Gates & Expanders: The Good and the Bad
Gates and Expanders are a type of dynamics processing used to reduce unwanted noise in audio recordings. They operate like compressors, except that instead of attenuating signals above a threshold (as a compressor would), gates and expanders attenuate signals below a threshold. They’re popular for use in radio, podcasts, television, film, live sound, and anything where vocals and noise are commonplace. Most expanders include additional controls for decay, threshold, and ratio, as well as side-chain filters. These additional controls greatly help to control how abruptly the expander attenuates sound, as well as the sensitivity to sound as it passes through the expander. In this post, we’ll focus on the potential advantages and drawbacks of using gates and expanders.
Expanders Vs. Gates
Expanders are an all-encompassing term, such that all gates are expanders, but not all expanders are gates. Gates are simply expanders that provide extreme output attenuation for signals that drop below the user-defined threshold. Gates act more like an on-off switch, whereas expanders offer some flexibility as to the degree to which output signals are attenuated. Expanders are sometimes conflated with gates even though they’re technically different. The same limitations of expanders apply to gates as well.
Expanders are simple algorithms with binary logic. If the signal level falls below the detector threshold, the expander attenuates the output signal. If the signal level exceeds the detector threshold, then the expander allows the signal to pass through to the output, unaffected. Time controls simply delay the onset of the expander or allow for more gradual attenuation, but the basic logic of the expander remains the same. As a result, expanders tend to work very crudely, and rarely strike a good balance between subtlety and noise reduction for vocal recording. The end results typically come across as abrupt and displeasing, even though they successfully reduce noise in a technical sense.
Expanders for Vocal Recordings
For vocal recording, expanders work incredibly well at reducing ambient noises – in fact, they work too well. Expanders tend to sound very abrupt and unnatural, allowing the signal to pass through suddenly, then cutting off quickly. Most audience accept prefer constant noise, however distracting, rather than abrupt transitions between no noise and sudden noise.
Setting the Threshold
Expanders often allow for user-defined thresholds that can be custom set for the program material in your recording. Unfortunately, most vocal recordings contain varying levels as the person speaks or moves away from or toward the microphone. You can set an appropriate threshold to allow the signal to pass through the expander unaffected for the majority of the recording, but if the person drops to a low speaking voice or whispers (without moving closer to the microphone), then the threshold should be adjusted to compensate. This can be especially difficult for a recording with a noisy background. For longer recording of twenty minutes or more, there’s a strong probability that several words or syllable might be lost as the person speaks. Unless the input threshold is set incredibly low, it’s typically more practical to edit out the silences by hand or simply leave the noise as-is. For video, this can be especially jarring as you may see lips move without hearing any sound. For the vast majority of audio production, we don’t recommend using expanders for vocal recordings.
When to Use Them
Oddly enough, expanders work quite well for recordings with low noise floors to begin with. A pristine, studio-quality recording in an isolated environment sounds just fine with an expander. The results are so good, that the differences between the expanded recording and raw recording might be indistinguishable from one another. Likewise, expanders sound great on someone addressing a microphone with precise, consistent microphone technique, who addresses the microphone closely and loudly for each syllable. For someone with this much attention to detail and experience, it’s unlikely that you’ll have problems with noise in the first place.
For music recording, expanders work incredibly well for instruments with fast transients. Kick drums, floor and rack toms, congas drums, and bongos all benefit from expanders, even in studio environments. These instruments all generate sound with fast, sudden onsets and rapid falloff, making them perfect for the naturally abrupt transitions that expanders offer.
Gates and expanders make excellent tools for reducing ambient noise levels, but the way they attenuate sound is often more distracting than the noise itself. More often than not, it’s easier to edit out or attenuate silences by hand rather than rely on an expander. Clean vocals recording in a studio or with excellent microphone technique rarely benefit from expanders, and the threshold setting may actually cut off the beginnings of words. For those of you doing music recording, use expanders on floor toms, rack toms, kick drums, congas, and bongos, but they’re best avoided for vocal recordings.
There are surprisingly very few things you can do to remove noise from a recording in post-production, hence why we’re staunch advocates for practicing the techniques that create great recordings to begin with. Refer back to our vocal recording basics series for information about how to create great recordings right out from the start.
If you found this helpful or tried using expanders and gates for vocal recording yourself, let us know in the comments below.