Cabinet bracing revisited…..

[lasse]

Hi,

I have posted before on this forum about my recent purchase of a pair of vintage AR-LST speakers from the 70ies.

The overall impression is impressive (!) When fed with a little more than ”casual-listening-power”, they come alive and deliver, especially after a period of burning-in after being in storage for many years (and som new caps and woofer surrounds)

But, as usual being an engineer, I investigate the system.

Now, this AR speaker is based on a principle, ”acoustic suspension”, that the air volume inside the box serves as a ”spring” and controls the 12” woofers behaviour. In this situation I believe that it is imporant that the cabinet is strong enough to hold the pressures inside and not to flex.

The back of the LST is 70x50 cm 18mm fiberboard and completely without bracing. On top of this, there is a internal plate in the center, holding the heavy XO. When feeding the speaker with bass-heavy music, one can physically feel that the backplate is flexing in/out. I feel that this is not the way that the woofer-part of this high-end speaker was intended to work.

What to do? I think that one has two ways to deal with this: One is to brace the backplate, probably on the outside. Doable but one has to dig into the box and attach screws from the inside to 2 or 3 glued 2”x2”-braces on the back.
Another option is to glue a 1” plate on top of the whole back-plate (and cut a hole for the connectors of course)

Please, give me your thoughts on this. Am I basically wrong in this? If not, which ”bracing-principle” do you recommend?

Best regards//lasse
Stockholm, Sweden
​

[andy19191]

Please, give me your thoughts on this. Am I basically wrong in this? If not, which ”bracing-principle” do you recommend?
​

It will almost certainly be inaudible. The flexing will be at low frequencies where are our ears are remarkably indifferent to all sorts of distortions. See figure 12 here for some data to help with estimates.

[billfitzmaurice]

Remove the woofer, glue two 30x30mm braces between the back and baffle. On the baffle place them at the driver screw holes at the 10:00 and 2:00 positions. This will stiffen both the back and baffle, while providing more secure fastening of the woofer. The effect on the back will be equivalent to doubling its thickness. Don't bother with spline braces, they're ineffective.

[andy19191]

Remove the woofer, glue two 30x30mm braces between the back and baffle. On the baffle place them at the driver screw holes at the 10:00 and 2:00 positions. This will stiffen both the back and baffle, while providing more secure fastening of the woofer. The effect on the back will be equivalent to doubling its thickness. Don't bother with spline braces, they're ineffective.

For the mode with the front and back moving together there will be no effective additional stiffness just lots of mass moving together. In addition this low frequency mode will radiate sound directly at the listener. Not a good idea. In order to stiffen the rear panel, braces would need to run from, say, the centre of the rear panel to, say, the sides, top and bottom panels, or the 4 corners of the baffle, or the centre of the 4 panel edges. It is unlikely to do anything audibly beneficial but at least it won't have the potential to do audible harm.

[andy19191]

Of course they move together if you add a couple of braces. What else are they going to do? To make matters worse the vibration of the driver is going to force this back to front mode efficiently. Side to side and top to bottom braces introduce similar kinds of modes but they don't get forced by the driver in the same way because of symmetry.

Tempted though I am to reply to your claim that you know how braces work it wouldn't be constructive. However I am currently developing, simulating and writing at some length on the engineering of speaker cabinets and will make a note to include a section on the pros and cons of the type of bracing you support. That should be a bit more constructive.

[billfitzmaurice]

Whether connected by braces or not the baffle and rear panel move in opposite directions, which is outward when the cone moves inward, inward when the cone moves outward, in response to the air within the enclosure being alternately compressed and decompressed. Tying them together with braces reduces how much they move. The baffle being of smaller area would have less movement than the back if the internal air pressure changes were the only force applied to it, but it's not. Thanks to Newton's Third Law the force that pushes the cone and voice coil in one direction also pushes the driver frame, and the baffle along with it, in the opposite direction. BTW, you've been claiming to be working on your opus magnum for at least five years. Tolstoy only need six years to write 'War and Peace'.

[Steve Lee]

"BTW, you've been claiming to be working on your opus magnum for at least five years. Tolstoy only need six years to write 'War and Peace'. "

Go get him, Bill!!

:D

HAHAHAHAHA!

[lasse]

Hi,

I thank you for your feedback.
Bracing is obviously something not totally aggreed upon.
I realize that there is a third option for me and that is to do nothing at all.

Regards//lasse
Stockholm, Sweden
​

[andy19191]

Bracing is obviously something not totally aggreed upon.
​

What bracing is/does can be independently and unambiguously evaluated from scientific principles or measurement. What is going on above is a difference in the knowledge bases people use to reason about bracing. I have put forward the engineering principles in terms of modes and how they are driven that I am using to reason about the problem. Bill has not been formally trained as an engineer and cannot debate as an engineer would by pointing out in engineering speak what assumptions of mine might be a bit wonky or what might not have been considered. (I am of course spot on with everything!). He puts it all aside in favour of what he considers from his interpretation of his past experience to have worked. It isn't resolvable unless we can all agree on a shared base of what is true on which to build a debate.

Having looked up the AR-LST it appears to be the side wall speakers for a 5.1 system with AR-3As as LCR speakers. Perhaps it was for one of the quadraphonic systems of the time?

[lasse]

Yes, as a naval architect I fully understand the complexity of vibration control in structures.

My professor always told us in structural design to "think in deformations" and that was why the external bracing was natural to me (like a ship webframe stucture)

I beleive one could see the LST as "a AR3a on stereoids" as they share the same drivers and XO apart frpm the transformer that enables the user to modify the energy distribution in different ways.

Regards//lasse

[andy19191]

Yes, as a naval architect I fully understand the complexity of vibration control in structures.

My professor always told us in structural design to "think in deformations" and that was why the external bracing was natural to me (like a ship webframe stucture)

If by that he meant examining how the modes deform the structure then yes that is the usual engineering approach. One also needs to consider which modes are or are not driven by the different kinds of forces present and in the case of speaker cabinets how those modes radiate sound.

To stick with the ship analogy the motion of the ship w.r.t. to the sea will induce large low frequency distributed forces on the structure that cause large deflections but this non-resonant movement doesn't result in much sound. This is what happens with the forces from the internal pressure in speaker cabinet which only become large enough to be significant in the stiffness controlled region below the resonances. If you look at the measurements of the sound from the cabinets in the low frequency stiffness controlled region in the BBC publication linked above it is so low it has disappeared off the bottom of the plot. The speaker measured by the BBC is likely to be both sealed and with unbraced panels although probably not using a woofer with a floppy suspension. So reasonably representative of your speaker. In addition an acoustic suspension speaker isn't going to be bothered at low frequencies whether sound comes from the driver or from the cabinet because it is effectively all moving together +/- 180 degrees when the wavelength of sound is so large compared to the size of the cabinet. The engineers that designed your speaker will have been more aware of this than we are because it is basic engineering knowledge.

Sticking with the ship analogy the majority of the sound heard comes from the engines driving the ship structure into various resonant motions. This is also what happens with speaker cabinets with the loudest sound coming from resonances that are primarily forced by the drivers vibrating the structure directly and not the forces from the internal air pressure which are relatively weak at these higher frequencies. In addition to this the ear is more sensitive to resonances at higher frequencies as shown in the cited figure which the BBC exploited in their design approach. Note the cabinet resonances are quite close to the threshold of audibility and are not particularly intrusive. Poor cabinet design w.r.t. resonance control doesn't usually create in-your-face audible deficiencies.

I beleive one could see the LST as "a AR3a on stereoids" as they share the same drivers and XO apart frpm the transformer that enables the user to modify the energy distribution in different ways.

For side speakers where the power response is more important they may well be a step up but when used as main speakers won't the large distance between the pairs of mids and tweeters cause significant issues for people sitting off axis?​

[djg]

Do you have any pictures?

[andy19191]

I was referring to the figures in the BBC paper cited in post 2. (This comment box doesn't seem to have a button to repeat the link). (This comment box did something weird when I used a hash).

[Billet]

If you decide to experiment with added bracing, I would suggest adding something that can be easily removed. It is quite possible that you will like the sound of the unbraced cabinets better.

[billfitzmaurice]

Since he has two boxes he can easily do an A/B comparison. Initially he can just screw in the baffle to back braces that I suggested, so they're easily removed. But if left permanently they should be glued as well, as screws have a tendency to loosen over time.