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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).
Its relatively low adoption in commercial speakers can mostly be attributed to the large resulting dimensions of the speaker produced and the expense of manufacturing a rigid tapering tube. The Voigt pipe was designed in 1934 by Paul G. A. H. Voigt and is also referred to as a tapered quarter-wave pipe (TQWP) or tapered quarter-wave tube (TQWT).
Some speaker designs also use a spiral or elliptic spiral shaped duct, usually with one speaker element in the front or two speaker elements arranged one on each side of the cabinet. Depending upon the drive unit, and quantity and various physical properties of absorbent material, the amount of taper will be adjusted during the design process ...
All speaker drivers have a means of electrically inducing back-and-forth motion. Typically there is a tightly wound coil of insulated wire (known as a voice coil) attached to the neck of the driver's cone. In a ribbon speaker, the voice coil may be printed or bonded onto a sheet of very thin paper, aluminum, fiberglass or plastic.
The effect is illustrated in Fig. 1, which shows a hard boundary at one end (the speaker) and the open-ended line vent at the other. The phase relationship between the bass driver and vent is in phase in the pass band until the frequency approaches the quarter wavelength, when the relationship reaches 90 degrees as shown.
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.
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This frequency is known as the free-space resonance of the loudspeaker and is designated by F s. At this frequency, the voice coil is vibrating in the speaker's magnetic field with maximum peak-to-peak amplitude and velocity. The back EMF generated by this movement is also at its maximum. The electrical impedance of the speaker varies with the ...