A loudspeaker enclosure is a cabinet designed to transmit sound to the listener via mounted loudspeaker drive units. The major role of the loudspeaker enclosure is to prevent the out of phase sound waves of the rear of the speaker from combining with the in phase sound waves from the front of the speaker. This results in interface patterns and cancellation, causing the efficiency of the speakers to be reduced; particularly in the low frequencies where the wavelengths are so large that interference will affect the entire listening area.
Most loudspeaker enclosures use some sort of structure, more like a box to contain the out of phase sound energy. the box is characteristically made of wood or, more recently, plastic, both for the reasons of ease of construction and appearance. Loudspeaker cabinets are sometimes sealed and sometimes ported. Ported cabinets allow some of the sound energy inside the cabinet to be released, and if designed correctly with proper attention to phase relationships, both increase bass response and reduce driver excursion.
Many other engineering variations on the basic box design exist, such as acoustic transmission lines. Enclosures always play a significant role in sound production in addition to the intended design effects, adding unfortunate resonances, diffraction, and other unwanted phenomenons. Problems with resonance are usually reduced by increasing enclosure mass and rigidity, by hightened damping of enclosure walls, or by adding absorption internally.
Bass Reflex or vented loudspeaker enclosure
Vented or bass reflex enclosures require special constructions due to the large forces that can be developed by the drivers installed inside that act on them. Vented loudspeaker enclosures have two primary functions - the separation of vibrations from the front and rear of the loudspeakers, and the containment of air so that the air can act as a resonating elastic medium inside the enclosure.
Vented enclosure operation is analogous to the way a bottle will behave as a whistle. In a tuned system it is important to avoid air leaks, since the vent produces most of the sound at the frequency of resonance and the pressure inside the enclosure can be substantial.
Air leaks in the seams or walls of enclosure can cause the tuning of the system to shift in frequency, producing other undesirable effects as well. The material used for enclosure walls should be solid and dense and should be free of voids or warps. The ideal loudspeaker enclosure would have no wall resonance at frequencies that fall within the frequency range of loudspeakers mounted in it. 25 mm solid lead plate would make an excellent loudspeaker enclosure.
Woofer and subwoofer enclosures
Enclosures used for woofers and subwoofers can be adequately modelled in the low frequency region, approximately 100 to 200 Hz and below using acoustics and the lumped component model. Electrical filter theory has been used with considerable success for woofer and subwoofer enclosures.
Conclusion
Before the 1950s many manufacturers did not fully enclose their loudspeaker cabinets; the back of the cabinet was typically left open and early on, it was observed that the enclosure had a strong effect on the bass response of the speaker. Previously loudspeaker enclosures were invented to either wall off the out of phase sound from one side of the driver, or to modify it so that it could be used to enhance the sound produced from the other side.
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