Background Noise are sounds that are not the main sound you want to record. Sounds from traffic outside, air conditioning, lighting and so on. Ideally sources of background noise are stopped, switched off or removed but what do you do if you had no choice but record the noise?
Record an atmos/room tone Wild Track this can be used to level any bumps in the background noise across edits in post production. When recording this wildtrack keep the microphone(s) and levels in the same positions as for the main recording. This will help ensure a close match between the two recordings.
Nowadays digital thingamajigs can use a sample to filter out the offending element. This is only really successful for consistent background noises. Mains hum, generator and fan noise, and lamp buzz are often most suitable. Most DSP based filters use a small sample of the “noise” and create a fast Fourier transform algorithm to extract the target elements from the waveform. The greater the level and frequency difference between wanted and unwanted sound, the more successful the extraction depth.
Over use and over-filtering produces quantising artefacts which are often, but not always, related to the money spent on the filtering plug-in. A frequent side-effect of overdone noise-filtering is an “underwater-helium-bubbling” sound. Better recognised once you’ve heard it than from a description.
Bottom line is this……
1 – do all you can to record it clean, and if you can’t, try and ensure that you keep the offending background as consistent as you possibly can, even at the expense of letting the wanted material drift a little bit.
2 – this is a process best sorted by an audio professional at the earliest possible stage. Don’t leave it to a well-meaning editor with cheap plug-ins and poor monitoring equipment.