You can do so much more with noise gates and expanders than nix unwanted noise...
Given the availability of advanced editing features and ‘strip silence’ facilities in pretty much all DAW software today, not to mention sophisticated noise‑reduction software such as iZotope RX, you’d be forgiven for thinking that the traditional ‘noise gate’ should by now have gone the way of the dodo, at least when it comes to mix processing, if not live sound. But gates, along with their close cousins expanders, make possible some incredibly useful sound‑design and mixing techniques. In this article, I’ll describe a number of them, but before we dive into the techniques themselves I’ll quickly explain how gates and expanders work, so everyone can understand what follows, and decide whether their gate/expander has all the controls required for each technique.
Conceptually, gates and expanders are similar to compressors and limiters — all they do is change the signal level. But while compressors and limiters attenuate signals that exceed a threshold, gates and expanders attenuate anything falling below it. A gate is essentially a downward expander with an infinite ratio. An expander’s ratio allows it to attenuate the below‑threshold signal in proportion to the signal level — the lower the signal and the higher the ratio, the more the signal will be turned down. A gate’s infinite ratio means any signal falling below the threshold is attenuated by a fixed amount, which may be full muting or a user‑defined amount of attenuation, as determined by the floor or range control. Attack, hold and release controls determine how quickly the processor opens and closes.
And that’s pretty much it for the basic concept, though some processors offer more controls than others. I’ve provided details of all the commonly found controls in the Glossary Of Controls box. OK, with our quick orientation session out the way, let’s consider what we can actually achieve with these things.
I’ll win no prizes for originality here, but no article about gates would be complete without explaining basic noise‑removal tactics! Obviously, you set the threshold so that the wanted signal keeps the gate open and when that signal isn’t present the gate closes. This usually means starting by setting the threshold just above the noise floor. In between wanted parts of the signal, when the gate is closed, you set the range/floor to allow as much noise through as you wish to, then turn your attention to the attack and release controls — you use these to fine‑tune the opening and closing of the gate where the sound transitions from noise to wanted signal, and vice versa.
While simple enough in theory, there are some crucial practical considerations. For continuous noise, like mains hum, the hiss from analogue tape, or the self‑noise of electronics, that noise will be present not only in between the wanted parts, but also during them. Assuming the part was adequately recorded, the wanted sound will be much louder than the unwanted noise and will largely mask it, or at worst distract the listener from it. (If it is a problem, then a gate isn’t really the right tool for the job, so revisit your recording technique or consider more sophisticated noise‑removal options!) However, the operation of the gate/expander means that the noise levels now vary, which can draw the listener’s attention to the noise, making the whole process counter‑productive. So you must pay minute attention to the contrast in noise levels...
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