The moving coil drive unit is as open to the air at the rear as it is to the
front, hence it emits sound forwards and backwards.
The backward-radiated sound causes a problem.
Sound diffracts readily, particularly at low frequencies, and much of the energy
will 'bend' around to the front. Since the movement of the diaphragm to the
rear is in the opposite direction to the movement to the front, this leaked
sound is inverted (or we can say 180 degrees out of phase) and the combination
of the two will tend to cancel each other out.
This occurs at frequencies where the wavelength is larger than the diameter
of the drive unit. For a 200 mm drive unit the frequency at which cancellation
would start to become significant is 1700 Hz, the cancellation getting worse
at lower frequencies.
The simple solution to this is to mount the drive unit on a baffle.
A baffle is simply a flat sheet of wood with a hole cut out for the drive unit.
Amazingly, it works.
But to work well down to sufficiently low frequencies it has to be extremely
large. The wavelength at 50 Hz, for example, is almost 7 meters. The baffle
can be folded around the drive unit to create an open back cabinet, which you
will still find in use for electric guitar loudspeakers.
The drawback is that the partially enclosed space creates a resonance that
colors the sound.