Hum, fans and the fridge
Noise that moves is where recordings are lost, and the difference is audible in ten seconds of silence.
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.
The useful question is not how to remove it but whether it sits still. Steady noise comes out cleanly, and most of it can be switched off for nothing before you record. Noise that moves is the expensive kind. Ten seconds of silence at the top of your next take will tell you which you have.
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.
How do you tell which one you have?
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 does steady noise come out cleanly?
Because it has one shape and it keeps it, so measuring it once during a pause is enough to subtract it everywhere.
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?
All of it is free, and all of it beats the software version.
1. The fridge. Switch it off at the socket, set a phone timer, keep the door shut, and hand the restart to a named person rather than to your memory. An hour is fine, and four is the limit worth knowing: the Food Standards Agency allows chilled food four hours out of refrigeration before it stops being safe to put back, and a food shoot reaches that faster than anybody expects, because the second setup and the vertical repeat happen after the cooking is done. Treat one hour as the target and four as the edge, not the plan. If you are filming anywhere food is sold or served, follow the local food-safety guidance rather than this paragraph. 2. The heating, twenty minutes before you record rather than as you press record, because pipes and radiators tick as they cool. 3. The extractor fan, the air conditioning, the dishwasher, the tumble dryer. Then find out what is on the other side of the wall you film against. 4. Chargers, cheap power supplies and dimmer switches. A dimmer set to anything other than fully on is a common source of buzz, one plug socket from being solved. 5. The computer. Stop the export that has the fan spinning, and take the machine off the desk your microphone stand is on, because a desk carries vibration better than air does. 6. Clothing. A clip microphone under a stiff shirt, against a necklace or with long hair across it will rustle for the whole take, and again after every break unless somebody checks.
Do not do this halfway. Turning the fridge off and leaving the fan running gets you nothing, because the fan was the louder. Use the ten seconds of silence, find the loudest steady thing, and deal with that first. And do not buy anything for hum: whatever you already edit in will very probably remove it.
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 genuinely 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 rather than a clean lift.
The options are short. Cut around it and cover the join with a shot of something else. Reduce it and accept the damage. Or record those lines again while the room still matches, which is the answer more often than anybody wants. A plane going over during your best take is not a technical problem, it is a retake.
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.
Which leaves the decision with you. Spend on the recording or spend on the repair, and only one of those is still working after this video is finished.
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