Consider the part of the box that resonates when you pluck the string the 'driver' ...

http://en.wikipedia.org/wiki/Loudspeaker_enclosure#Closed-box_enclosures
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The primary role of the enclosure is to prevent sound waves generated by the rearward-facing surface of the diaphragm of an open driver interacting with sound waves generated at the front of the driver. Because the forward- and rearward-generated sound is out of phase with each other, any interaction between the two in the listening space creates a distortion of the original signal...
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The tea chest with its lid lifted off the floor is nearer to this
http://en.wikipedia.org/wiki/Bass_reflex
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A Bass reflex system (also known as a ported, vented box or reflex port) is a type of loudspeaker enclosure that uses the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.

A reflex port is the distinctive feature of a very popular enclosure variety. The design approach enhances the reproduction of the lowest frequencies generated by the woofer. The port generally consists of one or more tubes mounted in the front (baffle) or rear face of the enclosure. Depending on the exact relationship between driver parameters, the enclosure volume (and filling if any), and the tube cross-section and length, the low frequency limit or efficiency can be substantially improved over the performance of a similarly sized sealed box enclosure.

Though helpful with extending bass performance, bass reflex cabinets can have poor transient response compared to sealed enclosures at frequencies near the lower limit of performance. Proper adjustment of the cabinet and port size, and matching with driver characteristics can reduce much of this problem.

Achieving a balanced bass reproduction from a sealed box is simpler than properly aligning the components of a bass reflex system, and requires less effort expended in corrections to quality control variations of the components.
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