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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. == Explanation == Unlike closed box loudspeakers, which are substantially airtight, a bass reflex system has an opening called a ''port'' or ''vent'', generally consisting of a pipe or duct (typically circular or rectangular cross section). The air mass in this opening resonates with the "springiness" of the air inside the enclosure in exactly the same fashion as the air in a bottle resonates when a current of air is directed across the opening. The frequency at which the box/port system resonates, known as the Helmholtz resonance, depends upon the effective length and cross sectional area of the duct, the internal volume of the enclosure, and the speed of sound in air. When this vent air mass/box air springiness resonance is so chosen as to lie lower in frequency than the natural resonance frequency of the bass driver, an interesting phenomenon happens: the backwave of the bass driver sound emission is inverted in polarity for the frequency range between the two resonances. Since the backwave is already in opposite polarity with the front wave, this inversion brings the two emissions in phase (although the vent emission is lagging by one wave period) and therefore they reinforce each other. This has the useful purpose of producing higher output (for any given driver excursion compared to a closed box) or, conversely, a similar output with a smaller excursion (which means less driver distortion). The penalty incurred for this reinforcement is time smearing: in essence the vent resonance augments main driver output by imposing a "resonant tail" on it. For frequencies above the natural resonance of the driver, the reflex alignment has no influence. For frequencies below the vent resonance, polarity inversion is not accomplished, and backwave cancellation occurs. Furthermore the driver behaves as though suspended in free air, as box air springiness is absent. When speakers are designed for home use, manufacturers often consider the advantages of porting to outweigh disadvantages. The design is popular among consumers and manufacturers (speakers cabinets can be smaller and lighter, for more or less equivalent performance) but the increase in bass output requires close matching of driver, the enclosure, and port. Poorly matched reflex designs can have unfortunate characteristics, making them unsuitable for settings requiring high accuracy and neutrality of sound, e.g. in monitoring facilities, recording studios etc. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Bass reflex」の詳細全文を読む スポンサード リンク
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