Why do hifi speakers needs several drivers to reproduce all audible frequencies whereas headphones seems fine with just one driver per hear?
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A lot of earphones actually have more than 1 driver. Typically could be say 1 dynamic drivers and 1 balanced armature or 2 of the same kind (some have 10!).
Bass is omnidirectional (well, relative to typical driver size) and thus fills the space. Your ear canal is a tiny space so a tiny driver is needed. A living room is giant by comparison, so larger drivers are needed.
Treble is directional (again, relative to typical driver size) and thus a small driver is all that’s needed. Treble is high in frequency and thus has lots of back/forth movement per second, and large drivers are heavy and can’t move like that without distorting. A large driver playing treble would also be highly directional, which gives no room involvement and makes group listening poor.
Let’s say you’re sitting 10 ft. away from that hifi speaker. The tweeter can produce the high frequencies easily, but low frequencies need to move more air to achieve the same volume. In other words, low frequencies don’t “carry” as well over longer distances. Those hifi speakers use a woofer with much more power for bass.
Now with headphones, there’s much less distance; so much less air needs to be moved for those low frequencies. At those short distances, even a small driver can produce “some” bass. It’s cheaper, easier, and weighs less on your head to use a single driver for headphones.
For the same reason you don’t have a head phone speaker setup on the table to listen from the couch. You have to move a lot of air back and forth for the waves to reach your ears from far away since a lot of it gets wasted over the rest of the room where your ears aren’t. Then when you want to move that much air is hard to do for the lower frequencies with a very small area moving back and forth without a lot of distortion.
Put your headphones next to the TV and sit on your couch. Now tell me how good they sound from across the living room, probably you can’t hear them or you hear a faint noise you can’t really understand.
Sound is air that is moving and headphones being so close to your ear require to move WAY LESS air so one driver (or two depending on headphones) are able to get the job done. But when sitting across the living room, you need to move A LOT of air, so multiple drivers are useful.
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