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I read somewhere that using an expander such as a gate increases the dynamic range of an audio signal. Does increasing the dynamic range of a song actually help the mix of the track at all while production? Can this be used in mastering?

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Gates and expanders are actually two different things (although an expander can be set to act as a gate). Both are used primarily as problem solvers and not sound quality improvers.

A true gate just turns completely off when the input level falls below a certain value, and then turns back on when it rises above that value again. Clearly it can sound weird for a signal to just turn on and off so to try to make it sound less weird, gates have controls that affect how long it waits before it turns off, and how quickly the output level drops to zero or rises back up again when the threshold level is passed. Most gates also allow for side chaining, which means a different signal can be used to control the gate open/close from the signal being changed. One use for gate side chains is to have a kick drum track turn on and off a bass track. The most common reason for using a gate is when a track only has occasional desired sound on it and the rest of the time the background sound on a track is undesired. In a band setting, a tom from a drum kit might be a typical gated track, since they are only hit occasionally and the rest of the time might only have undesired bleed from the rest of the kit.

A true expander does more than a gate. An expander is the opposite of a compressor: As the input level of an expander goes lower, and expander lowers it even more, and as the input level goes higher, and expander raises it. There is a least one input level or a range of levels that are not affected in the middle of the range of an expander. It is possible to set an expander to not change louder sounds and to lower the level of quieter sounds by a large amount, and that makes the expander work like a gate.

Expansion can truly increase the dynamic range of the source. I don't know a lot of examples of using an expander in rock music or in music production in general. Typically the dynamic range of a live instrument is so great that it is necessary to reduce it, rather than increase it. Expanders can be used in live sound, film and TV, and all kinds of speaking productions like radio or business meetings and panels to either act as softer gates or automixers or to change a sound to fit the situation. Expanders are also useful in sound restoration as old recordings can often have overly small dynamic ranges.

Expansion would not often be used in mastering except maybe for classical music or again to try to fix problems with a mix. A better solution with a problem mix is to have it remixed or to automate a level change through the mastering rather than use an expander.

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When talking about expanders, we usually mean upwards expanders. These basically work like a compressor with ratio smaller than 1:1 – below a certain threshold, they do nothing at all (or only apply a fixed gain), but when the signal crosses the threshold it gets boosted yet a bit louder. This will notably increase the dynamic range – but mind, in a way that's often counterproductive (as it tends to boost the very “freak peaks” that we like to cut with compressors). Upwards expanders have their use cases, but use them with care; generally it's better just to make sure you start out with a nice wide dynamic range and simply don't compress it too heavily.
Putting an expander in the master track is not something you'd normally do1, since it gives the whole mix a strange swaying sensation and reduces the perceived loudness.

A gate is indeed also a kind of expander, but a downward expander. I.e. it does nothing above the threshold, but reduces gain below the theshold. Again, this does also increase the dynamic range, but in a quite different way: sounds you would barely hear are reduced so you don't hear them at all. Generally only done to kill static noise or cross-talk. So, a gate does not really increase the dynamic range of the track it is applied to!
It does, however, indirectly increase the total dynamic range: e.g. in a mix with both quiet acoustic instruments and a sparsely used, loud distorted electric guitar, you may get significant hum from the guitar amp. In the passages where everything else is quiet this would be very disturbing, so you'd need to make the other instruments louder there (by way of compression, which reduces the dynamic range). If there's a gate on the electric guitar, this isn't so necessary, i.e. you retain more of the other instruments' dynamic range.

A gate should almost always be placed at the very beginning of the effects chain of an individual track. This gives it the most direct triggering, i.e. it can cut away exactly the static you don't want to hear at all, without accidentally removing something you did want to hear. When that happens, it sounds really weird – chopped off. Which is also the reason why you generally don't want any gates in your master track2.

My personal opinion is that whenever you feel like you need an expander, you'd probably better use something else to tackle the issue.

  • To remove hum or hiss, use dedicated FFT filters (e.g. ReaFIR in subtract mode). These remove the static, and only the static, without actually killing the signal in playing gaps. (For live applications, this may not be practical.)
  • To get rid of droning resonances (e.g. on bass drum), use narrow notch filters.
  • To fight crosstalk, set up your mics better. If that's not possible, scope out the crucial frequencies that get your instrument heard in the mix (and hopefully aren't so prone to cross-talk), and duck the remaining range.
  • To bring out accents, use a compressor. Yes, you've heard me right! A compressor with slow attack, low threshold & ratio actually boosts transients compared to the rest of the signal, while evening out the bulk level so the instrument itself can play with more dynamics without the quiet parts getting lost in the mix.
    Just make sure you don't use a limiter!

1That's not to say it is never a good idea. For instance it could be an interesting effect to put a fast expander before a hard distortion in the master track, to make the transients really crack up without getting the whole mix drowned in intermodulation artifacts. Just... use common sense when trying such stuff.

2Again, except as a deliberate special effect, when you want that particular “chopped up” sound.

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I'm still a novice at this, but my understanding is that having dynamic range is good so that the music has a groove or feel to it, I'm guessing because we are sensitive to volume changes. If there were no dynamic range the track would feel flat and would probably be less enjoyable, so I think increasing the dynamic range does help the track, but you have the 0 dB limit as your max volume and by increasing your dynamic range you are essentially pushing everything else down from this, lowering the overall volume of your track. This is not necessarily bad, I think it would just make your track much more quiet than others. I've heard that a dynamic range between 8 and 14 dB is usually good. Someone with more experience should feel free to correct me if any of this is incorrect.

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Like a compressor/limiter, an expander will alter the envelop of an signal. It can sharpen or dull the attack of the signal as well as gate out noise. By the way it does not turn off the sound below the threshold, it attenuates it to the level you set in the gain. If you are attenuating it 50dbs or 100dbs the gate will affectively be making that attenuated sound inaudible. How ever when used as an expander, one is only setting the range to a few DB to perhaps increase or define the attack of a signal. Opposite of the compressor/limiter where a slower attack time will increase the attack, a faster attack time setting on an expander will allow more punch to the signal. In general I find compressors work better for sustain control but an expander, with a fast release, can reduce the sustain and even reduce the reverb on a signal, particularly when combined with band pass filters so they may be tuned to the various bands uniquely. This is the philosophy upon which noise suppression is based. Four or six bands, for standard dialog noise suppression, to 512, or more, separate bands for sophisticated noise removal software. The artifacts commonly encountered when doing this sort of dramatic noise suppression, is the side effects of these numerous expanders opening and closing, producing a gurgling quality. The “smoothing” is a mater of changing the quality factors, “Q”s, on band pass filters requiring them to interact together a little more closely, reducing the amount of gurgle on the treated audio but this will also produce a bit more audible noise on the tails of the treated sound.

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