Acoustic suspension

Acoustic suspension (or air suspension) 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.

Acoustic Suspension in Mult-Way Speakers

While boxed hi-fi speakers are often described as being acoustic suspension, or ported (bass reflex) depending on the woofer it is also true that in typical box speakers with more than 2 drivers, the drivers between the woofer and tweeter are usually designed as acoustic suspension, with a separate, sealed air-space even if the woofer itself is not.

One notable exception to this was the Sonus Faber Stradivari Homage which uses a ported enclosure for the midrange.

Acoustic Performance Compared

The two most common types of speaker enclosure are acoustic suspension and bass reflex so they are worth comparing to each other. In both cases the tuning affects the lower end of the driver's response but above a certain point, the driver itself becomes the dominant factor and the size of the enclosure and ports (if any) become irrelevant.

In general, acoustic suspension systems (driver + enclosure) have a second order acoustic (12 dB/octave) roll-off below the -3 dB point. Bass reflex designs have a fourth order acoustic roll-off (24 dB/octave). Given a driver which is suitable for either type of enclosure the ideal bass reflex cabinet will be larger, have a lower -3 dB point, but equal voltage sensitivity in the pass band.

FaitalPRO 5FE120 Vented vs. Sealed

This is a simulation of a typical 5" mid-woofer, the FaitalPRO 5FE120 mid-woofer generated by WinISD. in ideal sealed (yellow) and ported (cyan) configurations. The ported version adds an octave of bass extension dropping the -3 dB point from 100Hz to 50Hz but the price is the cabinet size is more than twice as large, 8L vs. 3.8L. It is also worth noting that above 200Hz the simulations converge and there is no difference. The idea that a ported enclosure would be more efficient (improved voltage sensitivity) is a myth.

The acoustic roll-off of acoustic suspension designs makes them easier to integrate with other drivers with a crossover (passive or active). This makes it an ideal choice for midrange enclosures as well as satellite/subwoofer systems.

Theory

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.