[dfc]

"even outside the realm of speciality gear for more efficiently killing people."

FYI, this discussion is made possible because of DARPAnet and other innovation boosts drawn from the realm of "more efficently killing people". So handwaving/ignoring tech.mil seems a little short sighted at the very least. But let's return to:

"What good is a directional microphone and why (and lacking a microphone, how?) would a headphone distinguish between voices and background noise?"

How about headphones for factory workers? Construction workers? Soccer moms/dads who want to listen to a book on tape while little johnny and 25 other 5 year olds clamber up and down and all over metal playground equipment? Scientists on safari in africa or bird watching in the amazon? News camera man operating in a nousy environment that needs to hear what the reporter is saying while monitoring the police-band/other coverage/updates from main news desk?

That's five examples that I can think of off the top of my head. But its not clear what field you are restricting the discussion to. That is why I asked what you meant by "what good is"? Without a problem/application it is impossible to identify the value of a technology.

[morsch]

What would the directional microphone do in those circumstances?

[dfc]

When I came up with the list I was considering your initial question about a directional microphone and "why distinguish between voices and background noise?" So some of my examples were not restricted to directional microphones only. Given the new constraints:

Factory workers: Increase awareness about the sounds of the machinery the worker is operating and ignore the sounds of nearby machines. In a more hostile/mobile environment where the worker does not need to be so in tune with their own machine the directional microphone could provide the same benefit as it does with construction workers.

Construction workers: amplify noises from directions where the worker is not looking and therefore increase awareness of threats from blind spots.

Bird watching in the amazon: Isolate/focus scientists hearing in the direction of the bird that they are observing. Thereby reducing the possibility that they attribute sounds/songs from other birds to the bird they are observing.

News camera man: increase ability to hear sounds in front of camera man and/or directions from reporter/crew members while reducing distraction of environmental sounds.

Why are you moving the goal posts?