Do You Miss Acoustic Suspension Speakers?

[William Lemmerhirt]

I wasn't aware people actually used bing...

Lmao! Yep. When they don’t get the answer they want from googletron.
Just like when people don’t get the answer here. They hit avs. :neener:

 

[shkumar4963]

One of my favorite speakers ever were the Allison CD series. I remember my AR 38 and 58 very fondly. B&W and many subs are sealed acoustic suspension designs.

I still have Allison CD7 speaker. But I did not think it was an acoustic suspension speaker.

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[shkumar4963]

A sealed speaker is acoustic suspension! They have have been abandoned because ported designs have gotten much better and have for a long time not suffered from port "coloration". Ported designs offer better bass efficiency than acoustic suspension thus more db per square cm of driver. In essence they overcame the deficiencies they use to suffer from. As I pointed out previously they haven't disappeared at all, most subs still use today.

acoustic suspension - Bing

Intelligent search from Bing makes it easier to quickly find what you’re looking for and rewards you.

www.bing.com

Not all non-ported speakers are acoustic suspension. Acoustic suspension needs to provide speakers restraining force from sealed air.

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[shkumar4963]

Again read the attached article on acoustic suspension. If the box doesnt have a port, aka vented, aka bass reflex, its by definition a "acoustic suspension" design. And you are correct it is not a very common speaker design anymore but for subs its still prevalent. Its also part of the reason that good subs almost invariably include very high power class D amps. The latest DB1D, for example, has a 2kw amp! BTW acoustic suspension predated AR. One of AR's founders, Vilchur came up with the design a couple of years before AR was founded by himself and Kloss

This is the question you need to ask to decide if the design is acoustic suspension. What provides the restoring force to the speaker cone? Is it from the air or the from the spider? If the answer is air or mostly air, it is acoustic suspension speaker driver. If it is from the spider, it may be a sealed speaker design but not acoustic suspension.

Many modern sub are sealed design but not acoustic suspension.

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[Loren Banks]

Excellent article even though some of it is a little over my head. My first pair of speakers were AR speakers from the 80's. I've always preferred their tight clean bass and now I understand why.

 

[ciscocbee]

So I'm a little confused: I thought that Qts was basically inverse dampening, so higher numbers were "floppier" and better for sealed systems, while lower numbers gave stiffer drivers better for vented systems (for instance the Eminence speaker webpage but others as well). In this article you seem to be saying that lower Qts is better for sealed systems?

I also thought (possibly incorrectly) that as long as you matched Qts to the size of the sealed system to get a good Qtc then you wouldn't need extensive EQ to fix anything: I used 15" Dayton Ultramax (FS 19.5 Hz, Qts 0.47) and winISD to design my sealed box with a final Qtc of 0.65. Can't remember the volume now - they were built as end tables.

Regards,

 

[Kingnoob]

A sealed speaker is acoustic suspension! They have have been abandoned because ported designs have gotten much better and have for a long time not suffered from port "coloration". Ported designs offer better bass efficiency than acoustic suspension thus more db per square cm of driver. In essence they overcame the deficiencies they use to suffer from. As I pointed out previously they haven't disappeared at all, most subs still use today.

acoustic suspension - Bing

Intelligent search from Bing makes it easier to quickly find what you’re looking for and rewards you.

www.bing.com

Sealed subs work because a good driver is robust and powerful enough to overcome lack of efficiency. And do bigger boxes like 5! Cubic foot still acoustic suspensed?


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[kaiser_soze]

The goal of the acoustic suspension speaker is for the air spring effect of the sealed enclosure to supplant the suspension spring to the greatest extent practical. The driver's bowl-shaped "stiffness of suspension" curve is replaced by a curve that is V-shaped except for near the tips, where the suspension spring will always dominate. The restoring force thus becomes substantially more linear throughout most of the diaphragm's range of motion. Distortion is correspondingly reduced.

The driver's compliance may be expressed either in the natural style of distance per unit of applied force, or else in terms of equivalent volume. Vas is the volume of air that has the same stiffness and the same compliance as the driver, when the piston compressing the air has the same face diameter as the diaphragm. When the driver's compliance is expressed this way it is known as Vas. This way of expressing compliance allows direct comparisons of the driver's compliance with the compliance of the enclosure, which is simply its volume, Vb.

The goal is thus to make the Vas/Vb ratio as great as it can be made. This ratio equates to ((Qtc/Qts)^2 - 1), and is thus maximized by making Qts as small as it can be made (and potentially by allowing Qtc to be bigger, i.e., allowing the system resonance to be somewhat stronger). There is no defined threshold by which some sealed speakers are deemed acoustic suspension speakers and others not. All sealed speakers make use of the principle. The Vas/Vb ratio for sealed speakers will typically be in the ballpark of 2:1 or better even when no particular effort is made to improve this ratio. If maximization of the Vas/Vb ratio is identified as a primary goal in driver selection, this ratio can be as great as 6:1 for regular woofers and 4:1 for subwoofers, both with Qtc = .71. (These are good estimates that came about through years of perusing the T/S parameters of off-the-shelf drivers.) To achieve a Vas/Vb ratio much greater than these values it is generally necessary to adopt a higher Qtc value, e.g., .9.

The reason that subwoofers cannot generally achieve Vas/Vb ratios as great as the ratios that regular woofers can achieve is that deep bass extension prefers a large value for Qts. The reason for this is that the -3 dB point in the rolloff (F3) is related to the free air resonance (Fs) by way of a "stretch factor" that depends primarily on the value of Qts. The stretch factor is about 2 for Qts = .35, is less than 2 for greater values of Qts, and is greater than 2 for smaller values of Qts. For example if Qts = .35 and Fs = 20 Hz, F3 will be about 40 Hz. Deep bass extension demands both a low value of Fs and a small stretch factor; a small stretch factor demands a high value of Qts, which is at odds with the goal of a large Vas/Vb ratio.

(I've made things too simple. The only way to keep Fs low, when high compliance is not an option, is by increasing the mass of the diaphragm and cone assembly - or equivalently by using a bigger diaphragm to improve acoustic coupling, thereby increasing the effective mass of the diaphragm. If nothing is done to compensate for the increase in Mms, Qts will increase. Which isn't necessarily a bad thing, within limits, since a bigger Qts value means a smaller stretch factor, which is generally desired when a very low Fs is desired.)

Modern drivers that aren't nearly as compliant as those older drivers can still achieve Qts values as low as that type of driver that Kloss preferred, while simultaneously achieving similarly low values for Fs. However, there is a rub. The adjustments that are needed, to compensate for the low compliance, cause an increase in the oscillatory kinetic energy of the driver (at a given frequency and amplitude). This has to be dealt with via greater damping, i.e., a greater rate of extraction of oscillatory kinetic energy. Greater damping always implies a greater rate of energy extraction and therefore diminished efficiency.

The legitimate gripe with modern drivers, for anyone wanting to build speakers or subwoofers that adhere strongly to the acoustic suspension principle, is the diminished efficiency. Modern switching amplifiers are vastly more efficient than class A/B amplifiers (evidenced by the small size of the power transformers), and this is an overwhelmingly stronger effect than the diminished driver efficiency. The diminished efficiency remains undesirable, however the silver lining is that the enclosure will typically be less than half as large as the enclosures needed by those older, high-compliance drivers. If you are concerned about efficiency, then you need to pay attention to theoretical efficiency when selecting the driver. In general this will be a driver with a high Vas value, which generally implies a large enclosure.

 

[shkumar4963]

So I'm a little confused: I thought that Qts was basically inverse dampening, so higher numbers were "floppier" and better for sealed systems, while lower numbers gave stiffer drivers better for vented systems (for instance the Eminence speaker webpage but others as well). In this article you seem to be saying that lower Qts is better for sealed systems?

I also thought (possibly incorrectly) that as long as you matched Qts to the size of the sealed system to get a good Qtc then you wouldn't need extensive EQ to fix anything: I used 15" Dayton Ultramax (FS 19.5 Hz, Qts 0.47) and winISD to design my sealed box with a final Qtc of 0.65. Can't remember the volume now - they were built as end tables.

Regards,

No. Not all port free speakers are acoustic suspension. In fact nowadays, most ARE NOT.

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[Kingnoob]

No. Not all port free speakers are acoustic suspension. In fact nowadays, most ARE NOT.

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Yeah it’s sad but modern speaker designers just are too lazy to go back to basics , they want bigger badder , more rms more xmax .
This speaker design is probably extinct except in very professional speaker designers .

Most are ported these days , my sealed subwoofer is far too large to be acoustic suspension but I have no problems with bass .
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[kaiser_soze]

No. Not all port free speakers are acoustic suspension. In fact nowadays, most ARE NOT.

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Is this because you say so? Or is there a reason that you might be willing to share? You may find my previous post informative. If you are going to say that not all sealed speakers are acoustic suspension, then you need to share the criteria by which some are and some are not. This criteria would have to be based on the Vas/Vb ratio, i.e., if this ratio is at least X the speaker is acoustic suspension, otherwise not. Then you would have to justify the particular value of X that you chose. Most any sealed speaker will have a Vas/Vb ratio of at least 2:1 even if no particular effort was made to maximize the Vas/Vb ratio. Why shouldn't a speaker where more than half of the total restoring force is from the enclosure be considered acoustic suspension?

The acoustic suspension principle applies to all speakers that use sealed enclosures. In all such speakers, the restoring force, for low-signal behavior where the movement of the diaphragm is small, is obtained mainly from the air in the enclosure. As the signal increases and the excursion peaks become greater, there is a gradual transition such that eventually the driver's suspension becomes the stronger of the two (at the peaks of excursion). The point where the two partial restoring forces become equal is different for different acoustic suspension speakers. It would be silly to say that speakers where this transition occurs at comparatively low signal level are not true acoustic suspension speakers. But this is effectively what you are saying.

 

[kaiser_soze]

So I'm a little confused: I thought that Qts was basically inverse dampening, so higher numbers were "floppier" and better for sealed systems, while lower numbers gave stiffer drivers better for vented systems (for instance the Eminence speaker webpage but others as well). In this article you seem to be saying that lower Qts is better for sealed systems?

I also thought (possibly incorrectly) that as long as you matched Qts to the size of the sealed system to get a good Qtc then you wouldn't need extensive EQ to fix anything: I used 15" Dayton Ultramax (FS 19.5 Hz, Qts 0.47) and winISD to design my sealed box with a final Qtc of 0.65. Can't remember the volume now - they were built as end tables.

Regards,

One of the points of confusion that I often see in forums such as this one has to do with "stiffness". Damping is important, but stiffness refers to how strongly a driver refuses to yield to an applied force. This is what springs do, like the one in your ink pen, or the ones in your car's suspension. The stiffness of a driver refers to the strength of its spring, and nothing more. Stiffness and damping both influence the driver's total Q, i.e., its Qts. To oversimplify, stiffness is bad for an acoustic suspension speaker, because we want the stiffness of the air in the enclosure to dominate the stiffness of the suspension. Damping is good, for reasons much more esoteric. But even though it may not be apparent why damping would be good, it is intuitively apparent that high damping and low suspension stiffness are consistent in their effect on Qts. I.e., a spring that is less stiff implies a less severe resonance, and greater damping also implies a less severe resonance. Thus, it is good for the driver not to be stiff, i.e., good for it to be highly compliant, and also good for it to have a good amount of intrinsic damping, but only up to a point. The damping that is relevant, by the way, consists almost entirely of the electrical damping. Absorption of energy within and by the suspension is practically negligible. By and large, the suspension provides the spring and the motor provides the damping.

The extent to which the acoustic suspension principle is adhered is quantitatively expressed via the Vas/Vb ratio. Vas and Vb are both expressions of compliance, which is the inverse of stiffness. It is kind of odd to think of the enclosure volume as compliance, but the driver's compliance expressed in the natural manner as the reciprocal of its stiffness can be expressed alternatively in terms of the equivalent amount of air, i.e., the amount of enclosed air that has the same compliance as the driver (when the piston pushing on the air has the same diameter as the driver's diaphragm). By expressing the driver's compliance in this way, the enclosure's compliance is simply its volume. Thus, the ratio Vas/Vb is effectively the inverse of the ratio of the stiffness of the enclosure to the stiffness of the suspension. We want the stiffness of the enclosure to be much greater than the stiffness of the suspension, which is the same as saying that we want the Vas/Vb ratio to be as large as we can reasonably make it. The Vas/Vb ratio is:

Vas/Vb = (Qtc/Qts)^2 - 1

It is thus apparent that given a choice for Qtc, it boils down to a question of how small Qts is, which is to say, a question of how mild the driver's resonance is. The milder the driver's resonance, the more closely the speaker will adhere to the acoustic suspension principle. I'm not going to try to explain how Qts is fully determined, but it is necessary to keep damping and stiffness separate. Greater stiffness implies a stronger spring, lower compliance, a more severe resonance and a bigger value for Qts. Greater damping implies a less severe resonance and a lower value for Qts.

I'm tempted to delete all this, but I'm resisting the temptation on the off chance that this might be helpful. If not, and if all I managed to do here was to make things more confusing, I apologize. It really is better to just study the equations. The writeup on T/S parameters on wikipedia is very good, although it is the sort of thing that you have to tackle a little at a time.

 

[Verdinut]

The acoustic suspension principle applies to all speakers that use sealed enclosures.

That's inexact. A real acoustic suspension woofer depends solely on the entrapped air in the enclosure. I don't know of any manufacturer building such designs nowadays. Acoustic Research in the 1970's was the major company marketing such speakers.

Altec Lansing, the defunct pro audio and theater loudspeaker manufacturer, released a real acoustic suspension woofer for home use in the 1970's. The cone of that 15 inch woofer when you put it on a table would just drop and touch the table top, as there was no stiff spider to hold it. Only the foam surround kept it fixed to the basket:

http://greatplainsaudio.com/wp-content/uploads/2016/06/411-8A_lfspkr_spec_sheet.pdf

IIRC, today all sealed woofers have a stiffer spider, foam suspension has disappeared and drivers are built to withstand a lot more power than in the 1970's. I suggest that you peruse the posts which have been written above in this thread.

 

[kaiser_soze]

That's inexact. A real acoustic suspension woofer depends solely on the entrapped air in the enclosure. I don't know of any manufacturer building such designs nowadays. Acoustic Research in the 1970's was the major company marketing such speakers.

Altec Lansing, the defunct pro audio and theater loudspeaker manufacturer, released a real acoustic suspension woofer for home use in the 1970's. The cone of that 15 inch woofer when you put it on a table would just drop and touch the table top, as there was no stiff spider to hold it. Only the foam surround kept it fixed to the basket:

http://greatplainsaudio.com/wp-content/uploads/2016/06/411-8A_lfspkr_spec_sheet.pdf

IIRC, today all sealed woofers have a stiffer spider, foam suspension has disappeared and drivers are built to withstand a lot more power than in the 1970's. I suggest that you peruse the posts which have been written above in this thread.

I knew I should not have jumped in here to try and clear up some of this nonsense. I should have known that if I did, that no matter how much effort and time I put into giving an explanation that was fully correct and derived from a reasonably good understanding of elementary physics and of the physics of loudspeakers, that some person would respond with something like "That's inexact", or similar B.S.

Given that you did that, I will repeat, and this time in bold:

The acoustic suspension principle applies to all speakers that use sealed enclosures.

That statement is absolutely, 100% true. I will try to explain to you why it is, but it ought to be obvious to anyone who would bother to stop and think about it for a minute or two before opining.

Now I will explain. It is simply this. In ANY speaker that is sealed, the air spring effect contributes appreciably to the total restoring force. I explained this fairly well in the first post I wrote above, which you either didn't read or didn't understand. So I'll try again and I'll try to keep it simple, using an example. Let us suppose that the driver's Vas is equal to the enclosure volume Vb. In this case the compliance of the suspension and the compliance of the air sealed in the enclosure will be equal. The point of expressing compliance as equivalent air volume is so that direct comparisons of this sort will be possible. So if Vas and Vb are equal, the total restoring force will be twice greater than it would be with either the suspension or the enclosure acting alone, and the two partial contributions to the restoring force will be equal. Hopefully you understand this and agree up to this point, but if not, then there isn't much point of us trying to have a dialog. Now, if the two partial contributions to the total restoring are equal, what are we able to infer about a speaker where this is true? Since Vas and Vb are equal, the ratio Vas/Vb will equal 1. As such, the following expression, which equates to the Vas/Vb ratio, will equal 1:

1 = (Qtc/Qts)^2 - 1

Now, you can hopefully see that in this case, the ratio Qtc/Qts will be equal to the square root of 2, approximately 1.414. If we make a reasonable assumption about Qtc, that it is half of the square root of 2, then Qts will be equal to 1/2. Thus, if Qts is equal to .5 and Qtc is equal to .7071, the suspension and the air in the enclosure will have the same compliance and will contribute equally to the total restoring force. Now of course you can get different results by using a different value for Qtc. If you make Qtc bigger, the enclosure becomes smaller, and the air spring effect becomes stronger relative to the suspension. You could also make Qtc smaller, in which case the balance would shift the other way. But .71 is a typical value for Qtc, so it is reasonable to say that in general, the air spring effect will contribute at least 50% of the total restoring force so long as Qts is not greater than .5.

Even though I tend to be very exact with what I say and was careful here with what I wrote, you responded by saying "That's inexact", then you wrote:

That's inexact. A real acoustic suspension woofer depends solely on the entrapped air in the enclosure.

At face value that is preposterous, because at face value it is not possible. You are claiming that in a "real" acoustic suspension speaker that somehow the suspension's natural spring effect disappears altogether. How exactly do you propose this happens? It does not happen. In fact, the dominance of the air spring effect, in any acoustic suspension speaker, applies only for low signal behavior. As the signal level increases and excursion increases along with it, the stiffness of the suspension increases more rapidly than the stiffness of the air in the enclosure. Eventually, at the extremes of excursion, the suspension becomes the dominant source of the restoring force, no matter how strongly the speaker adheres to the acoustic suspension principle.

Then you wrote:

I suggest that you peruse the posts which have been written above in this thread.

That's blatantly insolent. I'm tempted to tell you where you can kiss me, but I'm fighting off the urge to do that. In all of the posts in this thread, in response to Mr. Feinstein's article, there was just one that was informed and that provided meaningful insight into this question. It was of course the first the post I wrote, which you obviously did not read, because, most likely, it was beyond your technical comprehension. You actually believed that in an acoustic suspension speaker that 100% of the restoring force derives from the air spring effect. If you had thought about it for even half a minute you would have realized that this cannot possibly be true.

It is further untrue that a driver absolutely must have a high-compliance suspension in order for it to be useable for an acoustic suspension speaker (even a "true" one). This is a false notion that circulates in Internet forums like so many other false notions. If you earnestly study the matter you will realize that what this is all about is this ratio:

Vas/Vb

Which is equal to this:

Vas/Vb = (Qtc/Qts)^2 - 1

It is thus apparent that any two drivers that have the same Qts value (and for which Qtc in the speaker is the same) will adhere equally to the acoustic suspension principle. If you study Qts, Qes, and Qms, you will find that although a big value of compliance is a huge, huge benefit in achieving a low Qts value, that it isn't absolutely essential. It can be done, and it is done, with stiff suspensions, however the speaker will not be as efficient. The true difference in the old and the new is the difference in efficiency, yet when people on the forums start lamenting the loss of the highly compliant suspensions, they almost never mention efficiency. The reason they don't is because they don't have sufficient technical ability to figure out that you can have the same Vas/Vb as those earlier acoustic suspension speakers and that when you do the true differences are that the speaker built using newer, stiff drivers will not be as efficient (and the enclosures will be much smaller).

That said, there is also the related but separate question of deep bass extension. There is an inherent tradeoff between deep bass extension and adherence with the acoustic suspension principle. This has always been true. It is no different now from what it was fifty or sixty years ago. To adhere strongly to the acoustic suspension principle you need for Qts to be small. But the smaller the value of Qts, the greater the factor by which Fs is multiplied to obtain F3. Herein lies the real challenge of building acoustic suspension speakers. It is not especially difficult to build a midrange speaker or a mid-woofer speaker or even a conventional woofer speaker, adhering very closely to the acoustic suspension principle, because there are plenty of off-the-shelf drivers with adequately low Qts values, many below .2. Where it becomes difficult and challenging is with building a sealed subwoofer that will be flat in response right down to 20 Hz (or thereabouts) while at the same time adhering strongly to the acoustic suspension principle. This is a difficult thing to do, and this may well be more difficult today than it was fifty or sixty years ago, because it is more difficult to achieve a low value Fs without the benefit of a big value of Cms, the compliance.

Thus, if you want to lament the loss of high compliance drivers, and you want to do it in a way that will inform other people who understand this stuff that you also understand it, you should make the lament specifically about efficiency and about the difficulty in achieving a suitable low Fs value.

 

[Verdinut]

I knew I should not have jumped in here to try and clear up some of this nonsense. I should have known that if I did, that no matter how much effort and time I put into giving an explanation that was fully correct and derived from a reasonably good understanding of elementary physics and of the physics of loudspeakers, that some person would respond with something like "That's inexact", or similar B.S.

Given that you did that, I will repeat, and this time in bold:

The acoustic suspension principle applies to all speakers that use sealed enclosures.

That statement is absolutely, 100% true. I will try to explain to you why it is, but it ought to be obvious to anyone who would bother to stop and think about it for a minute or two before opining.

Now I will explain. It is simply this. In ANY speaker that is sealed, the air spring effect contributes appreciably to the total restoring force. I explained this fairly well in the first post I wrote above, which you either didn't read or didn't understand. So I'll try again and I'll try to keep it simple, using an example. Let us suppose that the driver's Vas is equal to the enclosure volume Vb. In this case the compliance of the suspension and the compliance of the air sealed in the enclosure will be equal. The point of expressing compliance as equivalent air volume is so that direct comparisons of this sort will be possible. So if Vas and Vb are equal, the total restoring force will be twice greater than it would be with either the suspension or the enclosure acting alone, and the two partial contributions to the restoring force will be equal. Hopefully you understand this and agree up to this point, but if not, then there isn't much point of us trying to have a dialog. Now, if the two partial contributions to the total restoring are equal, what are we able to infer about a speaker where this is true? Since Vas and Vb are equal, the ratio Vas/Vb will equal 1. As such, the following expression, which equates to the Vas/Vb ratio, will equal 1:

1 = (Qtc/Qts)^2 - 1

Now, you can hopefully see that in this case, the ratio Qtc/Qts will be equal to the square root of 2, approximately 1.414. If we make a reasonable assumption about Qtc, that it is half of the square root of 2, then Qts will be equal to 1/2. Thus, if Qts is equal to .5 and Qtc is equal to .7071, the suspension and the air in the enclosure will have the same compliance and will contribute equally to the total restoring force. Now of course you can get different results by using a different value for Qtc. If you make Qtc bigger, the enclosure becomes smaller, and the air spring effect becomes stronger relative to the suspension. You could also make Qtc smaller, in which case the balance would shift the other way. But .71 is a typical value for Qtc, so it is reasonable to say that in general, the air spring effect will contribute at least 50% of the total restoring force so long as Qts is not greater than .5.

Even though I tend to be very exact with what I say and was careful here with what I wrote, you responded by saying "That's inexact", then you wrote:



At face value that is preposterous, because at face value it is not possible. You are claiming that in a "real" acoustic suspension speaker that somehow the suspension's natural spring effect disappears altogether. How exactly do you propose this happens? It does not happen. In fact, the dominance of the air spring effect, in any acoustic suspension speaker, applies only for low signal behavior. As the signal level increases and excursion increases along with it, the stiffness of the suspension increases more rapidly than the stiffness of the air in the enclosure. Eventually, at the extremes of excursion, the suspension becomes the dominant source of the restoring force, no matter how strongly the speaker adheres to the acoustic suspension principle.

Then you wrote:



That's blatantly insolent. I'm tempted to tell you where you can kiss me, but I'm fighting off the urge to do that. In all of the posts in this thread, in response to Mr. Feinstein's article, there was just one that was informed and that provided meaningful insight into this question. It was of course the first the post I wrote, which you obviously did not read, because, most likely, it was beyond your technical comprehension. You actually believed that in an acoustic suspension speaker that 100% of the restoring force derives from the air spring effect. If you had thought about it for even half a minute you would have realized that this cannot possibly be true.

It is further untrue that a driver absolutely must have a high-compliance suspension in order for it to be useable for an acoustic suspension speaker (even a "true" one). This is a false notion that circulates in Internet forums like so many other false notions. If you earnestly study the matter you will realize that what this is all about is this ratio:

Vas/Vb

Which is equal to this:

Vas/Vb = (Qtc/Qts)^2 - 1

It is thus apparent that any two drivers that have the same Qts value (and for which Qtc in the speaker is the same) will adhere equally to the acoustic suspension principle. If you study Qts, Qes, and Qms, you will find that although a big value of compliance is a huge, huge benefit in achieving a low Qts value, that it isn't absolutely essential. It can be done, and it is done, with stiff suspensions, however the speaker will not be as efficient. The true difference in the old and the new is the difference in efficiency, yet when people on the forums start lamenting the loss of the highly compliant suspensions, they almost never mention efficiency. The reason they don't is because they don't have sufficient technical ability to figure out that you can have the same Vas/Vb as those earlier acoustic suspension speakers and that when you do the true differences are that the speaker built using newer, stiff drivers will not be as efficient (and the enclosures will be much smaller).

That said, there is also the related but separate question of deep bass extension. There is an inherent tradeoff between deep bass extension and adherence with the acoustic suspension principle. This has always been true. It is no different now from what it was fifty or sixty years ago. To adhere strongly to the acoustic suspension principle you need for Qts to be small. But the smaller the value of Qts, the greater the factor by which Fs is multiplied to obtain F3. Herein lies the real challenge of building acoustic suspension speakers. It is not especially difficult to build a midrange speaker or a mid-woofer speaker or even a conventional woofer speaker, adhering very closely to the acoustic suspension principle, because there are plenty of off-the-shelf drivers with adequately low Qts values, many below .2. Where it becomes difficult and challenging is with building a sealed subwoofer that will be flat in response right down to 20 Hz (or thereabouts) while at the same time adhering strongly to the acoustic suspension principle. This is a difficult thing to do, and this may well be more difficult today than it was fifty or sixty years ago, because it is more difficult to achieve a low value Fs without the benefit of a big value of Cms, the compliance.

Thus, if you want to lament the loss of high compliance drivers, and you want to do it in a way that will inform other people who understand this stuff that you also understand it, you should make the lament specifically about efficiency and about the difficulty in achieving a suitable low Fs value.

Have a look at this thread:

Sealed is Not Acoustic Suspension in Loudspeakers

Some vented speakers offer port plugs for sealing the cabinet and making them act like sealed speakers. This does not result in a true acoustic suspension. What's the difference? Read on to find out.

www.audioholics.com

 

[kaiser_soze]

I came back to clear up something I wrote before that I wrote too hastily. I wrote:

"... . The driver's bowl-shaped "stiffness of suspension" curve is replaced by a curve that is V-shaped except for near the tips..."

It will be best for me to erase this and write over it, so to speak.

In driver test's such as the excellent tests that Vance Dickason does, you will usually see a "stiffness of suspension" graph, which is the graph of Kms. By definition Kms is the ratio of the force (the restoring force of the suspension) to the displacement (the distance the diaphragm has moved from the rest position). A glance at any of these graphs reveals that Kms depends greatly on the displacement, which is to say that it is not an ideal spring, i.e., it does not obey Hooke's law, which assumes a constant ratio of force to displacement, i.e., a constant Kms value as applied to a loudspeaker suspension.

To obtain the force value itself, or a graph of the force value itself, from the Kms graph, you multiply each Kms value by the corresponding value of displacement (the value on the x-axis). The condition for avoidance of distortion is for the force curve to be linear. If you graph a linear force curve it will be a straight line with constant positive slope, located in the lower left quadrant (of a Cartesian coordinate graph) for displacement in the negative direction and in the upper right quadrant for displacement in the positive direction. (It doesn't really matter which direction of displacement you deem positive and which direction you deem negative.) This straight line will pass through the origin of the x/y graph.

Stiffness, or Kms, is force divided by displacement. Since force will be negative when and only when displacement is negative, it follows that Kms will be positive everywhere. Since Kms is the slope of the force graph, it follows that if the force graph is linear, as required in order for distortion to be avoided, the Kms graph will be constant, i.e., a flat line, at every point equal to the constant ratio of force to displacement. The graph of Kms, showing the ratio of force to displacement for all values of displacement, should be a flat line, because the ratio of force to displacement should be the same no matter the displacement, if the restoring force is to be linearly related to displacement.

What we see in real Kms curves is nothing like a flat line. What we see is a bowl-shaped curve. So what happens to this funky curve when the driver is mounted in an acoustic suspension enclosure? Since the stiffness of the air in the enclosure is almost perfectly linear with respect to displacement, the Kms curve gets pushed upward by a uniform amount throughout the curve, and by an amount that is several times greater than the nominal Kms value, depending on the exact value of Vas/Vb. The shape of the Kms curve doesn't change. It does not become flatter, however in EFFECT it becomes flatter. Why? Because the correct way to interpret the deviation from flatness, in the Kms curve, is relative to the nominal value, i.e., the change in Kms as a percentage of the nominal value. Since the midpoint of the graph is pushed higher, the nominal value is increased by a factor that depends on the actual Vas/Vb value. The nominal value of Kms increases according to that ratio, the Vas/Vb ratio, which means that the deviation from flatness, which occurs as displacement increases in either direction, becomes a smaller percentage of the nominal Kms value. In other words, the Kms curve is made flatter in effect, by pushing the curve upward by a factor that is equal to the Vas/Vb ratio.

As I have said repeatedly, the extent to which this effect occurs is determined by the Vas/Vb ratio, which in turn depends on the Qtc/Qts ratio. In most any sealed speaker where no particular effort has been made to adhere closely to the acoustic suspension principle but where reasonable choices have been made with respect to the Qts value, toward obtaining a cabinet that isn't huge for example, the Vas/Vb will most likely be 2 in the least. Thus, for most any sealed speaker, the effective Kms curve, of the driver mounted in the enclosure, will be at least twice as flat as the driver's true Kms curve, when the deviation from flatness is correctly interpreted as a percentage of the nominal value. The improvement factor will probably be greater than 5 for most sealed speakers that are designed specifically toward close adherence to the acoustic suspension principle. As best as I've been able to determine, the Vas/Vb ratio for the original Advent (later the Large Advent) was in the ballpark of 7. This is partly because Kloss liked to use a somewhat large Qtc value of about .9. This is what the "voicing" of the speaker was really about, choosing how small to make the enclosure, equivalently choosing how large the Qtc value should be, such that the amount of boost in mid-bass would be deemed pleasant and not so excessive as to be overt. Consumer Reports gave the Advent very good ratings in the early '70s, because of the unusually low level of bass harmonic distortion, but later in the early '80s, they downgraded the Large Advent because of the mid-bass emphasis. This greatly annoyed Kloss, and they ended up in a sort of public brawl. Kloss steadfastly defended the mid-bass emphasis on the grounds that it flattened the in-room response.

 

[shadyJ]

An interesting Youtube video that I found which relates to the subject of this article:

 

[dlaloum]

I enjoy my Gallo Reference 3.2 mains, and my Gallo Reference AV Center and TR1 sub.

All are acoustic suspension / sealed designs for Sub/Woofer and Mids.

The still current Gallo Strada speakers are similar - but are designed as satellites... no (real) woofer.

With the arrival of active reflection cancelling methods, such as Dirac ART, and the latest Trinnov, the gentle rolloff of AS speakers is about to become a major advantage.

The active cancellation does not require high output, but it does take advantage of extension - so the lower output but more extended performance profile of typical AS woofers, is going to be something that people can truly take advantage of... paradigm shifting stuff...

 

[fmw]

All B&W subs are sealed enclosures so are Sunfire among others.

Current ones perhaps. I have a 12" ported B&W sub in my bedroom.

 

[highfigh]

Here is Steve's statement:
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.

That is inexact. Low compliance has no direct relation with a high Qts:

For example, Altec Woofers: 411A Rs: 18.3 (high compliance) Qts: 0.33

515-8GHP Rs: 37 (lower compliance) Qts: 0.19

The Qts is related to the ratio magnet weight to moving mass/suspension. The magnet strength is the important factor. A bigger magnet means more sensitivity, better transient response and a lower Qts.

Altec Thiele-Small Parameters

And to make it somewhat understandable, this is found by using the formula Qts = (Qes x Qms/Qes + Qms)