Sealed is Not Acoustic Suspension in Loudspeakers

There are lots of great ported subwoofers and full-range tower speakers from many great companies that come with “port plugs” so the user can block up the ports and turn these systems into “sealed” systems. Many of today’s audio enthusiasts make the mistake of thinking that these newly-sealed speakers are now bonafide sealed systems and that their performance is now representative and indicative of what sealed loudspeakers are all about.

That’s not true. In fact, nothing could be farther from the truth. A ported speaker with its ports sealed off is a strange mutation that is neither fish nor fowl: It no longer performs like the vented speaker it was designed as, nor is it a properly-executed sealed speaker. It is an aberration, a nothing. There may be some understandable reasons you might want to do this under very specialized conditions (which we’ll touch upon later), but in the main, sealing a vented speaker does not create a proper sealed-box system.

But first, some historical context.

The History of Acoustic Suspension

With all due apologies to Gene Roddenberry, audio enthusiasts definitely seem to rally around the thought of, “Bass—The Final Frontier.” The crusade for better bass response has been an ongoing quest of audiophiles since the very earliest days of this hobby. In the late 1940’s/early 1950’s—right after the end of World War II in 1945—‘hi-fi’ audio became a major hobby and pastime in America. As millions of returning GIs got married, raised their families and bought houses in the newly-burgeoning suburbs that were springing up everywhere, home music systems took their place alongside television as the staples of home entertainment electronics.

Those very early post-war home music systems were not exactly barn-burners of wide-range, accurate reproduction. They were mono systems (single channel), comprised of tube electronics of limited power and bandwidth. Ten or twenty watts maximum was common, with a frequency range of about 50 or 60-15,000 “cps” (cycles per second, as it was called then). THD at maximum power, if it was specified at all, was usually on the order of 10%, especially in the bass end.

 

 1950’s mono music system

But, a low-end limit of 50 or 60 Hz was just fine in 1951 because the vast majority of home loudspeakers couldn’t reach any lower than that anyway. Speakers in those days were typically some sort of bass-reflex or infinite-baffle design, woofers that had some kind of accordion/pleated surround and a very stiff, low compliance suspension system. Names like University, Bozak, Jensen, Electro-Voice and others dotted the scene.

Many people didn’t even buy commercially-designed and manufactured finished speakers. Instead, a lot of music consumers simply installed raw, standalone speakers (perhaps full-range drivers with a ‘whizzer cone’ to extend the response a bit or an actual coaxial design) into walls or closet doors by cutting a circular hole in the hollow-core wood door and mounting the speaker. When the closet door was closed, the “enclosure” was ready to go. That’s really the way it was done. Today’s rabid home theater aficionados, meticulously positioning their 7.2.4 Dolby Atmos speaker system, aided by sophisticated digital Audyssey room-correction EQ, undoubtedly have a hard time believing this is how it once was. But it’s true.

  

1950’s Staples: Full-range speaker with whizzer, infinite-baffle co-ax 

In order to reach down to even to 40 or 50 Hz, the best speakers of that time had to be huge, refrigerator-sized infinite baffle designs with multiple woofers or big ported or horn-loaded speakers. The Bozak Concert Grand used four (4) 12-inch woofers and a line array of cone tweeters in a mammoth box in order to reproduce what it could of the full musical frequency range. The Concert Grand series—made from around 1951 until the mid 1970’s—was emblematic of those huge top-of-the-line speakers from audio’s early years.

But then something remarkable happened in the home-music reproduction realm: Stereo was invented. Now, everything would be two channels, not one. Left, right and a phantom “center” channel. Imaging. Left-right panning. Three-dimensionality. Depth. Two channels opened up the concept of the lifelike soundstage in home listening, in a way that single-channel (mono) systems never could.

Two-channel stereo was indeed a great advancement in the realism and enjoyment that could now be attained from a home music system. Unfortunately, however, one of those refrigerator-sized monstrosities was difficult enough to squeeze into the typical family room of a 1950’s split-level home on Shady Lane. Two of those speakers (for stereo) would be absolutely out of the question. Impossible.

Refrigerator-sized Bozak Concert Grand speaker

Quite fortunately for us, an enterprising young inventor named Edgar Villchur had an idea for getting solid, extended low-distortion bass from an enclosure barely 1/8 the size of the best existing speakers. Although not a formally-trained acoustics engineer, Villchur was one smart, savvy individual. He looked at the current loudspeakers of the day and saw that their cone’s motion (the so-called “restoring force” that brought the cone back to its original resting position once the input signal had ceased) was controlled by that speaker’s suspension—the surround around the outside of the cone and the pleated “spider” that centered and supported the voice coil. Together, these two components were responsible for most of the cone’s control as it traveled back and forth in response to the input signal. Because of their suspension design and the requirement that the suspension exercise such tight control over the cone’s motion, these drivers had a very low compliance (the driver moved stiffly back and forth). Mathematically, in Thiele-Small parlance (language that wouldn’t be commonly spoken in the loudspeaker universe until the early ‘70’s, almost 20 years into the future from the time of these speakers), these low-compliance drivers are known as having a high total Qts, usually well above .4 or so.

Villchur saw the inherent limitations that a man-made suspension had on driver control. Both the edge surround and the spider were subject to fairly significant unit-to-unit variations, manufacturing irregularities, material inconsistencies, labor/assembly errors and so on. Villchur found a better way...

Air.

He used air as the restoring force for the driver’s cone, not a mechanical man-made suspension. Air is “perfect” from an engineering standpoint, because it behaves in an absolutely predictable, consistent manner and there are no unit-to-unit variations or manufacturing irregularities. In order for air to supply the cone’s restoring force, it has to be “springy” enough to bounce back, if you will, when it’s compressed. The nice thing is that it will do so with essentially perfect linearity.

So Villchur designed a driver with the opposite characteristics from the drivers that existed at the time, in the very early 1950’s. His driver would have extremely high compliance (a low Q driver). The edge surround and spider would merely hold the cone physically in place, but they would not exert dominant control over the cone’s motion. The air spring trapped inside of the sealed cabinet would do that. As the driver moved, the enclosure’s air spring would control its motion, with far greater linearity and lower distortion than the mechanical suspension of a conventional speaker.

By adjusting the driver’s mass (thus raising or lowering its free air resonance or FAR), the strength of the magnetic field around the voice coil and making the enclosure slightly bigger or smaller to maintain the proper air spring, the designer could achieve a desired low-frequency extension for his particular design goal. The famous “Hoffman’s Iron Law” was always in play (LF extension, sensitivity and smaller enclosure size—pick any two at the expense of the third), but within reason, a designer could obtain strong, deep bass response with very low distortion in an enclosure 1/8 the size of a Bozak Concert Grand.

Note: Hoffman's Iron Law stated you could you could only have 2 of the 3 in relation to bass in loudspeakers: 1) Bass Extension, 2) Efficiency, and 3) Small Enclosure

 Acoustic Suspension Enclosures

Villchur’s “acoustic suspension” design made it possible to get truly accurate, powerful bass into the low 30 Hz-range from a cabinet that was only about 1.5 cu ft in volume—only 25 x 14 x 11” deep. Truly a “bookshelf” speaker. Any normal-sized room could easily accommodate two of his AR-1 speakers, and their bass response was superior to those refrigerator-sized dinosaurs—tighter, cleaner, more extended.

AR quickly introduced smaller, less expensive versions of their first model and other companies followed suit, so good-sounding compact speakers that could be used in pairs in any room became readily available and affordable at the same time that solid-state stereo electronics with reliable silicon transistors began to really take off.  AR was followed quickly by the KLH Corporation, and then by Advent and EPI, all offering compact speakers utilizing the acoustic suspension design principle of low-frequency loading. These were meticulously-engineered speakers, with their designers—long before the days of instantaneous computer simulations working through thousands of iterations—painstakingly jockeying woofer parameters, enclosure size, cabinet stuffing schemes, etc., all in a frantic competitive effort to wring an additional 3 or 4 Hz of bass extension while maintaining acceptable efficiency and impedance characteristics at a given price point.

If the high-performance bookshelf speaker as pioneered by Villchur’s AR-1 hadn’t been available right at the beginning of the stereo era, the market would likely have developed quite differently.

 

AR-3 bookshelf loudspeaker from 1958 - image courtesy of Tom Tyson

In the 1960’s thru the 1990’s. audio stores were commonplace. Stores that specialized in the display and demonstration of “stereo equipment” seemed to be everywhere and they featured “speaker walls,” where customers could choose any two sets of speakers and the store salesman would perform instantaneous comparisons, switching rapidly between the two while the music played. These stereo store showroom “A-B” comparisons ruled the day. When a 19-year-old student from Boston University wandered into Tech Hi-Fi with his favorite Emerson, Lake and Palmer album under his arm, the $125 ea. speaker that had the strongest bass line on “Lucky Man” would win the day—and the dollars. A -3dB down point of 46 Hz or 41 Hz could make the difference. In those days, speaker engineers didn’t have the luxury of on-board, high-powered amps that could be EQ’d at will to achieve a desired LF extension and response shape. In the 1960’s thru ‘90’s, speaker engineers got their speakers to reach their design goals “organically”—in response to a flat-response external signal input, not with internal EQ that could wallpaper over design errors, laziness or lack of fundamental design understanding.

Acoustic suspension held some other advantages over vented designs as well. For one, a sealed box rolls off at 12dB/oct below the system resonance. In contrast, a vented system drops off the cliff below its tuning frequency at 24 dB/octave, so there is generally more usable low bass in a passive acoustic suspension system than in a passive vented system, below their respective -3 dB points. In addition, because of the linearity and predictability of the trapped air in a properly-designed acoustic suspension system, its transient response tends to be better and above the system’s resonance, its THD is usually lower. Group delay is also better in an acoustic suspension speaker than in a vented one.

 

Sealed Rolloff (12 dB/oct, yellow) vs. Vented Rolloff (24 dB/oct, blue)

Bass Reproduction & Group Delay

There was an intriguing article by the late Peter Mitchell in the December 1995 issue of Stereo Review on the subject of group delay in loudspeakers. Group delay is a measure of how sharply the phase of a signal changes with frequency. Mitchell was an acclaimed audio expert, reviewer, and commentator whose views and writings were very highly regarded. He was a founding member of the nationally-known Boston Audio Society. I remember he caused quite a stir at a Bose press conference several years ago with his pointed, relentless questioning and was summarily banned from all future Bose press events. His passing several years back was quite a loss for the audio enthusiast community.

In this article, Mitchell puts forth the idea that there is a definite correlation between the subjective quality of bass reproduction and superior group delay performance. Speaker systems can be thought of as minimum-phase filter devices, so the magnitude response (amplitude vs. frequency) of a speaker will determine its phase, time (impulse) and group delay characteristics. All things being equal, sealed speakers (2nd -order high pass filters) have superior (lower) group delay characteristics than ported or bandpass systems (4th - or 6th -order high pass filters). As Mitchell’s article states, “Sealed-box…speaker systems consistently have the least group delay (under 10 milliseconds), and they usually deliver the tautest bass transients, the deepest-sounding bass tones, and the most clearly resolved bass textures. Bass reflex and bandpass systems often exhibit substantial group delay [in excess of 50 milliseconds]…and their sound tends to be thicker, fuller, and ‘slower’.”

This is very thought-provoking, and goes a long way to explaining why the “tightness and crispness” of the bass of an acknowledged “perfectly done” acoustic suspension system like the classic AR-3a, or AR-9 with dual 12-inch woofers seems so much better than many other systems’.

  

AR-3a with FR of -3dB at 35 Hz

Note, however, that there are lots of factors that are not “equal.” The system designer has many choices. How he chooses to damp the system affects the magnitude response, and thus affects all the other factors, including group delay. An under-damped sealed system will exhibit poorer group delay characteristics than a properly-damped ported system. 

With the advent of computer design and instantaneous iterative testing, it’s routine these days for skilled engineers, armed with precise Thiele-Small parameters that didn’t exist in the 1950’s and 60’s, to deliver vented systems that maximize all of that design approach’s theoretical advantages while minimizing their drawbacks. Early vented systems got an undeserved bad rap for being boomy and inaccurate mostly because speaker designers had to try every variation of vent tuning, driver characteristic and enclosure size in real time, creating a new physical prototype each time they wanted to try something new, and they simply lacked the resources that enabled them to quickly zero in on the exact design aspects needed for deep, tight, accurate bass response.

It can be convincingly argued that the benefits of extended bass of vented over closed box is actually of more benefit than sealed’s lower group delay, since even though the ultimate group delay of vented systems may be higher, it is usually deferred to a lower frequency where it is of less audible importance.

The sealed box AR-3a and AR-LST have a Q of .707, which is optimally damped. The AR-9 has a Q of .5, bordering on critically damped. The choice of these Q values by the designer results in a very flat, non-peaked response down to the system’s –3dB point, which implies a very low group delay. The 9 actually begins to roll off a little before its 3dB down point, but it’s an excellent design choice, because the 9’s natural 3dB down point is so low anyway (an honest 28 Hz!) by virtue of its two 12-inch drivers’ 18Hz free-air resonance and the amount of bass energy the system produces with that big enclosure (a floor standing cabinet of over 4 cubic feet). By choosing a Q of .5 the designers have elected to intentionally “throw away” a little bass energy—which they can easily afford because the 9 has so much to begin with—in exchange for super tight, clean bass. A lower Q means greater damping, and lower group delay.

 

System Q comparison in acoustic suspension loudspeakers

 

AR-9 with Dual 12-inch Woofers

This notion of low group delay is also presented by another well-respected source, Siegfried Linkwitz on his website. In a feature entitled “Frontiers,” in section F under the heading “Group delay and transient response,” he states: "…I am not certain what happens in the range below 100 Hz and I have strong suspicions that this is the region where delay distortion is audible. It is also the region where delay really accumulates via vented and bandpass woofers, and the great numbers of dc blocking capacitors in the signal chain from microphone to speaker terminal."

The delay issue in vented systems is particularly important around the “handoff frequency” (the point in a vented system at which the port’s LF output begins to dominate over the woofer’s actual output) because the backwave of the woofer that exits through the vent is literally “delayed” some amount of time relative to the direct frontal output from the woofer. Below that frequency, the port’s output dominates over the woofer and therefore any “delay” relative to the woofer is insignificant due to the relationship of the speed of sound (1130 feet/second) to the very small distances involved (perhaps a 12”-long port internal port). Nonetheless, some highly-accomplished and well-regarded audio engineers (such as the late, brilliant Peter Mitchell) have theorized that these admittedly small differences in group delay in favor of acoustic suspension are a contributing factor to the long-held view that a properly-executed acoustic suspension speaker—like the AR-11 or ADS 1290—has bass best described as “fast, tight and clean.”

It needs to be pointed out that most of today’s high-profile speaker engineers do not share Mitchell’s contention that group delay is a major contributing factor to the audible character and quality of loudspeaker bass reproduction. The general feeling is that group delay is not anywhere near as audible to the untrained ear as Mitchell implies and there are many other factors that dominate perceived bass quality, such as frequency response, distortion and audible mechanical artifacts such as voice coil bottoming against the backplate, surround “snap” at very high SPL levels and port chuffing in vented loudspeakers. Nonetheless, the group delay issue is being presented here because its impact on audible bass character is a plausible possibility, especially being presented by such a highly-educated, scientifically-minded expert like Mitchell.

Sealing the Ports of Vented Loudspeakers

Quite frankly, there may be valid reasons as to why some manufacturers these days provide port plugs. In a full-range tower, making a ported tower into a “sealed” system changes its low-frequency rolloff from 24 dB/oct to 12 dB/octave, while also raising the system’s effective -3 dB LF limit. So a ported pair of $1500 towers that get down to 45 Hz all by themselves with a very steep rolloff below that now go to, say 70 Hz when sealed, with a much more gradual rolloff. This higher -3 dB point and shallower rolloff can very well be a much better blend when used with a subwoofer and a 60-80 Hz sub crossover. In a case like this, the goal of “sealing” the tower is not to produce a full-range acoustic suspension speaker, but instead, to create a more flexible speaker that works better in conjunction with a subwoofer.

Likewise, there can be good reasons for offering ported and sealed options in a subwoofer. Make no mistake, a ported sub is optimized to be a ported sub. It will have its deepest LF extension and greatest SPL capability when used as a wide-open unrestricted ported system. But modern-day high-powered ported “super subs” can easily overwhelm a medium or small room with excessive output, especially if it’s located too close to reinforcing room boundaries, like in or near a corner. Plugging the port (or one or both, if it’s dual-ported) will reduce the sub’s LF SPL output and ultimate frequency extension, which will often result in a better, smoother in-room frequency response. Sometimes, these subs have a choice of internal EQ shapes, depending on whether port plugs are used or not, thus ensuring good response and accurate sound.

The Appeal of True Acoustic Suspension Speakers

However, the contention here is that subwoofer designers are missing the boat by not recognizing what makes for a truly excellent acoustic suspension system, designing drivers that have the correct Q, compliance and FAR and utilizing that driver in an advantageous way in a self-powered system. Consider this:

Many of today’s “sealed” powered subwoofers have an internal cabinet volume of around 1.5 cu. Ft. What a coincidence—1.5 cu. ft. happens to be the exact same internal cabinet volume of perhaps the most famous and highly-regarded acoustic suspension speakers of all time: AR’s AR-3, 3a, 11 and 10π series. These 12-inch 3-way speakers achieved a -3dB down point of 35 Hz, which all by itself is significantly lower than the low E on an electric bass (41.2Hz). These speakers were legendary in their day for clean, powerful low bass.

AR-11 spec sheet showing 1.48 cu. ft. enclosure volume, 18 Hz woofer FA

Do the math: If an AR-3a or AR-11 is -3 dB at 35Hz, with a rolloff of 12 dB/oct, that means it will only be down 15 dB at 17 Hz. With a very modest LF boost of about 5 dB centered around 30Hz, the 3a/11 is flat (-3 dB) down into the low-mid 20’s, with useful response still available at 20 Hz.

Today’s best subwoofer drivers from top manufacturers far exceed the old AR and KLH woofers in terms of heavy-duty construction, greater power handling, higher-temperature adhesives, longer excursion, bumped-out backplates, and so on. Today’s woofers are designed for digital LF inputs at levels and frequencies AR’s and KLH’s designers could never have foreseen. A modern version of the 3a’s woofer could tolerate 5 dB of boost with its eyes closed.

But quite unlike today’s woofers, the Q and FAR of those old AR and KLH woofers were optimized for their small sealed enclosure. The key is to look at the FAR of the woofer. The AR 12-inch woofer—essentially unchanged from the AR-3 of 1958 right through the AR-9Lsi of 1984—had an FAR of around 18 Hz. Peruse the pages of Parts Express or Madisound or any other raw driver supplier and you’ll be hard-pressed to find many 12-inch woofers—even so-called subwoofer drivers—with an FAR much below 25-30 Hz. The driver’s FAR really tells most of the story, because in order to have a really low FAR, the driver’s compliance has to be very high, the cone mass also has to be appropriately high and the driver’s magnetic strength has to be correct for the application intent of the driver. So if you look at the FAR, you can pretty much tell if it’s going to work well in an acoustic suspension system. There are very few good offerings, which perfectly mirrors the fact that there is an almost complete lack of real acoustic suspension subwoofers (or full-range speakers either, for that matter) on the market today. Remember, we’re not talking merely “sealed.” We’re talking real acoustic suspension, where the drivers have extremely high compliance and low FAR and the air spring in the box—not the driver’s mechanical suspension—is providing the great majority of the restoring force.

SB Acoustics SB34SWPL-76-4 12" Subwoofer Driver

But this is far more typical of today’s subwoofer drivers:  FAR of 28 Hz and a Qts of .48. Beautiful driver—beefy, well-made, aluminum cone, no doubt a monster of a woofer, as long as it’s mounted in a vented cabinet of the appropriate size (3.0 cu. ft. or more). But as physically “impressive” as this driver is, it would never reach -3 dB into the low 30-Hz range passively—unaided by rather drastic response-shaping electronic EQ—in an enclosure of 1.5 cu. ft.

Drivers such as these with an FAR above 23-30Hz are simply unsuited to being used in true, optimized acoustic suspension systems. If they are used in a sealed system, excessive electronic EQ is required to achieve a reasonable LF extension because the “organic” (passive) response of a sealed system using a driver like this in an enclosure of 1.5-2.0 cu. ft. will have a -3 dB point of 40-60 Hz at best. It’ll take a lot of amplifier power to EQ its way down to 20 Hz, power that will no longer be available for overall system SPL. Remember, every additional 3 dB of SPL requires a doubling of amplifier power. The difference between 6 dB of LF EQ and 12 dB of LF EQ is 4 times the power! Like former president John Adams said, “Facts are inconvenient things.”

The on-board power in a subwoofer (an expensive and quite finite commodity, after all) is being “wasted” on EQ because the system’s passive response gives up the ghost at 55 Hz instead of 35 Hz. And you can be sure these new woofers—optimized for vented cabinets—wouldn’t even be in a nearby universe of -3 dB at 35 Hz in a sealed enclosure as small as 1.48 cu. ft.

The drivers used in today’s “sealed” subwoofers are not acoustic suspension drivers. The majority of the restoring force is not provided by the air spring in the subwoofer’s enclosure, as would be the case in a true acoustic suspension system using a very high-compliance driver. In fact—amazingly, unbelievably—many of these manufacturers use the exact same driver in both the ported and sealed versions of their subwoofer. Since it is used in the vented system (and that system performs so well), one can safely assume the driver’s Q is well above .4-.45. That means that it is optimized for ported use and badly mismatched for a sealed application, or that both of the applications are unsatisfying compromises. A driver is either optimized to be a ported driver or an acoustic suspension driver. It can’t be both.

But….if these manufacturers had optimized a second version of their woofer specifically for use as a true acoustic suspension driver, what a truly great product that  would be. Less LF EQ would be necessary for the system to reach the same-3 dB LF point than is currently the case with a high-Q woofer in a small sealed enclosure, so more power would be available for sheer output. The system would play louder with the same bass extension. Or, you could make the enclosure even smaller, which would require a bit more EQ, probably on the order of what those actual small sealed subs are using now. So a true acoustic suspension compact subwoofer would either play louder at the same size or play as loud, but in a smaller enclosure. Either way, it’s a better product in my opinion.

The only obstacle to getting a great product like this is that the engineers would have to know the ins and outs of real acoustic suspension design. Which they don’t (or they choose to ignore, unfortunately). So instead of great small acoustic suspension subs that will play loud and low from a small enclosure, with great transient response, low THD over their usable band and a very gradual 12 dB/oct. rolloff—perfect for real-life family rooms of, say, 2000 cu. ft, what we have instead are ‘sealed sub’ compromises, which are merely very good products.

 

Editorial Note about Sealed vs Acoustic Suspension Enclosure by James Larson:

As excursion for bass drivers have grown larger, high compliance suspension of the type needed for acoustic suspension design becomes more difficult to engineer for reliable performance. The thin suspension components needed for high compliance suspension are susceptible to self-destruction at high excursions, not to mention increased distortion from rocking modes and deformation. Furthermore, air doesn't have enough damping force against the effects of progressive suspension for high excursions. At high excursions, the suspension must be able to damp itself, and that means that it has to have lower compliance than an acoustic suspension system would call for. Acoustic suspension was a more viable solution in older eras of loudspeaker design when one-way linear excursion could be around 5mm, but modern designs aim for much larger excursions where high compliance suspensions become impractical.

This article has been technically vetted by several very experienced and highly-respected acoustics engineers:

• Mark Nazar—In his distinguished career spanning more than four decades, Mark has held senior Project Engineer, Transducer Engineer and Systems Engineer positions at major companies such as McIntosh, Apogee, Acoustic Research and Boston Acoustics.

• Gerry Sheetoo—Gerry was a loudspeaker systems engineer for close to 20 years at Boston Acoustics, where he was directly responsible for the design and voicing of several of the company’s (and thus the audio industry’s) best-selling and most highly-reviewed music and home theater speakers, covering all types from acoustic suspension to vented to infinite baffle, both active and passive designs.

• Paul Ceurvels—Paul was head Electrical Engineer at NAD USA and then went on to become Senior Design Engineer at Atlantic Technology until his retirement in 2017. He has extensive experience in all aspects of product design and engineering, from electrical to acoustics to mechanical to materials. Eating lunch every day at his desk, he’d download the latest technical data sheets and research papers to read for entertainment, “just to keep tabs on what those sons-of-b’s are up to.”

Please share your thoughts in the related forum thread below about Acoustic Suspension speakers and whether or not you think they deserve a come back.