Bass-reflex vs. acoustic-suspension: need refresher course!

Ever since getting into audio (@1977) basic speaker theory says that when using the same size woofer and same enclosure volume an acoustic-suspension configuration (i.e. sealed) will go lower frequency-wise than a bass-reflex (i.e. ported) design.

But over the last few years, I see more and more people writing that the ported configuration will have the more extended bass.* But except for active EQ systems, I have not heard of any new/significant developments with either design, so to me anyway this increasingly common viewpoint is more a result of mis-information than actual fact, worsened by the fact that the internet allows such information to be spread quicker than ever before.

Any thoughts?

Btw I realize sealed designs are in general less efficient and usually produce less bass (volume-wise) than an equivalently-sized ported speaker. I am really more concerned here with the bass extension issue.



* IIRC from my speaker-building days, a ported design can go lower when its enclosure is optimized for that particular woofer but that meant a much larger enclosure volume when compared to a sealed design using the same size of woofer. This is the main reason Henry Kloss and Edward Villchur designed the acoustic-suspension system, so people wouldn't have to deal with those enormous cabinets one sees in 1950s vintage speaker advertisements.

 

Generally speaking, most well designed woofer drivers today will give you lower extension in a ported enclosure. And also, the larger the enclosure, the greater the cone excursion, providing more low-frequency. But ported designs tend to have a peak tuned to the port and should be controlled/filtered at lower levels to avoid distortion. You will often hear people describe a sealed sub design as being more “musical.”

But there is so much to consider in designing a speaker for the lowest frequencies including desired cabinet size, efficiency, quality of the driver that it’s not a simple “one mode is best for all” approach. Additionally, good subwoofer drivers today have much lower distortion and much greater excursion capabilities.

 

Acoustic Suspenders

The Acoustic Suspension system is , by nature ineffecient, and does not provide the same amount of lower bass response as a ported system.

Acoustic Suspension speakers were developed for one reason ONLY...to save $$$ on cabinets by making them smaller. Manufacturers knew that they were sacrificing bass response but they saw a bigger profit there by cutting cabinet expense (and quality, when MDF board reared ITS ugly head.)

Over the years, nothing has beaten the original math of the 1950's when they defined the correct size ported enclosure for a specific size speaker. Its one of the reasons those old 50s speakers STILL sound so good, because the cabinets were built correctly to maximize their frequency response curves .

I've always hated AS speakers.......a little vibration from the music, the screws loosen a bit and the bass response changes!!! No thanks!!

 

I'm an AS guy...I'm not sure if AS goes lower, but I think it goes lower with less distortion than BR designs. To my ears, AS when configured correctly, has a tighter, cleaner bass than BR designs that I have heard. But yes they are not as efficient, require more power.

http://www.stereophile.com/interviews/105villchur/

- Sushimaster

 

The drivers themselves are drastically different from the drivers used during the dawn of "Acoustic Research." Speaker cones are both lighter and stiffer. Use of materials unavailable in the sixties results in mid/bass-drivers capable of moving more air with less break-up and better efficiencies. I personally prefer the more dynamic sound from ported enclosures to sealed designs.

At the same time, the satellite/sub has become more common as more audio gear is designed around home audio. Ported speakers generally work better in these kinds of set-ups.

 

I guess the real question is, if you used these modern drivers in a sealed box, would the bass sound better than the same modern drivers in a BR design, apples to apples...

- Sushimaster

 

Well, if Edgar Villchur hadn't invented the AR-1 acoustic suspension loudspeaker, we'd need big cornerhorns for full range bass. And Stereo would have been delayed in it's acceptance. The advantages of acoustic suspension designs are cleaner, lower bass in a smaller cabinet. The disadvantage is the need for more powerful amplifiers. I don't care for the bass quality in most modern bass-reflex designs and dislike most subwoofer-satellite systems. I like my midrange free of being sucked out.

 

An acoustic suspension system will have a slightly more "taut" bass than a ported system. That's because in order for the port to augment the bass response of the system, it has to be 360 degrees out of phase with the woofer, so ported bass always has a lag of one cycle.

 

Early speaker designs had a 'stiff' suspension and needed a large box to attain the adequate low frequency response. When manufactures analyzed this they realized that they could lower the woofer suspension stiffness allowing them to use a smaller (=more popular & cheaper) box size.

If one adds a port to a given box size, that alone lowers the resonant frequency due to the added moving mass of the air. Note however that manufactures today make the same size (diameter) woofer with specific characteristics that will suit a closed or open box. The driver free air resonance frequency, resonance amplitude magnification and compliance are selected to fit the enclosure type/size and to obtain a specific bass responses or 'alignments'.

Some people prefer the ported enclosure due to the higher efficiency and acoustic output plus the ability to 'tune' it by varying the port design. I find that either design when done carefully can achieved excellent results.

 

some reference equations and a graphic rep of sealed/ported enclosure

(no affiliation & have not verified accuracy)

http://www.ajdesigner.com/phpsubwoofervented/minus_three_decibel_equation.php

graphic
http://www.ajdesigner.com/fl_subwoofer/subwoofer.php

 

Kinda wrong, sorry....

Acoustic suspension was not designed to Save money as you state, but to make VERY deep bass in a very small cabinet. A bass reflex will never equal a good acoustic suspension speakers deep bass, unless its very big or tuned very low, then it will often overload and lose control of the speaker very easily, resulting in very high distortion.


All bass reflex does is allow a higher efficiency around the tuning frequency and allows the bass to be extended a bit more than if it was a closed cabinet design. Acoustic suspension is designed to have a VERY low resonance frequency and the cabinet stiffens the load the back of the speaker sees.

You forget one MAJOR thing, huge cabinets in the 50s were because they DIDNT know how to design speakers well back then.

Bass reflex speakers roll off at a much steeper rate than an acoustic suspension ever does. Not sure where you are getting your ideas from, but most of what you said is totally wrong.

 

An example of measured frequency response graphs of the same woofer unit (F free res. = 53 Hz; D eff. = 160 mm) in enclosures of different types but of the SAME VOLUME = 80 litres:

1. vented port
2. passive radiator
3. sealed enclosure.

 

Ported has several other bad side effects.......

Its main advantage is about a 3db gain in sensitivity.

Below the tuning frequency driving it at high levels will cause the woofer to go out of control very easily and distort. Also the roll-off is approx 18 decibels per octave, so really deep bass is often sacrificed at the expense of loud bass at a higher frequency.

Bass reflex has lower distortion at part of its operating range, but much higher at the extremes of deep bass.

Try playing a pipe organ or synthesized bass with really loud deep bass and a bass reflex will distort readily at high volumes, not so with a good acoustic suspension.

Of course amplified and controlled drive woofers in subs are another story.

 

Your chart is good, EXCEPT you would never use the same woofer in an acoustic suspension speaker system. You would design it with a free air resonance of more like 20 Hz or so and the small box would raise it to around 35-40 hz, then the rolloff would start a lot lower.

That type of woofer would never be used in an acoustic suspension design, but in your case you are describing more of a closed box design. Using a high free air resonace bass driver in a sealed box isnt exactly what acoustic suspension means. It means using a very low free air resonace driver and the box raises the frequency to a reasonable point.

 

You can't really say that one is more efficient than the other. Efficiency is determined by the driver, not the box, as long as it's a direct-radiating system (i.e. no horns).

It's not really quite fair to compare the results of different box designs for the same driver, because a driver made for Acoustic Suspension is going to be quite different than one made for a ported box.

To paraphrase Hoffman's Iron Law: high efficiency, low frequency extension, or small size - pick any two!

 

VERY true..!!

Its one of those things you say USUALLY...!

I think back to the OP's original question, a well designed acoustic suspension will indeed have better very deep bass extension, but, but it depends on a lot of things. Like you said driver design, box size and so many things factor into this. No matter what though, a very small box will favor an acoustic suspension for deep bass, but it will be inefficient. You cant have all things


Think back to some of those bigger ININITY speakers in the 80s, and back in the 70s to the ADVENT LARGE. Deep bass, great sound, but not extremely efficient.

 

Right. But the graph was not to recommend a woofer of 53 Hz (or any other) to use, but to demonstrate clearly the character of the extra low frequency response of the SAME WOOFER in enclosures of the SAME VOLUME, but different typical conceptions!

And then it’s easy to imagine that mounting such a woofer “with a free air resonance of like 20 Hz” in a bass-reflex cabinet on the same volume, tuned to some 17 Hz, there will be NO raising of its F free res., and the roll-off starting at 15 Hz… In any case, the “general image” of the FR sound pressure output will be the same as the graph cited… With, once more, a much more efficient extra-low frequencies segment (by the way, very appreciated for a full-range, linear sound spectrum), than in the same sealed box.

 

For a manufacturer designing from scratch all those choices are available. But for DIY or small mfg who must use off-the-shelf devices, there is very little out there in aftermarket woofers that have the right specs for Acoustic Suspension.

 

I knew someone would get around to mentioning the Advent Large speakers, excellent example indeed.

From my memory I believe the design challenge was to emulate an infinite baffle from an acoustic suspension design. Deep bass reproduction is the inherent characteristic.

Bass-reflex designs are popular from a cost basis because they inherently require a smaller enclosure. Perhaps this from wiki will help bass-reflex fans relate to the design criteria:

Infinite baffle
A variation on the 'open baffle' approach is to mount the loudspeaker driver in a very large sealed enclosure, providing minimal 'air spring' restoring force to the cone. This minimizes the change in the driver's resonant frequency caused by the enclosure. Some infinite baffle 'enclosures' have used an adjoining room, basement, or a closet or attic. This is often the case with exotic rotary woofer installations as they are intended to go to frequencies lower than 20 Hertz and displace large volumes of air.

Acoustic suspension
A variation of the closed-box enclosure, using a smaller box to exploit the almost linear air spring which results. The "spring" suspension that restores the cone to a neutral position is a combination of an exceptionally compliant (soft) woofer suspension, and the air inside the enclosure. At frequencies below system resonance, the air pressure caused by the cone motion is the dominant force. Although no longer popular in commercial designs, the acoustic suspension principal takes advantage of this relatively linear spring. The enhanced suspension linearity of this type of system is off-set by rather low efficiency. Drivers for these designs rely more upon the enclosure characteristics than typical drivers, and most modern woofers are not well suited to acoustic suspension use.

Bass reflex enclosure
Also known as vented (or ported) systems, these enclosures improve low-frequency output, increase efficiency, or reduce the size of an enclosure, using cabinet openings or passive radiating elements to transform and transmit low-frequency energy from the rear of the speaker to the listener. As with sealed enclosures, they may be empty, lined, filled or (rarely) stuffed with damping materials. Port tuning frequency is a function of cross-section and length. This enclosure type is very common, and provides the maximum deep-bass output for a given enclosure volume. Vented system design using computer modeling has been practiced since about 1985, when researchers Thiele and Small first systematically applied electrical filter theory to the acoustic behavior of loudspeakers in enclosures. While ported loudspeakers had been produced for many years before computer modeling, they rarely provided optimum performance, which is a complex sum of the properties of the specific driver, the enclosure and port, because of imperfect understanding of the assorted interactions. These enclosures are sensitive to small variations in driver characteristics and require special quality control concern for uniform performance across a production run.

http://en.wikipedia.org/wiki/Loudspeaker_enclosure

 

Great info here - thanks!

I figured if companies like B&W and Dynaudio use ported enclosures, that bass system can't be all that bad, though personally speaking I generally better enjoy the bass produced by sealed speakers.

I have noticed many sealed designs back then used heavy cones (I think AR's was made of a paper+felt hybrid) which lowered their resonance point but also contributed to lower efficiency. My own pair of Smaller Advents use a 9" woofer - yep nine inches - with a very large @5" dust cap made of a thick felt-ish material to add mass to the cone, which itself is not all that thick. Maybe this was Henry's solution to the light/stiff cone with a low-resonance issue?

Modern subs are what partly motivated my question. You see many subs today with say a 12" woofer but the enclosure is quite small compared to a conventional speaker with the same size woofer.....but then again, usually the woofer itself has much longer excursion capabilties; and I think many subs include some active EQ to extend their bass output (nothing wrong with that - whatever sounds good is fine with me).

The last sealed subwoofer I experienced was when I sold HT gear in the early 90s and we sold Cambridge SoundWorks' then-new sub with a 12" driver. It's amp only generated 120 watts so it couldn't provide high levels of bass, but the bass it *did* have was deep, clean and very smooth. This sub matched up well with music, though I thought movie soundtracks also sounded very good but volume-wise it lacked the power to keep up with the ported subs.

 

Thx for posting the chart. That helps explain the different behaviors I was wondering about.

Usually the woofers I've seen used in most ported enclosures utilize lighter cones with relatively less excursion vs. those used in a sealed enclosure (for example the 6.5" woofer in my sealed Baby Advents [1984 model year] had a thick cone with a very "mushy" surround; the Baby's had a low 86dB SPL rating but in return their bass was excellent for such a small speaker).

OTOH.......the 8" woofers in my ported Boston CR9s use a light cone, but their surround seems to be just as compliant as for a woofer used in a sealed enclosure. Confusing!

 

There are arguments to be made for all types, all dependent on price-point. I've heard many ported systems that can dish out bass but they don't sound convincing compared to any proper acoustic suspension I've heard. There was a great technical article/publication on the merits of each design type on I think it was audiokarma where the gist was that acoustic suspensions have significantly less distortion, and get significantly more bass, than the ported designs. The ported designs are capable, but they rely upon a trick that isn't perfect across the entire range. Essentially at some point the effect upon which they rely no longer works (non-linear performance) so the bass rolls off really quickly and distortion is significantly higher whereas the entire purpose of the acoustic suspension is to produce as linear a performance as possible.

Yes you can get low distortion ported speakers, but I have found the ones that actually pull this off are REALLY expensive.

My take is that bass reflex is just a technique to decrease size of the enclosure to save on shipping/transport cost. It is more expensive to implement a system that would beat a comparable acoustic suspension, so it seems silly. Acoustic suspension created the monster receivers of the 70s, and it was possible because the transistor was much much cheaper than tube power, but in the modern era much of stuff out there is just wimpy and can't really dish. I don't mean *our* stuff, I mean go to best buy... so it makes sense in a way.

Shame really, people hear the port noise and think its deep bass.

 

I set out to design a folded horn loudspeaker system once. The enclosure size required to meet my low bass requirements was far too impractical. I opted for a two-way bass reflex system with an L-pad controlled mid/high horn driver. My interest was not in linear response but rather as much sound as I could get with the cabinet designed around the resonant frequency of a 15" Pyle driver woofer. The enclosures ended up 4ft high, 3ft wide, and approximately 2ft deep. I use these as field monitors at the farm and over long distances (several acres) the sound very nicely fills the field. They really start to kick well with about 400 to 600 watts peak service. Over so many acres the distance smooths out non-linear response issues. The entire expense incurred building them was $500 back in the 80s and I suspect they'll last forever.

I think the bass-reflex design wasn't so much created to move away from acoustic suspension performance. I think it became popular as an alternative to folded horn designs.

 

perhaps, but it did come after AS right? Wasn't bass reflex a 60s era introduction while AS was in the mid to late 50s?

 

I think audio reproduction probably was prominent in theatres prior to systems for home use. I suspect the theatres had horn systems built into cavernous walls. My highschool auditorium was constructed this way.

I may be wrong in my critical assumptions, however as an audio enthusiast I would see the pursuit of powerful sound through bass reflex systems as a substitute for horns. Seconday refinements would have been a pursuit of accuracy. I would see that pursuit of accuracy as a reaction to the superiority of AS systems. I think the culture of home audio was another variant that acted somewhat independently from how these design directions were playing out. I think people were looking to break new ground in many ways. I think a most important element in AS design was the use of two way systems and not three way. There were many schools of thought bumping heads back then. I don't think what transpired was the result of objective qualification as much as it was faith based.