It's the perennial "cocktail party problem" - standing in a room full of people, drink in hand, trying to hear what your fellow guest is saying.
And that matters when it comes to using audio evidence in court cases. Voices in the background can make it hard to be certain who's speaking and what's being said, potentially making recordings useless.
Electrical engineer Keith McElveen, founder and chief technology officer of Wave Sciences, became interested in the problem when he was working for the US government on a war crimes case.
"What we were trying to figure out was who ordered the massacre of civilians. Some of the evidence included recordings with a bunch of voices all talking at once - and that's when I learned what the "cocktail party problem" was," he says.
"I had been successful in removing noise like automobile sounds or air conditioners or fans from speech, but when I started trying to remove speech from speech, it turned out not only to be a very difficult problem, it was one of the classic hard problems in acoustics.