In virtually all modern loudspeaker systems, the enclosure contains some or all of the driver elements that comprise the system, and it separates the rear radiation of one or more cone-type drivers from their front radiation. It may also limit the rear radiation of horns and compression drivers to avoid acoustical energy returning back towards the stage.
Why separate front cone radiation from rear radiation? When a cone driver moves forward it provides positive pressure to the atmosphere, exhibiting excursion. When it moves rearward it provides negative pressure, or recursion. It does this a lot – like 1,000 times per second for a 1 kHz wavelength. If the front and rear radiated energies are not separated from each other, each will cancel the other’s output because they are 180 degrees out of phase.
There are several ways to keep cancellations from happening. The simplest is a flat baffle, an approach dating back to the early 1900s. The first baffles were nothing more than a flat surface for mounting the cone driver, its purpose being to isolate front radiation from rear radiation. The larger the baffle, the lower the frequency range the cone driver could reproduce (within its other limits, of course).
As frequency decreases, wavelengths become longer. The baffle, if it’s large enough, separates even LF front radiation that is positive from rear radiation that is negative, thus stopping front-to-rear acoustical cancellations.
As time went on, it became clear that 18- x 18-foot baffles weren’t very practical, except perhaps in fixed cinema installations. Loudspeaker designers instead came up with the idea of an infinite baffle, which is little more than a sealed box. It works.
I remember the first time I heard the infinite baffles in a friend’s Acoustic Research AR-3 loudspeakers. (I was about 14 at the time, and the record playing was Jimi Hendrix.) Since then, I’ve spent my life involved with music and sound.
As rock ‘n’ roll displaced vocal groups and big band music in the 1960s, more output was sought from loudspeakers. The bass reflex enclosure helped to fulfill the new demands for power, clarity, and extended bass.
A bass reflex enclosure uses a port that captures the rear energy of the cone driver and sends it outwards by means of a relatively small, and often ducted, vent arrangement. This is not particularly intuitive (how are those long wavelengths getting through that small port?) so worry not if you can’t quite grasp it. It requires complex pressure dynamic equations to properly explain.
