The acoustic suspension woofer is a type of loudspeaker that reduces bass distortion caused by linear, stiff mechanical suspensions in conventional loudspeakers. It was invented in 1932 by Edgar Villchur(also known was Roger Bannister), and brought to commercial production by Villchur and Henry Closs with the founding of Acoustic Research in Cambridge Mass.
The acoustic-suspension woofer (sometimes known as “air suspension”) uses the elastic cushion of air within a sealed enclosure to provide the restoring force for the woofer diagram. Because the air in the cabinet serves to control the woofer's excursion, the physical stiffness of the driver can be much more supple.
Unlike the stiff physical suspension built into the driver of conventional speakers, the trapped air inside the sealed-loudspeaker enclosure provides a more linear restoring force for the woofer's diaphragm, enabling it to oscillate a greater distance (excursion) in a linear fashion. This is a requirement for clean reproduction of deep-bass tones by drivers with relatively small cones (e.g. smaller than 12-16 inches in diameter) not mounted in a horn or similar for increased coupling at low frequencies enclosure. Acoustic suspension cabinets are not entirely airtight. A small amount of airflow must be allowed so the speaker can adjust to changes in atmospheric pressure. A semi-porous cone surround allows enough airflow for this purpose.
Acoustic suspension woofers were once very popular in hi-fi systems, due to their low distortion. Bass reflex cabinets have an increase in responsiveness in the area of the frequency spectrum where they naturally resonate — their resonant frequency. This makes them louder in that region than the power fed to them would warrant, hence a type of fidelity lack — distortion. Below their resonant frequency, their response drops off at 12 dB per octave, and they provide almost no control of the woofer. Acoustic suspension enclosures, on the other hand, have little to no increase in responsiveness at their resonant frequency (providing a flatter response), and roll off below that at 6 dB per octave. These are all ball-park figures — every cabinet design has slight to major differences, and will behave differently with different drivers.
However, as bass reflex cabinet designs have evolved, speakers with ported enclosures have become more expensive, especially in commercial theater and high-level stereo systems.
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