Again see previous post for 'driver' meaning in this context ...

Raison d'être for a speaker enclosure

http://www.silcom.com/~aludwig/Loudspeaker_construction.html

QUOTE
A primary function of a speaker enclosure is to keep the sound coming from the back of a driver cone from going into the room. The sound from the back of a driver is 180 degrees out of phase with the sound from the front. For bass frequencies the sound from the back would cancel the sound from the front, destroying the low frequency performance. At higher frequencies where the wavelength is smaller than the driver diameter, the situation is more complex. The sound may add, cancel, or something in between. The sound from the back is also delayed in time by a fraction of a millisecond, which can interfere with the stereo imaging (see discussion on source location). Preventing all sound from the backs of all drivers from going into the room is by far the cleanest way to obviate these problems.
UNQUOTE

With an 'instrument' as distinct from a 'sound reproducer (loudspeaker) cabinet', you are actually relying on various aspects of the 'tone and timbre' of the instrument deriving from some of these effects that you normally want to avoid when designing a loudspeaker cabinet. Any good luthier should be able to explain...