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Old August 31st, 2010, 01:47 PM   #1
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Removing Air Conditioning Noise from Audio

I don't know if this belongs in the audio section, but since I'm using FCP any post work I do will use the filters it has.

Anyway, I'm cutting together a scene where audio recording on the day was a nightmare. When we were filming one actor's reverses the air con unit in the location came on and the people there couldn't/wouldn't turn it off. End result: his shots have clean audio, hers have this constant hum in the background.

Using FCP's audio filters, how do I go about removing it? Someone mentioned in another thread a cancelling technique where you copy the noise, invert it and lay it under the shot in question to cancel out the frequency. Makes sense, but I've had a look at the tools I've got and can't find a way to "invert" the sound?

Any ideas?
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Old August 31st, 2010, 04:29 PM   #2
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There are a couple of ways of doing this - and they don't always work - especially if the A/C is at voice frequencies.

One way is to use the graphic equaliser filter and adjust the frequency range that the A/C is in to reduce it. The problem is that you will also remove any voices etc in 'that' frequency range.

The other one is not in FCP but is in Soundtrack Pro that comes with the Final Cut Suite. This is an excellent but alas very under used tool. It takes a little bit of learning, but it's well worth it. If you need to get up and running quickly spend the $25 for a one month subscription to Lynda.com. Larry Jordan does a great job of stepping through it all.

In Soundtrack Pro there are tools to let you sample sound and set it as the 'noise print'. You can then have STP remove that noise print from the rest of the audio. If you over do it then things can start to sound too digital, but the nice thing is that the tool allows you to adjust things in real time, while the audio is looping and listen to the 'before', 'after' and also 'just' what sounds are being isolated and removed.

In addition there an excellent tool that allows you to 'see' frequencies and let you edit them visually. Using this tool you can do things like remove a police siren from the audio. The siren frequencies will be obvious when you see them, as will the air conditioner! Whether you can remove them or not is up to how well you learn how to use the tool.
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Old August 31st, 2010, 05:08 PM   #3
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Thanks Dave. I'm using quite an old Mac (and an old version of FCP) so Soundtrack Pro doesn't always play well, but if I can't sort things with EQ removal I'll give the noise print method a try. I assume it'll do the frequency inversion trick I've heard of and cancel itself out.
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Old August 31st, 2010, 09:12 PM   #4
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I hope you can remove it, but I have my doubts. Hollywood loves to have audio types wave a wand and isolate every sound in a recording, but the reality is almost never thus ...

You may have more luck _adding_ the a/c noise to your clean audio track if the goal is to establish a consistency. Recording 'room tone' is a good habit for this purpose, but even if you didn't intentionally lay down some room tone you can probably find a few seconds and create a loop long enough to layer in the sound.

An aside: I was startled and initially confused when I lived in the UK and the heating system was referred to as 'air conditioning' -- in North America a/c is always and only the cooling system, and if the heating system is also a 'forced air' system it is still called a heater, or a furnance ... but never the air conditioner.

Cheers,
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Old August 31st, 2010, 10:49 PM   #5
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Here's the old analog way to do this type of noise removal...
1) low cut (or high pass) to get rid of all the frequencies below the lowest dialog
2) High Cut( or low pass) to get rid of all the frequencies above the highest dialog
3) EQ Notch filters
- A) Q (Frequency Width) as high as it'll go
- B) Amplitude/Gain/Volume as high as it'll go
- C) Scrub frequencies until the sound you want to get rid of becomes horribly loud
- D) Drop Amplitude/Gain/Volume as low as it'll go
- E) Adjust the Q slightly to catch the frequencies just above and below if necessary.
- F) If it cuts into your dialog too much, ADR the scene.

On set sound capture in non-event shooting should only be to capture dialog on set. Often stuff like this pops up, however. Room tone will help pad out the high and low frequencies after having them sound hollow from the High/Low cut filters and EQ notching you do by refilling those frequencies lost with audio that you control. Foley will help cover the non-dialog bits that happen on set that can be seen (or help motivate cuts and expand the space).
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Old August 31st, 2010, 10:52 PM   #6
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Oh... and audacity has a spectral analyzer as well... the one in STP is phenomenal and is accessed in the upper right hand corner of the window as a choice between waveform or spectral graph. It shows audio the way I've seen it in my head since using reel to reels with mixers and EQ's on a board to do these same things.
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Old September 1st, 2010, 07:06 AM   #7
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I've had the same issues on a number of shoots (why do the difficult ones always come to me?) and all the suggestions here are great. A couple of suggestions I would like to add:

1) Try the simplest solution first, high pass filter / low pass filter. This has proved to worked with air conditioners (heaters included) that I've had to filter out in the past. Not all but well tuned ones. I shot an event recently in a coffee shop with a malfunctioning counter refrigerator that was all over the place sonically and was very loud. That thwarted the noise removal tool in Audacity and Soundtrack Pro. I ended up doing a drastic manual EQ and the resulting sound wasn't great but you could at least listen to the voices without a headache. And fortunately the video was for archival purposes not broadcast.

2) After you apply whatever audio filter you end up using to the woman's soundtrack, consider applying it, or a variation of it, to the man's sound as well. Have full sound on one voice and filtered sound on the other may be noticeable. If the entire scene has the same sound then the filter actually becomes part of the acoustic environment of the scene and the viewer will not notice. That's why the suggestion to add air conditioner noise is a good one if you can't filter the sound out without ruining the woman's voice.
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Old September 1st, 2010, 02:19 PM   #8
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Audio Noise Removal

I've found a great program called Soundsoap that is cheap and does a great (and quick) job of finding the noise and removing it. It gives you control as to the noise vs good audio you want to keep. Hope that helps!! I've been using it for years.
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