A simple noise gate is a sound processing effect (equipment or software) that modifies the sound signal by allowing this signal to pass through unchanged, if the amplitude of the signal is above a certain threshold, and by disallowing that signal (zeroing it out), if the amplitude of the signal is below that threshold.
Imagine that you record vocals and get a bit of noise throughout the track. If the amplitude of the noise is small relative to that of the vocals, you can apply a noise gate whether the threshold of the noise gate is somewhere between the dBVU level of the noise and the vocals. Any time the vocals are audible, the signal will pass through as its amplitude is larger than the threshold (the gate is open). At any time there are no vocals, the signal will not pass through as the noise, the only remaining portion of the signal, has amplitude which is smaller than the threshold. Hence, the noise when there are no vocals will be eliminated (the gate is closed). The one thing that will remain, however, is the noise during the vocals.
What is described above is a simple noise gate. More complex noise gates can do one or more of the following.
- A noise gate can decrease the amplitude of a signal rather than just cut it off, which basically makes the noise gate a compressor (actually, an expander);
- A noise gate can have attack and release, so that there are no clicks when the signal is cut off or restored;
- A gate can have different thresholds for opening and closing.