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Two-inch port tube installed in the top of a Polk S10 speaker cabinet as part of a DIY audio project. This port is flared. Unlike closed-box loudspeakers, which are nearly airtight, a bass reflex system has an opening called a port or vent cut into the cabinet, generally consisting of a pipe or duct (typically circular or rectangular cross section).
Exploded-view diagram showing the IMF Reference Standard Professional Monitor Mk IV speaker by renowned transmission line loudspeaker pioneer John Wright (of IMF/TDL), from the 1970s. The complex shape of the transmission line allowed a full frequency range of 17 Hz to "beyond audibility" and loudspeaker sensitivity of 80 dB (specified as 96 dB ...
Older descriptions explained the design in terms of "impedance mismatch", or pressure waves "reflected" back into the enclosure; these descriptions are now considered outdated and inaccurate as technically the transmission line works through selective production of standing waves and constructive and destructive interference (see below).
Moving iron speaker. The original loudspeaker design was the moving iron. Unlike the newer dynamic (moving coil) design, a moving-iron speaker uses a stationary coil to vibrate a magnetized piece of metal (called the iron, reed, or armature). The metal is either attached to the diaphragm or is the diaphragm itself.
The design is often described as non-resonant, and some designs are sufficiently stuffed with absorbent material that there is indeed not much output from the line's port. But it is the inherent resonance (typically at 1/4 wavelength) that can enhance the bass response in this type of enclosure, albeit with less absorbent stuffing.
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The two most common types of speaker enclosure are acoustic suspension (sometimes called pneumatic suspension) and bass reflex.In both cases, the tuning affects the lower end of the driver's response, but above a certain frequency, the driver itself becomes the dominant factor and the size of the enclosure and ports (if any) become irrelevant.
A common use of Thiele/Small parameters is in designing PA system and hi-fi speaker enclosures; the TSP calculations indicate to the speaker design professionals how large a speaker cabinet will need to be and how large and long the bass reflex port (if it is used) should be.