What is a noise gate? And what's an expander?

On a technical level, a noise gate is a type of dynamic audio processor that attenuates the amplitude (lowers the volume) of an audio signal when its level falls below a specified threshold. It continuously monitors the input signal and compares its amplitude against the set threshold.

A noise gate controls audio signal flow by allowing sounds above a set level to pass through while blocking quieter sounds.

If the input signal's amplitude drops below this threshold, the gate reduces the gain by a certain level, often completely muting the signal. When the input signal exceeds the threshold (is louder), the gate doesn't change the signal.

Now, let us have a listen.

Below, you can find 2 sound samples from a live studio session. A snare drum mic and an electric guitar recorded through 1 microphone on an amp. You can find the entire session to download in myLEWITT (St. Agnes - move like a ghost).

You will hear that both microphones get a lot of "bleed" from sources they don't want to capture. The gated versions have a simple noise gate added to eliminate unwanted sound.

Typical parameters for a noise gate are:

• Threshold: The level at which the gate opens or closes. Signals above this level pass through, while signals below it are attenuated.
• Attack: The time it takes for the gate to fully open once the signal exceeds the threshold.
• Release: The time it takes for the gate to close after the signal drops below the threshold.
• Hold: The amount of time the gate stays open after the signal falls below the threshold before starting to close.
• Range: The degree of attenuation applied to the signal when the gate is closed, determining how much the signal is reduced.

Noise gates come in all shapes and sizes. They can be software plugins, DSP effects in an audio interface, or guitar pedals.

A noise gate can sometimes sound choppy and unnatural by abruptly cutting off quieter sounds; to smooth things out, using an expander instead of a gate can yield better results. An expander reduces the volume of signals below a set threshold more gradually, resulting in a more natural sound. An expander is like a reverse compressor.

It is very handy to have an expander built into your audio interface to make things easy for yourself!

An expander is a dynamic processor similar to a noise gate, but instead of completely cutting off sounds below the threshold, it gradually lowers their volume, making the quiet parts even quieter. 

This creates a more natural and smooth transition between louder and softer sounds compared to a gate's abrupt cutoff. 

An expander is like a reverse compressor.

When the signal falls below the threshold, the expander reduces the gain based on a set ratio, which determines how much the volume decreases relative to the original level. It works the same way as a compressor.

Expanders can be used to reduce background noise, making louder sounds stand out more while keeping quieter sounds audible but less prominent. You can also set an expander to function exactly like a noise gate.

The typical parameters for an expander are the same as those for a noise gate (see above), with the addition of a ratio.

Ratio: Determines how much the signal below the threshold is reduced. Unlike a gate, which fully attenuates the signal when closed, the expander applies gradual reduction based on this ratio (2:1, 4:1, for example).

So, when should you use a noise gate? Here are a few examples where a noise gate will make your life easier and your recordings better:

• Electric guitars: When recording electric guitars, there is often a lot of noise between the players playing. A noise gate is the perfect tool to get rid of this noise.
• Drum recordings: Mics that focus on a specific drum, such as tom microphones, can have bleed from other drum elements you don't want in that specific microphone. A noise gate works wonders here.
• Live vocals: You can reduce feedback and bleed by using a noise gate to mute the microphone when nobody is singing.

In the following use cases, using an expander is better than a noise gate:

• Cleaning up vocal takes: You don't want your vocals to sound choppy, so using an expander is the better choice to reduce background noise.
• Live broadcasts: An expander is also better for streaming or live podcasts. Your signal will keep its natural sound, but you will eliminate background noise.

Our audio interfaces come with an expander and are great for vocal recording, podcasting, and streaming.

Using an expander is very straightforward. Start with the threshold. Adjust it until the main sound is not affected. Make sure you're listening to quiet parts.

Next, adjust the ratio. A 1: ∞ ratio is a gate. You can start there and then slowly reduce the ratio until you are happy with the sound. A fast attack, like 0.2 seconds, is a good starting point. Set the release to your taste. Listen and adjust accordingly. Trust your ears, and you will be fine.

A noise gate doesn't exactly "remove" background noise. Instead, it mutes or reduces the audio signal when it's below a certain volume level, which can make background noise less noticeable during quieter moments. 

When the main sound (like a voice or instrument) is loud enough, the gate opens, letting all sounds through, including any background noise.