Getting SFX from film for a prop

[zmaster]

Hello everyone, I'm looking for advice.

I'm thinking about making a prop with a sound effect. The main problem is getting the sound effect file. The audio in the film is 5.1 channels, one of them is mainly vocals and the SFX I want, but there's still some background music in it.
How would I go about "isolating" the sound effect? Is there some software/method?

It sounds tricky, but I figured it must be a common problem when making props...

 

[Shipbrook]

A couple of ways you could go about it. If the music is also present in other channels, you could use DAW software to remove the audio:

1. Reverse the polarity of the soundtrack channels. (Not a Star Trek technobabble joke, I swear!)
2. If the dialogue/SFX is on a mono track, then either mix down the soundtrack channels into a single track, or link the volume sliders for them so that they move together. Adjust the volume of the soundtrack channel(s) until you can't hear the soundtrack any more (since the polarity is reversed, you should be able to find a point where they exactly cancel out the copy on the dialogue track.
3. If the dialogue/SFX is on a stereo pair, then don't link anything; mute the right channels and adjust the volume of the left soundtrack channel as above. Then do the same thing with the right channels.
4. Export the clip with the SFX.

There's some free DAW software out there, ranging from fairly basic (Audacity) to professional-grade (Cakewalk by BandLab).

Or you could try some stem separation software ("stem splitter"). These are usually used for separating music into parts: vocals, drums, bass, and all other instruments. There are some free ones, and even some that work online. One of them might isolate your SFX on the vocal stem, depending on how "musical instrument-y" it sounds.

 

[zmaster]

Thanks for the advice. I understand the idea and just tried it, but it doesn't seem to work in practice. Perhaps the music is a bit out of phase in the other channels (delayed) because it never cancels out.

I have already tried some of those stem splitter software but they don't do a good job with this SFX. It was free stuff, but I don't think I'd have much luck even with commercial software because it's not instrumental.

I got a decent result with Audacity's spectrum editing on one effect, but the second one becomes "metallic".