Wiktionary
n. 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 frequency, as compared to a typical closed-box loudspeaker or an infinite baffle mounting.
Wikipedia
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 efficiency can be substantially improved over the performance of a similarly sized sealed box enclosure.
Though helpful with improving efficiency, 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.
Usage examples of "bass reflex".
She began in the middle of her range, using the most sensuous songs she could remember, augmenting them subtly in bass reflex but keeping the volume tantalizingly low so that he had to crouch forward to hear her.
He punched a preset and the pitch of the static dropped a few octaves, overwhelming the bass reflex in his speakers.