The logical extension of the baffle and open back cabinet is to enclose the
rear of the drive unit completely, creating an infinite baffle.
It would now seem that the rear radiation is completely controlled. However,
there are problems:
- The diaphragm now has to push against the air 'spring' that is trapped inside
the cabinet. This presents significant opposition to the motion of the diaphragm.
- Sound will leak through the cabinet walls anyway.
- The cabinet will itself vibrate and is highly unlikely to operate anything
like a rigid piston or have a flat frequency response. (Of course, this happens
with the open back cabinet too).
At this point it is worth saying that the bare drive unit is often used in
theater sound systems where there is a need for extreme clarity in the human
vocal range. Low frequencies can be bolstered with conventional cabinet loudspeakers.
Despite these problems, careful design of the drive unit to balance the springiness
of the trapped air inside the cabinet against the springiness of the suspension
can work wonders.
The infinite baffle, properly designed, is widely regarded as the most natural
sounding type of loudspeaker (electrostatics excepted).
The only real problem is that the compromises that have to be made to make
this design work result in poor low frequency response.
Points of order:
- 'Springiness' is more properly known as compliance.
- Another term for 'infinite baffle' is acoustic suspension.
- You would need a very deep understanding of loudspeakers (starting with
parameters of drive units) to be able to design a loudspeaker that would work
well for studio or PA use. Electric guitar loudspeakers are not so critical.