure 2).
Problems can arise with a sustained sound that dies away very gradually, such as a piano or guitar, for as the signal gets quieter, the noise becomes more evident, consequently, gating is often most successful with short percussive sounds that decay rapidly.
Furthermore, it will be appreciated that in attenuating all signals below the threshold level, the unit makes no distinction between noise and wanted signal components, and therefore a high threshold setting, used to combat a high noise level, must also result in the loss of a substantial portion of the sound (Figure 3).
The action of an expander/gate in reducing noise, can be seen to be very different to that of a noise reduction system such as 'Dolby' or 'dbx', for they can only reduce noise that is imposed on the original signal (ie. tape hiss), whereas a gate can tackle noise inherent in the source.
Although it was originally developed as a problem-solving device, the noise gate has acquired a number of creative uses, and has become accepted as an essential tool for some contemporary recording techniques.
The controls on a noise gate can vary considerably in number and type, according to the sophistication of the unit, but some sort of Threshold control will certainly always be present, for the level at which gating occurs must be optimised for different types of signal and varying noise conditions.
Many devices will also feature an Attack control, which governs the amount of time taken for the circuit to respond to a signal crossing the threshold. To operate effectively with all types of signal, the unit must obviously be capable of very rapid action, in order to avoid audibly clipping the initial transient qualities of percussive signals, which have a very fast rise-time.
However, the fastest possible attack time sometimes introduces a 'click' into the audio output, and although this might often be masked by a wideband percussive signal such as a snare drum, it would be unacceptably noticeable with the sound of string or wind instruments. Therefore, low cost units, which often dispense with an Attack control in favour of a fixed value, are obliged to use a compromise attack time of half a millisecond or more, whereas adjustable units may offer attack times down to a few microseconds (μs - millionth part of a second), for use with appropriate signals.
A Release control, determining the time taken for the gate to close after the signal has fallen to the threshold level, could be considered essential, for the rapid cut-off that would be appropriate for a drum, would produce a very unnatural effect on a smoothly decaying cymbal. Release times from a few milliseconds up to several seconds are commonly available, with the best results usually being achieved by matching the release to the natural decay of the instrument being processed.
A Hold control is sometimes also featured, which allows the user to introduce various amounts of delay before the release phase is initiated.
An Attenuation or Depth control, usually scaled in decibels (dB), is used to determine the precise amount by which signals below the threshold are reduced. In some situations, a complete cut-off into silence is too drastic, and only succeeds in attracting attention to the gating action, but by selecting an intermediate depth setting, a worthwhile reduction in noise or crosstalk can still be achieved whilst side-effects are kept to a minimum.
Noise gates have always been closely associated with the production of a tightly controlled 'studio' drum sound. The close-miked technique with a microphone for every drum, plus overhead or cymbal mics, affords the engineer the luxury of being able to fine-tune the sound of each element of the kit in isolation. However, the total effect when all mics are open at once, can very often be an uncontrolled and disappointingly weak sound that has lost all of the qualities that were carefully engineered into the individual signals.
The problem is that the separate elements of a drum kit are, of necessity, in close proximity to one another, and with perhaps nine or more microphones placed around a kit, it is inevitable that serious levels of 'crosstalk' will occur. Tom-tom mics will pick up not only each other's signals, but plenty of snare drum and cymbals as well, while the snare drum mic cannot possibly avoid picking up plenty of the adjacent hi-hat.
Imperfections in the off-axis response of the type of microphone often used in this application tend to cause this spill to sound coloured and restricted, resulting in the loss of overall quality when the signals are combined. But by introducing noise gates into the system, it is possible to arrange for each microphone to be 'open' only when its appointed drum is actually struck, with its gate remaining closed to all other signals, thus eliminating crosstalk.
It can sometimes be difficult to arrive at a threshold setting that prevents the gate opening to the wrong source, whilst still permitting subtle use of the drums, but the best compromise must be determined by the circumstances of use.
When working with large track formats, twenty-four tracks or more, it is common to record each drum on its own track, thus facilitating gating 'off-tape' at the mixdown stage, whereas, at sixteen track and below, it is most likely that elements of the drum kit will have to be combined at the recording stage. Processing for extra separation must, therefore, obviously take place before they are mixed, which means gating the individual mic channels.
However, if a gate is inaccurately triggered on mixdown, you can simply adjust the controls and make another attempt, whereas a misaligned gate at the record stage, with its threshold set too high, for example, could result in a completely missed drum beat - a problem that is very difficult to fix at a later stage. When processing during recording it is preferable to employ the lowest acceptable gate threshold, for the occasional burst of crosstalk from accidental triggering is far more acceptable than a missing signal.
The fast rise-time of all percussive instruments naturally requires that a very short attack time (500 μs or less) is selected for gating drums, but the optimum release time will vary according to the size of the drum, the tuning of the head, and the amount of damping in use. With tom-toms it is important to avoid cutting off all the resonance that gives the sound its character, as too short a release setting combined with a high threshold will leave only the transient impact, with no real tonal qualities. Experimenting with an attack time around 100 to 150ms can often provide a reasonable compromise between separation and a natural decay.
Snare drum is sometimes gated in a deliberate attempt to 'tighten' the sound, rather than just to cut down spill. A very short release (some units offer under 50ms), that cuts off the signal as soon as it drops to the threshold level, can effectively eliminate all unwanted 'ring' and snare rattle, producing a crisp and controlled snare sound, without needing to damp all the response out of the drum head with old dusters and gaffa tape. This approach does require a consistent playing technique from the drummer, however, for if the variation in the power of the snare stroke is too great, it can be impossible to find a threshold level that adequately tightens the sound and yet still always passes the signal.
A gate can occasionally be useful on a bass drum mic, enabling the engineer to artificially shorten the decay of an underdamped or oversized bass drum, by using a fast release (50ms). Very short attack times will impart an audible click to the start of each note, but a longer setting (300μs or more) can usually be adopted to avoid this, without adversely affecting the leading edge of the sound.
Where overhead mics are employed solely for picking up cymbals rather than an overall kit balance, the very high level of sound spill that they inevitably suffer can make gating, without obvious side-effects, rather difficult. An expansion setting may often be more successful than a gating action, for an attenuation of 10 or 20dB combined with a gentle release over a period of 1 or 2 seconds, can produce a worthwhile reduction in the level of the unwanted signals, whilst still retaining a smooth cymbal decay, and with no unnatural cut-off of the ambient (room) sound of the kit being evident.
Gating can be useful when working with amplified instruments, for the combined 'hiss' of several noisy guitar amps will cause a significant deterioration in the apparent signal-to-noise ratio of the tape recording. As the background noise level from the amp is fairly constant, this type of signal can usually be quite safely processed at the recording stage, thus leaving the noise gates free for other, more critical applications during mix-down.
Attack time must be reasonably short for this application, 1ms or less, to avoid losing any of the transient qualities of a plucked string, whilst the release may often need to be very long, perhaps 2 or 3 seconds, to avoid causing an unnaturally abrupt cut-off of a sustained note or dying chord.
When a guitarist is overdubbing a solo, using a heavily overdriven amp or footpedal distortion effect, the extreme sensitivity of the system usually causes hum and buzz picked up by the instrument, which is subsequently reproduced at an obtrusively high level, along with all accidental fingering noises and the vibration of undamped strings. If left unattended, this sort of noise can build up during the course of several overdubs, to obscure low level information in the mix, and contribute to a general lack of definition in the overall sound.
If heavy compression is used on the guitar amp's microphone signal, all such low level sounds may be raised to within a few dB of the wanted signal's average level, where they can be removed by employing both a high threshold setting and a fast cut-off (release). Under these conditions it is important to assess the acceptability of the gating in the context of the whole mix, for very often gross switching effects that were evident with the gated signal heard in isolation, can be completely inaudible when other sounds are present.
The heavy compression invariably employed in the recording of vocals, has the effect of dramatically increasing the level of any background noise, such as the vocalist's breathing between lines, and especially leakage of the foldback signal from the vocalist's headphones.
When gating is applied, patched in after the compressor, it is necessary to use a release time that does not allow the gate to close down between words, which otherwise produces an unnatural 'clipped' effect - a smooth release over 1 or 2 seconds is often preferable. Combination units which include a compressor/limiter and an expander/gate in one integrated device, can be especially helpful when recording vocals.
Most gating devices offer a 'Key' or 'Trigger' input facility, which allows the action of the gate to be controlled by an external source, rather than by the signal passing through it (Figure 4).
A number of interesting effects can be created by having the dynamics of one signal controlled by another. Usually this involves triggering the gate with a percussive sound, in order to modulate the level of a more sustained sound passing through the gate. Only a few decibels of attenuation are required when the gate is in its 'closed' state, just enough so that when the device is triggered, a discernible increase in level occurs, in time with the triggering source.
Bass drum can be used to affect the bass guitar level, or the snare drum to alter the backing guitar part, and add a heightened rhythmic emphasis to the performance. More dramatic effects can be generated by using the full attenuation setting, with a fast release, which gives short bursts of sound, or by completely altering the envelope shape of a familiar instrument by adopting an unusual attack and release combination.
External 'keying' can be used to good effect with multitracked vocal or instrumental parts, in order to achieve superior synchronisation. By arranging for the original recorded line to trigger a gate set for maximum attenuation and fast cutoff,which is patched across the overdubbed parts as they are recorded, simultaneous phrase endings are ensured, creating the impression of a very 'tight' performance.
A relatively recent addition to the spectrum of modern drum sounds is the interesting effect of 'gated reverb'.
Gates are placed in the reverb return lines, set for a fast attack and release, with a feed from the drums that are to receive the effect patched into their Key inputs. The auxiliary sends on the drum channels of the mixer are then used to send signals to the reverb device, as normal, with the gate threshold controls being adjusted so that they are accurately triggered by the drums. A burst of reverb will briefly accompany each drum, before being cut-off by the gates, but because the reverb decay is artificially shortened, it is possible to use a very high level of effect, giving the drums a hugely ambient 'live' quality, without the sound becoming uncontrollably swamped. The drum track on 'Intruder' from the Peter Gabriel 3 album is a superb example of this technique.
One of the more frustrating aspects of the performance of some noise gates is a tendency to switch on and off a number of times as a decaying signal dies away. Cyclic level variations inherent in the sound can cause it to rapidly cross and recross the threshold, and when a short release time is employed, the gate switching will become very evident, either as severe modulation or complete interruptions of the signal. Most modern units successfully combat this problem by arranging for the closed threshold to be lower than the open threshold during the release phase.
The popularity of the excellent Drawmer Dual Gate has generated much interest in 'frequency conscious gating'. By including equalisation in the side-chain, or keying circuit, a gate can be made to respond only to signals of a specific bandwidth (frequency range). For example, by lowering the unit's sensitivity to middle and bass frequencies, it is possible to reduce the tendency for drums to accidentally trigger a gate on a hi-hat mic. This is an extremely useful facility which can significantly improve the performance of gates in difficult recording situations.
Any noise gate unit that has an external Key facility can, in fact, be used in this way, by taking a parallel feed from the input, through an equaliser, and then into the Key input. The Drawmer's onboard EQ is certainly more convenient, however, and also offers the valuable 'key listen' facility that enables the user to accurately 'tune in' to the wanted sound by being able to hear the filtered key signal.
Many units now incorporate a form of LED display, to give a visual indication of whether the gate is open or closed. This can be of considerable assistance in helping the user to visualise the envelope (amplitude variations) created by the control settings, and can also facilitate a fairly accurate preliminary setting up of the unit, before switching it into the audio path - fairly important when you consider that certain control settings could result in a complete interruption of the signal.
Gating is certainly not a perfect solution to all noise problems, for the process can only be said to be truly free from adverse side-effects when there is a clear margin between the lowest wanted signal level and the noise floor. Nevertheless, with multi-mic and multitrack recording methods placing a premium on good sound separation and the suppression of all types of unwanted signal, the noise gate has become an essential aid to the production of tight, clean, professional-sounding recordings.