Blue Mangoo released Noise Gate & Downward Expander, a unique noise gate that promises to sound more natural than typical efforts. Instead of acting as a simple on/off gate, the Downward Expander effects the output signal by a factor of the ratio. If your input goes below your specified threshold it won't cut it out entirely, but will instead reduce it by your ratio amount. There's also a sophisticated side-chain in here!
Noise Gate & Downward Expander App Store Description:WHAT DOES THIS PLUGIN DO THAT OTHER NOISE GATES DON'T?
Most of the iOS recording apps already come with a built-in noise gate effect. So why do you buy another noise gate? There are 2 main reasons: clearer sound and more natural gate-closing sound.
1 - DOWNWARD EXPANSION SOUNDS MORE NATURAL THAN OPEN & SHUT GATING
The simplest noise gate plugins simply set the volume to zero when the input level drops below a threshold. The abrupt change in volume from that process sounds unnatural and makes it obvious that you are using a noise gate plugin on the signal. Instead of using a gate that fluctuates between two states (open, closed), this plugin uses downward expansion to gently and smoothly reduce the gain as the input drops below the threshold setting.
An example to illustrate this:
With the downward expansion ratio set at 2:1 and the threshold at -10 dB, when the signal drops below the -10 dB threshold level, the plugin doubles its distance from the threshold. So a signal that comes in 2 dB below the threshold goes out 4 dB below; 10 dB below the threshold comes out 20 dB below, and so on. The big advantage to this method is that when the signal is just slightly below the threshold, say 1/2 dB for example, it won't be reduced all the way down to zero volume, so you don't hear an abrupt change as the signal drops below the threshold. Instead it just seems that the quiet sounds are even quieter, and they decay faster than they normally would. This allows you do eliminate a lot of background noise without the listener knowing that you are using a noise gate.
2 - SMOOTHER FILTERING IN THE ENVELOPE FOLLOWER MEANS LESS DISTORTION
The key challenge that all noise gate designs struggle to overcome is how to make sure the gate doesn't flutter open and closed several times at the end of a sound. This is a serious challenge because the input signal is a wave, meaning that it is constantly moving up and down and it crosses above and below the threshold with every up and down motion. How to keep the gate from opening and closing in time with the oscillation of the sound waves? There are many ways to do it, but the basic idea is to slow the gate down somehow, so it can't open and shut too quickly. Of course, slowing the gate down or delaying its closing is not desirable because we need it to close as quickly as possible when the input signal ends and open immediately when it starts up again. Each noise gate plugin handles this problem in a different way, and therefore each one has its own unique sound. To hear how a noise gate performs in this respect, set its release very fast, pluck a low guitar string (or hit a bass note on the piano) and let ring out, listening as the note decays down to the threshold level and the gate starts to activate. If your noise gate allows a fast enough release time, you'll hear a buzzing noise as the gate rapidly flip-flops open and closed for a few moments before settling in the closed position. The audibility of the buzzing sound as the gate closes determines the quality of the plugin. Any gate can avoid buzzing if its release time is set very long, but only the best of them can close down quickly and smoothly without buzzing.
This noise gate plugin has a unique smoothing filter to ensure that the gate makes as little noise as possible when opening and closing, while still being able to open and close very quickly.
3 - SIDECHAIN HIGH-CUT AND LOW-CUT FILTERS ALLOW YOU TO GATE OUT PART OF THE SIGNAL WHILE KEEPING OTHER PARTS
The side chain input is the signal that the plugin follows to measure the input volume. If you have bass drum bleeding into your hi-hat mic, you can gate out the bass by cutting bass frequencies out of the side chain. The result is that the gate only opens when the hi frequencies from the hi-hat are sounding, not when the bass drum is bleeding through.
The official Blue Mangoo YouTube channel doesn't have a traditional demo for this one. Instead there's a 23 minute long exploration of the design and philosophy behind their approach to noise gating.