[Mik James]

Your probably thinking i'll be suggesting multiple drivers for "moar surroundz" but no I have something else in mind

 
Incorporate a bass and treble driver in the same earcup with a crossover?
Whenever I read about the average open headphone in a reasonable <300$ price range there is usually the tradeoff of bass vs open sound.
 
So why not give a multi driver design a shot? The drivers currently used in headphones will have minimal excursion capabilities for the size, allowing for a full range sound due to the frequency gain curve afforded by the small space within the earcup. This works well until that enclosure is lost leading to less bass.
 
A multi driver design with a very small tweeter for the highs and a higher excursion (more bass capable) style of driver for the lows could minimize this limitation and still be perceived as point source due to the distance of the drivers from your ears.
 
None of this should be taken as a statement of absolute fact on my part of course. Just looking to spark a discussion on why this may not have been done before.
Complexity could certainly be a factor when you introduce crossover networks.
 
With the wide availability of cheap passive crossover networks these days it shouldn't be to much to ask to include them in a 100$+ pair of headphones, considering they are included in 40$ pairs of speakers (albeit unlikely to be tuned for the peak performance of the loudspeaker.

 

[castleofargh]

there have been and still are some multidriver headphones. to my knowledge none deserved to spend too much time on them.
 
I would suspect that the main problems are space and airflow. that could weight on both the design looking insane, and the 2 drivers not interfering with one another too much.
on speakers you don't care about space too much and can do what you need to do and most of all, use different physical outputs. with IEMs, the principle for most multi drivers is just to vibrate in a sealed space and it works ok, but they do suck bad at actually having good treble extensions however how many drivers they put in it. so it's not all nice and easy to deal with small space.

 

[Mik James]

Yeah, a few minutes after posting this I found a massive thread regarding this topic

 
Sounds like there may be some obstacles to work around but I didn't see anything definitive, just theory's of why it might not sound to good.
 
My understanding is that singular drivers capable of accurate full range sound in headphone design are expensive to engineer and produce.
You can likely cover the same frequency range more efficiently with multiple drivers at a lower cost of design/production, all the while experimenting with interesting designs like wide open enclosures.
Might be something to tinker with if I ever take a crack at designing a pair of budget headphones.
Nothing like hearing the theorized disadvantages for yourself