What is Acoustic Suspension?
By: Mike Perez (Audio Arkitekts)
Edgar Villchur and Henry Kloss brought the innovation of acoustic suspension design in loudspeakers to the mass market via their first project Acoustic Research out of Cambridge, Massachusetts. Edgar Villchur described acoustic suspension in 1960, “The first aim of the acoustic suspension design, over and above uniformity of frequency response, compactness, and extension of response into the low-bass range, is to significantly reduce the level of bass distortion previously tolerated in loudspeakers. This is accomplished by substituting an air-spring for a mechanical one.”
That quote from over 60 years ago sums up the goal of acoustic suspension and its benefits.
The desire to provide a better bass response from a loudspeaker is not a new trend. Since the 1950s, when Villchur and Kloss were working on this recent advent, there was a desire for better designs that provided that sweet bass everyone wanted. Granted, mainstream music back then wasn’t precisely bass-heavy, but specific instruments did go down below the 50Hz range, which is the lowest point most speakers of that era could reach. The newfound audiophiles wanted to experience a fuller range of sound. Back in the 1950s, there were plenty of infinite baffle and bass reflex speakers, but they couldn’t get that low, and if they did, most were riddled with distortion and didn’t sound too pleasant.
Villchur was aware of the era’s limitations in loudspeaker design and had an idea.
Why not use air?
So, instead of using a mechanical suspension design, he utilized air, a readily available element, as the driving force for the speaker’s cone. Air is advantageous for engineers since it’s predictable and acts without deviations and abnormalities. The goal was to accomplish a design with perfect linearity, which worked by utilizing this Villchur’s radical idea.
Acoustic Research was able to design a small form factor cabinet, well considerably smaller than what was available in the 50s, which was a massive cabinet that commanded a large part of your listening area. The goal of creating a compact speaker, utilizing only 1.5 cubic feet of volume and reaching the low 30Hz range, was finally achieved with their AR-1 speakers. This was around the same time when sound was transitioning into a two-channel stereo environment, so when people saw this new speaker with clean, tight bass that sounded fantastic, it became an instant sensation. As with all new innovations, Acoustic Research saw the opportunity, and they subsequently released more compact and less expensive iterations of their original AR-1.
They weren’t the only ones; following in tow were the KLH Corporation and Advent, all adopting similar designs and principles. KLH was a collaborative effort founded in 1957, along with Malcolm Low and J. Anton Hofmann, who had also been involved with Acoustic Research in an investment capacity. Essentially the 50s were a paradigm shift in sound since, at roughly the same time, amplification was transitioning from the vacuum tube to solid-state designs as well.
An acoustic suspension design can provide crisp high frequencies, a clean mid-range, and tight bass if engineered correctly. An enclosure combining a bass driver, a proper mid-range speaker, and a tweeter can provide the listener with a straightforward, three-way system with precise, efficient frequency response and reduced distortion. Ideally, an acoustic suspension speaker is sealed tight. In the past, companies used a form of plumber’s putty to seal the speaker frame edge to the enclosure to ensure there was no room for error. Acoustic Research was just as prudent. The enclosed air acts as a spring to control the woofer’s linearity. If the enclosure leaks, the speaker Q (quality factor) is lowered, failing to extend the output to the lowest frequencies and compromising other variables. So, it was and is the manufacturer’s best practice to assure that a serious level of quality control was implemented when sealing the enclosure.
The natural roll-off of low-end frequencies, less likelihood of unwanted interaction with your listening room, and convenience in positioning allow an acoustic suspension design to integrate much more readily into a sound system.
Through the 60s and 70s, acoustic-suspension loudspeakers seemed to be everywhere. But by the early 80s, only KLH, Infinity, KEF, Advent, Celestion, Boston Acoustics, and a handful of others in the US continued to sell them. Then by the mid-90s, Advent and Cambridge SoundWorks, both companies founded by Henry Kloss, were the last few that sold these types of enclosures. People began to feel that the acoustic suspension design was too inefficient compared to the bass-reflex design, which was gaining more and more popularity. Consumers became exasperated at paying extra for more powerful amps that could drive acoustic-suspension speakers to their full potential. People migrated to the sound of a bass-reflex design and accepted it as the new normal.
Another factor that pushed the transition into the bass-reflex era was the publication of Neville Thiele’s work in the Journal of the Audio Engineering Society. The first paper was published in 1971 and the second in 1972. The pieces were first published in Australia several years earlier and went “under the radar” for almost a decade until the JAES published them. Those papers may have allowed people to build reflex enclosures with much more confidence as they now had an established technical basis.
In modern days, acoustic-suspension loudspeakers have begun to resurface in the mass market due to their ability to achieve more extended low-frequency responses and get more sound pressure levels (SPL). An acoustic suspension design normally rolls off on the low end at 12dB per octave versus the 24db per octave slope on bass-reflex speaker designs creating more bass in the lower frequencies rather than a complete waterfall as the bass reflex speakers tend to provide. The monotony of the same type of speaker has become quite dull. It was refreshing to see that Kerry Geist, Chief Engineer at KLH, was paramount in resurrecting the original Model Five loudspeaker. He brought back an enclosure with a traditional acoustic suspension design that carried the heritage of Henry Kloss, who championed this design throughout his years in the industry. The announcement of the Model Five and the Model Three that followed was well received because now, in the roaring 2020s, there is an insatiable thirst for vintage style. Many manufacturers are releasing vintage-style loudspeakers from their heritage lines utilizing the technology provided by today’s standards.
Where can we expect this to go?
Well, the problem with manufacturing an acoustic suspension speaker is cost. Not many companies manufacture drivers that play well with this type of enclosure. Therefore, most off-the-shelf drivers are best paired with bass-reflex enclosures. A standard acoustic suspension woofer has low compliance (large Vas), high magnet strength (low Qes), and an overall low fs (free-air resonant frequency). These types of speakers aren’t readily available, so if a manufacturer wants to create an accurate acoustic suspension loudspeaker, they will more than likely design and fabricate the drivers themselves to make sure they suit the proper specifications to create that beautiful sound that you hear from an acoustic suspension design.
With the Model Five and Model Three, KLH has created a marvel in both aesthetic and sound quality. I have had experience with both speakers and found them to be articulate. The bass was impressive, especially in the Model Five’s, which boast a 10″ paper cone woofer with a unique inverted roll rubber surround. They are unique, and I feel that KLH came out ahead of the game since JBL was the only other option for a mid-century style loudspeaker at the time of release. In my opinion, they weren’t a perfect representation of JBL’s offerings of that era.
Even though they aren’t as popular as they once were and are not the most cost-effective option for manufacturers to pursue, I believe there is a place for acoustic suspension loudspeakers within the industry. The more companies adopt the format; the more speaker driver manufacturers will be motivated to make drivers that work well with the acoustic suspension design.