Hum, fans and the fridge

Play a take back and there it is underneath everything. A low buzz. A wash from the extractor fan. The fridge, which you have never once noticed while standing in your own kitchen.

First listen for whether the noise is steady or changes over time. Steady noise is often easier to reduce, but the result still depends on its level and overlap with the voice. Remove a source before recording where it is practical to do so.

Is your noise steady, or does it move?

Steady noise keeps the same shape from one second to the next. Mains hum, meaning a low tone tied to the electricity supply and arriving through a poor earth connection or an adapter near a cable. Hiss, meaning a wash of high frequency, usually the recorder amplifying its own electronics after the input level was set too low. A fan, air conditioning, a computer, a fluorescent tube, a dimmer left at half.

Noise that moves changes second by second. Traffic. A plane going over. A boiler firing. Footsteps upstairs. Wind on a clip microphone, the small one worn on a collar. A chair creaking, a hand on the desk, a shirt brushing against that microphone.

The fridge sits in an awkward middle. A compressor is steady while it runs and does not run continuously, so a take that begins with it on and ends with it off holds two backgrounds, one more than a single profile can handle.

Telling them apart

Record ten seconds before you speak, every time, and listen back on headphones with your eyes shut. A pitch you could almost hum along to is hum. A steady wash with no pitch is hiss or a fan. Anything with events in it, things that start and stop, is the moving kind. You will very often find all three.

Do this even when you are certain the room is quiet, because you are the last person able to judge it. Your hearing stops reporting a constant sound within a minute or two of your entering a room, and a microphone has no such mechanism.

Why is steady noise often easier to reduce?

Because it has one shape and it keeps it, so measuring it once during a pause is enough to model it and take most of it out everywhere, even where the voice sits on top.

Mains hum is the easiest of the lot. In the UK the mains alternates fifty times a second, so a true hum sits on that pitch and the multiples above it, with the first multiple, a hundred hertz, usually the loudest of them. It comes out with narrow filters that barely touch anything either side, which is why nearly every editing package has a button for it that works. Not every electrical buzz obeys this, though: a cheap charger or a dimmer can sit off those pitches, and the button that clears a real hum does nothing for it. Hiss and fans are broader and want more care, and they are still tractable, because they are still standing still.

What can you switch off before you record?

Start with the sources you can safely quieten or move away from. A quieter recording usually gives the edit better options.

1. Listen for appliances, fans, power supplies and vibration before choosing the recording position. 2. Pause non-essential noisy tasks, such as a dishwasher cycle or computer export, where practical. 3. Move the microphone or setup away from noise that needs to stay running. Do not turn off food storage or other essential equipment just to follow a filming checklist. 4. Check clip microphones for contact with clothing, jewellery and hair, including after a break. 5. Record a short speech test and room tone, then listen before the main take.

Use the test recording to identify the most intrusive source first. Reducing one noise can still help even when another remains. Listen again before deciding whether repair or a setup change is needed.

What happens to noise that moves?

Harder, and less certain. The classic denoiser learns a fixed shape and subtracts it, so a moving target defeats it: every guess about which part of this instant is noise is also a guess about your voice. The newer adaptive and machine-learning tools do better here, tracking noise that changes and pulling speech out of it without a fixed profile, and they are useful now. They are not magic either: where the noise moves like the thing you want to keep, they still trade a little of the voice for a little less noise, so the result is a judgement.

For changing noise, try a sample repair or record the affected lines again while the setup is available. The material, available time and acceptable result decide which approach is worth taking.

One habit makes all three easier. Record thirty seconds of the empty room before you pack up, every shoot. It gives an editor a matching background to patch holes with, and without it a repair leaves a gap where the room stops.

Where the fixing stops and the planning starts

This studio automates the steady noise, treats what moves by hand, and tells the partner how to stop it happening next time, which is the part that changes anything. A cooking partner was sent a written list to check before filming, and one line concerned hair touching the clip microphone. A line in a document is cheaper than a repair and it works on every future shoot rather than one file.

Sources and evidence

What here is established signal processing, a safety fact, and our own method.

What is established. Steady, unchanging noise can be reduced more cleanly than noise that moves, but even steady-noise removal is estimation and trades against the voice (peer-reviewed speech-enhancement research, Loizou and colleagues). One practical caution: switching a fridge or freezer off to record has food-safety limits, set out by the UK Food Standards Agency.

Our method. Switching off at the source whatever can be switched off before recording, and leaving a little steady noise rather than over-processing the voice, is the practice here.

References.

• Kim, G. and Loizou, P. C. Gain-induced speech distortions and the absence of intelligibility benefit with existing noise-reduction algorithms. pmc.ncbi.nlm.nih.gov. Noise suppression introduces amplification/attenuation distortion; conventional NR does not by itself raise intelligibility. Constraining amplification distortion CAN yield gains; does not establish a viewer-retention effect.
• Food Standards Agency (gov.uk). Food hygiene for businesses: chilling and freezing; and How to chill, freeze and defrost food safely. gov.uk . Catering display/serving allowance: chilled food out of the fridge for a single period up to 4 hours, then discard; home guidance = shortest time, chill leftovers within 1-2 hours. The 4-hour figure is a catering display rule, Not a safe home-fridge window.