Acoustic suspension

The acoustic suspension (or air suspension) woofer is a type of loudspeaker speaker enclosure design which uses a sealed box. Acoustic suspension systems reduce bass distortion caused by stiff motor suspensions in conventional loudspeakers. It was invented in 1954 by Edgar Villchur, and brought to commercial production by Villchur and Henry Kloss with the founding of Acoustic Research in Cambridge, Mass.

The acoustic-suspension woofer uses the elastic cushion of air within a sealed enclosure to provide the restoring force for the woofer diaphragm. The cushion of air acts like a spring or rubber band. Because the air in the cabinet serves to control the woofer's excursion, the physical stiffness of the driver can be reduced.

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 and loud reproduction of deep bass by drivers with relatively small cones.

Even though acoustic suspension cabinets are often called "sealed box" designs, they 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. Most Acoustic Research designs used a PVA sealer on the foam surrounds to enable a longer component life and enhance performance. The venting was via the cloth spider and cloth dust caps, not so much through the surround.

Acoustic suspension woofers were once very popular in hi-fi systems due to their low distortion. Compared to bass reflex cabinets, acoustic suspension has a flatter frequency response and slower rolloff below their resonant frequency. Bass reflex cabinets are generally more efficient, however, and the use of a vent or port in the cabinet provides improved low-frequency response. Many subwoofers, bass amplifier cabinets and sound reinforcement system speaker cabinets use bass reflex ports, rather than a sealed box design, in order to obtain better efficiency and low-frequency response.

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