Loudspeaker Cabinets - Closed or Open
It is the fundamental characteristic of the moving coil loudspeaker that it produces an output from both the front and back of the cone. These two outputs are in opposite phase. When the cone pushes forward it sends out a wave of compressed air, and simultaneously a wave of negative compression from the back. If these two waves can meet they will cancel and the result is zero audio.
The strategies for mounting the loudspeaker drive unit come from the need to deal with the output from the back of the speaker in some way or another. There are two main methods - 1) Attempt to suppress the output completely, 2) Time delay the output sufficiently to reduce the effect of cancellation. Method (1) involves totally enclosing the back of the loudspeaker. This is technically quite difficult, Hi-Fi manufacturers have taken years to get it right, usually adopting narrow and deep cabinets made of heavy non resonant materials and filling the internal air space with a damping fibre, almost completely the reverse of a guitar amplifier cabinet that is usually wide, shallow, made of resonant materials and with no internal damping. Method (2) involves leaving the back of the cabinet open, but using a cabinet large enough for the path from back to front to have sufficient time delay for the bass response to be maintained down to a useful frequency.
The enclosed box can have a good bass response, but the function of the drive unit in a guitar amplifier is not the same as that in a Hi-Fi music system. For a start the drive unit is not ideal, the large relatively thin cone allows appreciable reflected audio from inside the cabinet to pass through and combine with the forward audio. This is why Hi-Fi drivers often have relatively small cones of a dense material, which to together with heavy cabinet damping reduces colouration but then necessitate an amplifier of several hundred watts in order to fill the average living room.
Enclosing the back of a small, 1 x 12 cabinet, does not improve the very low frequencies, but has the effect of producing a resonant peak at about 150 Hz (Guitar open A) that gives the impression of added bass. If this is a nuisance then trying to reduce it with simple tone controls ends with an overall a loss of the extreme low bass required for the A and bottom E strings. Additionally, closed back speakers have a low frequency response which is omni-directional and this is more likely to cause room resonance than the open back speaker with a figure of eight response.
Open back speakers have less extreme bass than closed back, but produce a more natural sound, a frequency response that is ideally suited to projecting the guitar as a baritone instrument. This together with reduced excitement of room resonance also make it suitable for amplification of acoustic guitars, where feedback can become a problem, especially in small venues.