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Here's a neat trick that can be used to split the recorded background sound from the stuff you are actually aiming to record.
You just need to find the song/movie or tv/radio playing in the recording and ad it up to an inverted version of the background sound.


Things needed:
Audacity
Recorded sound
Background sound


The music im using in the background is Pronobozo-Fullflex

I added two sound files, the recording and the song. You need to open them both in audacity invert one of them, and play them together perfectly in sync.
I have cut them both to the same size, synced to make this easier.
In an real situation, you could have more background noise and more dificulty adding the two sounds up, but this is just the principle.

Sound is a sequence of waves of pressure that propagates through compressible media, so if you add a inverted equivalent the waves cancel out.This effect is also used in noise canceling headphones and some cars to bring the noise down as much as possible.The problem in actual space is that the waves go trough and bounce of things, slightly changing as they do , so while the noise canceling headphones can work perfectly, the canceling in a car  or space around us is not perfect. Another point worth mentioning is that if you slightly miss the timing the sound you intended to cancel will be amplified.

Step 1: Using Audacity


Start  the installed Audacity
Go to :
File-Open-Downloads(or navigate to your files) - Recording.wav -ok -ok.
File-Open-Downloads(or navigate to your files) - Song.wav -ok -ok. (it will open in a new window)
One random click somewhere on the song.wav sound wave.
On the bottom of the same window check snap to
go to selection start and type in 00000000
go to end and in this case type 000022
Click on the copy button and go to the recording.wav's  Audacity window.
Tracks-Add New-Stereo Track
Paste button
Effect-inverse
Play button
File-export (optional)
Always been fascinated by this! Nice instructable!
Tnx!

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Bio: Electronics geek, Openwrt, Linux, hardware hacker, maker, SF fan.
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