Sealed vs. Ported

[Steve F]

>And....what ever happened to acoustic suspension design?

>Seems like almost EVERY high end audiophile speaker these

>days (B&W, KEF, Monitor Audio, Dynaudio, etc.) are all

>ported. I always loved the bass of acoustic suspension; can

>you get that with ported?

Ported designs came of age a few years ago, and were greatly improved. It is probably less expensive, both in terms of amplifier power required and box design, to make a good ported speaker than a good acoustic-suspension speaker. The acoustic-suspension woofer is still considered better in terms of lower harmonic distortion and less "boominess" and so forth, but the ported designs of today are nevertheless excellent. Hope you still have your Allisons.

--Tom Tyson

Even though this is a response to a 9/26/02 Allison thread, I decided to post this in the AR section of the Forum for two reasons:

1. The debate of ported vs. sealed is intrinsically relevant and interesting, regardless of the arbitrary brand involved, and

2. It’s an important enough subject that I felt it deserved to be read by the larger AR Forum audience.

When AR introduced it original acoustic suspension system in 1954, it represented a quantum leap forward in the advancement of the reproduction of accurate, extended, low-distortion bass from a home loudspeaker. Using the air trapped inside a sealed cabinet as the restoring force to the woofer cone resulted in a clean, linear response that has not been significantly improved upon, even today.

But as research and computer technology have advanced through the ensuing years, better ported designs are now possible—to the point where acoustic suspension’s pure performance advantages, such as low distortion and linear response (lack of "boominess") are less apparent. In fact, the trade-offs in greater efficiency and lower 3dB down point often argue in favor of a ported approach to system design.

What has changed over the last 50 years to close this gap? Several factors:

1. Quantifiable, known research and documentation of driver, port, and enclosure parameters. Specifically, the work of Thiele and Small in the late ‘70’s removed virtually all of the previous uncertainty regarding vented speaker design. In the 40’s, 50’s and 60’s, designing a vented speaker involved a lot of cut ‘n try guesswork. Designers had to be happy with ‘close enough’ or ‘in the ballpark,’ because the precise calculations needed for optimum outcomes hadn’t yet been done. Sealed designs were more forgiving by nature, and didn’t sound offensive or low-fi when they were less than perfectly done. A poorly done vented system, in contrast, could sound like a bad acoustic joke.

2. Computer-aided design programs. With today’s design programs, such as Finite Element Analysis (FEA) and 3D modeling programs such as Pro/Engineer, hundreds or thousands of driver/enclosure prototypes can be built in the virtual world of the computer before even one is actually built in the lab. A virtually unlimited number of iterations of magnet structure, cone shape, cone thickness, surround/spider compliance, voice coil length, coil winding, and pole plate variations can be tried and evaluated, before any physical parts are ever used. Different port lengths and geometries can be tried, vibrational analysis can be applied to ‘virtual’ cabinet walls to discover resonant modes and ascertain the structural integrity needed to optimize cabinet design within the cost target of the project. By the time a physical prototype is constructed, it is already 95% of the way there, with no surprises.

In contrast, in the good ‘ol days of the big bass reflex floorstander, how many iterations and prototypes did they do? Two? Maybe three? Each time, hand-cutting a new pole piece, hand-winding a new coil, ordering and waiting for delivery of new magnets….

(BTW, vented enclosures are more expensive to manufacture than sealed enclosures. A ported speaker has the extra cost of the port tube and flared ends, the extra manufacturing steps of routing the cabinet for the port opening, and the extra labor cost of installing the port. In terms of bracing, all good speaker companies brace and reinforce their cabinet, vented or sealed, as much as possible, within project budget. And it’s just as important—if not more so—to seal a ported enclosure everywhere but the port. "Unintentional" ports like air leaks around drivers will ruin a ported system’s tuning and cause audible turbulence.)

3. Advanced manufacturing processes and materials. Today’s most advanced loudspeaker manufacturing plants (the two or three big American speaker manufacturers and the big OEM driver companies who supply most other major brands) utilize very close-tolerance procedures with advanced adhesives and materials technology that were simply unavailable a generation ago.

It’s interesting to note a few basic factors that still fuel the sealed vs. ported issue, even today. At low frequencies, increasing SPL requirements demand ever-greater woofer excursion from a sealed system, which limits output and can increase distortion. In a properly tuned vented system, the port is providing the majority of low-frequency output, and the driver’s excursion—and hence its distortion—is very low.

On the other hand, such notable audio experts as the late Peter Mitchell have found a direct correlation between a sealed system’s superior group-delay performance (compared to vented and bandpass systems) and its characteristic "tight, clean" sound.

The upshot of all this is that today’s speakers CAN be much better performers than yesterday’s IF the company and/or designer so chooses. The blueprint for excellent performance is better known than it was in years past, and a skilled designer with clear, worthy goals working with today’s best tools can achieve astonishing performance at incredibly modest prices. It’s less a question of whether to take Sealed Avenue or Ported Road, and more a question of what you’ve chosen as your destination.

Steve F

[ToastedAlmond]

Sir, your knowledge on matters regarding speakers is apparent. But to this ragged pair of 50 year old ears, acoustic suspension designs....

just plain sound better. They always seem to "go the extra mile" when it comes to bass extension. The ported designs give up the ghost too quickly. Adequate power to drive smaller acoustic suspension designs is affordable to a larger group of listeners now. Ports and horns may have had a reason to exist when adequate power was at a premium.

Just my take. Plus, deep down, I think I WANT the sealed boxes to sound better. Therefore, they do.

Increasing SPL levels? Relating to perhaps home theater? When it comes to planets exploding, buses crashing, and D-Day Normandy Beach invasions, I have no point of reference. I don't witness those things live and in person on any type regular basis. For HOME THEATER applications, the average Joe may like the port. For musical applications, STUFF THAT PORT!

Between yourself and Tom Tyson, a person would have to have their eyes closed here not to learn about speakers.

Toasted Almond

[Guest Nigel]

I have AR9's for my music listening, and our home theatre system (7 channel) has recent AR HiRes speakers.

I am slowly working towards getting a whole set of AR9x for HT.

My wife is not an avidmusic listener, but she like the AR9 and AR91.5 a whole lot better than the HiRes speakers (mains are AR7, center AR4C, surrounds AR17). The AR7's are bi-amped, an Adcom 545 II drives the woofers, and the Onkyo 787 receiver (100w per channel) drives the rest of the speaker. My AR9's are presently driven by an Onkyo TX8500 ('75-'77 vintage). The HT system will probably play louder due to the 6dB efficiency advantage, but no matter what the level, the 9's are better.

I have a friend who used to play in a jazz band, he brought over a Diane Krell CD, which we played on the 9's. He said they were the best he ever heard. The reproduction of the piano, bass and solo was just like being there live.

Nigel

[tysontom]

>

>It’s interesting to note a few basic factors that still fuel

>the sealed vs. ported issue, even today. At low frequencies,

>increasing SPL requirements demand ever-greater woofer

>excursion from a sealed system, which limits output and can

>increase distortion. In a properly tuned vented system, the

>port is providing the majority of low-frequency output, and

>the driver’s excursion—and hence its distortion—is very low.

Well, it could not be spoken more precisely regarding the technologies of sealed-vs.-ported enclosures! That is a great synopsis on the two designs. Obviously, a tremendous amount of progress has been made since the days of "How to build Speaker Enclosures," by Alexis Badmaieff of Altec-Lansing, or "Hi-Fi Loudspeakers and Enclosures," by Abraham B. Cohen of University Sound! In the heady days of the 40s, 50s and well into the 1960s, speaker-box design was indeed hit or miss (except pretty much for the acoustic-suspension design in 1954). I think one of the first loudspeakers to use quantified, ported design was the Electro-Voice Interface:A speaker, an equalized, hemholtz design with passive radiator. It could go down to 35 Hz. with no problem, and with relatively low distortion. But since this speaker used equilization, it also had protection against sub-sonic energy that is lacking in standard, unequalized vented systems. Therein lies a small problem for vented systems.

If you look at an AR-3a, for example, the bass rolls off at 12-dB-per-octave below resonance (42-43Hz normally). In an acoustic-suspension system, such as the AR-3a, cone velocity -- for a given acoustic output -- doubles with each lower octave below "radiation resistance" down to resonance, and the excursion of the cone is quadrupled for each lower ocatave. However, below resonance, the excursion does not further increase, but the acoustic output begins to "roll off." The woofer is therefore protected against over-excursion or bottoming under normal circumstances at very low frequencies.

Vented enclosures, which incidentally roll off much faster on the low end than a/s woofers, have no such "compliance control," and are vulnerable to damage from energy at very low frequencies, especially at those frequencies below which they are designed to operate. Prior to 1983 or so, if you used a record player -- and with indiscreet amplifier volume you accidentally dropped the tone arm on the record -- you could easily damage a vented woofer. Granted, this was a much more serious problem in the early days before digital, but even with today's musical sources, there is opportunity for very high energy at very low frequencies.

--Tom Tyson

[Steve F]

>But to this ragged pair of 50-year-old ears, acoustic suspension designs....

>just plain sound better. They always seem to "go the extra mile" when it comes to bass extension. >The ported designs give up the ghost too quickly. Adequate power to drive smaller acoustic >suspension designs is affordable to a larger group of listeners now. Ports and horns may have had a >reason to exist when adequate power was at a premium…

>Just my take. Plus, deep down, I think I WANT the sealed boxes to sound better. Therefore, they do.

>Toasted Almond

I want to return to this very interesting sealed/ported issue. Please bear with me as I expound upon this subject for 1500-odd words.

Ported speakers are by their very nature more difficult to design properly, and even more important, extremely difficult to measure, as demonstrated by Don Keele in his 1973 paper to the AES entitled “Low-Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement”. The engineer must measure the contributions of the port and woofer separately, and then correctly weigh their outputs, taking into consideration the ratio of their radiating areas and the overall system tuning. For example, if the diameter of the port is one-half the effective piston diameter of the woofer, the port’s output must be reduced by 6dB relative to the driver’s measurement before their outputs can be summed.

This produces a complex composite curve of the system’s overall far field low-frequency response. The system’s response must then be verified by double-checking with other measurement techniques such as placing the microphone in the near field equidistant between the port and the woofer. If the port and woofer are on the same surface—which is often the case—it can be impossible to reliably measure the separate woofer and port contributions without severe crosstalk between the two. Poor sounding ported speakers are often the result of something as infuriatingly simple as the engineer measuring and reacting to the wrong data. Designing good speakers is not easy stuff, and as much as we hobbyists would like to think we have it all figured out, we don’t.

A parallel situation occurred recently when someone asked me why oval speakers sounded worse than round speakers. My reply was that oval speakers don’t have to be inferior per se to round drivers, they just end up being worse for reasons totally unrelated to their ultimate design potential. If my friend has heard bad-sounding ovals in the past, he's probably right, but not for the reasons he thinks. They're bad simply because they were poorly designed drivers. Maybe they were intended only as rear-deck original equipment speakers in a Chevy Caprice, where ultimate audio quality was not the primary goal. So the speaker didn't have an optimized cone shape to reduce destructive resonances, its surround didn't allow long, linear excursion, the voice coil couldn't handle much power so its distortion was high, etc.

This would certainly lead to a bad-sounding speaker. The fact that it was oval is coincidental. But since the oval shape fits in a lot of places where high-fidelity is not a priority (AM pocket radios, 13" TVs, computers, standard-issue factory car audio systems, etc), it's natural to begin to associate "oval" with "low-fidelity." This is specious logic. There is no cause and effect. There is nothing that says the shape of a driver is the sole or even the primary determinant of sound quality. The original Infinity EMIT tweeter was a 3 x 5" rectangle, but it sounded great. The oval 9 x 13" KEF B139 bass driver of the 1970's was a great woofer. Martin-Logan electrostatics are long and rectangular--do they sound airy, natural, and transparent? They sure do, and they're not round.

If it rains three Tuesdays in a row, does Tuesday cause the rain? No, it's just a coincidence. The weather conditions cause the rain, not the day of the week. If you hear three bad-sounding speakers, is it because they're oval or rectangular or round? No, it's because they're just bad speakers.

Similar reasoning can be applied to ported speakers. If you hear a series of bad ported speakers, it’s natural to associate boomy, floppy bass with ported design. But this isn’t necessarily the case. If the designer is good, if he uses the proper measurement techniques, if the system is tuned correctly, if the driver has the appropriate electrical and mechanical parameters, then a superb system can be produced. That’s a lot of “IF’s”, and it’s much trickier to do it well than with a sealed system. As I said in a previous post, if you miss by a little in designing a sealed system, you’re still ok. If you miss in a ported system, welcome to Boomy, One-note City. My own feeling is that the lower 3dB down point of a small ported system and its higher efficiency are thought by many companies to be worthy advantages in today’s less critical, less hobbyist-driven audio environment. When Mr. And Mrs. Gen X’er wander into the Big Box Mass Merchant and listen to speakers, chances are they’ll be impressed with the one that has more “bass” and plays louder. Those are bad ported speakers. My point is, they don’t HAVE to be bad.

Now I’m going to completely contradict myself and go against all this carefully laid-out reasoning I’ve just spewed forth. There was an intriguing article by 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 from one frequency to the next. 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. His passing a few 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 acoustic suspension system like the AR-3a, 11, or 9 seems so much better than other systems’.

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.

(The 3a and 11 have a Q of .707, which is critically damped. The AR-9 has a Q of .5, bordering on over-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 by virtue of its two drivers’ 18Hz free-air resonance and the amount of bass energy the system produces with that big enclosure. 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.)

This notion of low group delay is also presented by Siegfried Linkwitz on his website linkwitzlabs.com. 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."

So who knows? Maybe sealed IS inherently better. (Truth be told, I kind of “root” for them also, just like my friend Mr. Almond.) Maybe we’re just not measuring the right data. But I have heard terrific (and lousy!) systems of all types, so I'd be willing to believe that it's more a matter of the designer's goals and their skill at execution that makes the difference, rather than any arbitrary design approach.

Steve F.

[Guest]

Hi Steve F,

"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 …and their sound tends to be thicker, fuller, and ‘slower’.”"

- - - - -

Thanks for all the very good info; a question/comment....

While the acoustic suspension design seems to inherently minimize the timing issues (if I understood your post correctly), it would seem that the Dahlquist DQ-10 takes another approach to resolving these matters. It has an acoutic supsension woofer, but the drivers are not ported or sealed (I don't think they are considered sealed, but maybe you can comment on that.) On the DQ-10s the remaining four drivers (above the woofer) are placed in an open area on their own baffle boards which are physically stagered so to as to account for the time delays as the signal passes through the cross-over; hence Dahlquist's "phased array." This combined with minimizing the size of the baffle boards to avoid (diffraction and dispersion?) problems does seem to result in the taut/transient characteristics you reference, along with very good overall definition and imaging. - However, I realize I'm very close to your "rain on Tuesdays" logic loop

You and Tom are the engineers, I'm just suggesting that as good as the acoustic suspension speaker is (and while I have historically favored it over ported designs, your port design improvement descriptions make a lot of sense), maybe there are other design factors and approaches which should also be considered beyond the staight head to head issue of sealed vs. ported?

Thanks to everyone for the good posts; this is a neat site.

[Steve F]

>Thanks for all the very good info; a question/comment....

>

>While the acoustic suspension design seems to inherently

>minimize the timing issues (if I understood your post

>correctly), it would seem that the Dahlquist DQ-10 takes

>another approach to resolving these matters. It has an

>acoutic supsension woofer, but the drivers are not ported or

>sealed (I don't think they are considered sealed, but maybe

>you can comment on that.) On the DQ-10s the remaining four

>drivers (above the woofer) are placed in an open area on

>their own baffle boards which are physically stagered so to

>as to account for the time delays as the signal passes

>through the cross-over; hence Dahlquist's "phased array."

>This combined with minimizing the size of the baffle boards

>to avoid (diffraction and dispersion?) problems does seem to

>result in the taut/transient characteristics you reference,

>along with very good overall definition and imaging. -

>However, I realize I'm very close to your "rain on Tuesdays"

>logic loop

>

>You and Tom are the engineers, I'm just suggesting that as

>good as the acoustic suspension speaker is (and while I have

>historically favored it over ported designs, your port

>design improvement descriptions make a lot of sense), maybe

>there are other design factors and approaches which should

>also be considered beyond the staight head to head issue of

>sealed vs. ported?

The issue that I presented was concerning the perceived quality of bass reproduction, and whether or not group delay was a contributing factor. The staggered upper-frequency driver array of the DQ-10 was attempting to correct for a different set of problems in a different frequency range. Note, however, that its staggered array would only be optimal for a single listener seated at exactly the correct height, directly on axis. If the listener were slightly more towards one side than the other, they would be positioned at different distances from the speakers’ drivers than the design intent. This would also be the case if the listener stood up, or slouched down in their chair. It would also be the case if the speakers were or weren’t toed in towards the listening area, if there were any large reflective surfaces in the near field (like a bookcase or other piece of furniture), etc., etc. In the end, the sound of the speaker will be primarily determined by its composite frequency response (“power response”) at the listener’s ears. If this is good, then other factors can come into play and further refine the sound. If the overall response is not good, its deficiencies will predominate the perceived sound quality, and the secondary and tertiary factors cannot save it.

Since the Dahlquist had a sealed-box woofer, it would have as good or mediocre group delay characteristics in the bass region as its design choices allowed. I have never seen detailed measurements of the DQ-10’s low-frequency performance, so I can’t really comment on that aspect of its behavior. From my limited listening exposure to the speaker, I remember that overall, it sounded pretty good.

Remember, magnitude response (amplitude vs. frequency) will indicate a speaker’s performance in other areas. It really comes down to getting the frequency response correct. If this is right, the other things fall into place automatically. An engineer doesn’t decide to go for flat frequency response, and then try for a low group delay, and then try to get a great impulse response. They’re a package deal.

As practical proof of the importance of frequency response, you can have a subwoofer quite a distance away from a satellite, and as long as the level is right and the overall system response is right and the low-passing to the sub is right (low enough so it’s non-localizable by ear), it will sound correct and seamless, virtually every time. Delay and the design's 'transient' response just do not matter as much if the other, accepted-to-be-important variables--like response, distortion, and uniform dispersion--are right.

It's amazing how overwhelmingly important good FR turns out to be, every time, no matter how much we'd like some other, more exciting, esoteric factor to be the ONE. (A very strong argument can be made that “differences” in the sound of amplifiers, speaker cables, interconnects, and so on, turn out to be--after CAREFUL, HONEST evaluation--differences in frequency response, from various causes.)

It could be that the original AR-3, 3a, 11, LST, etc. had such properly-designed woofer/enclosure systems that their low-end response was just about perfect (within their chosen frequency range), so all the other indicators like group delay, etc. were spot-on also. As I said before, I’ve heard good and bad systems of all kinds, so I feel what really counts is proper execution—MUCH easier said than done.