BBC style thin wall enclosure - how thin can one go?

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Milus

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Joined 2011
#1
I am about to order plywood for my speaker project, a subwoofer assisted two way speaker, with BBC-style thin wall enclosure. When researching the subject I found that as far as panel thickness goes Harbeth/Spendor/Rogers (so BBC contractors) used from 8mm in two way monitors to 12 mm plywood for larger boxes.

Now, from my local supply I can order only 7 mm plywood, even though its just 1 mm thinner than BBC spec, its as much as 12,5% difference from 8 mm panels used in commercial designs. So how thin can I go? I know that reducing panel thickness will lower the resonance frequency of the panel, which is a good thing according to BBC's approach, but there has to be some limitation, otherwise people would be using 3-5 mm panels to push the resonance below speaker's passband, which I haven't really seen in forums.

My two way design will cross over to sub, with Satori MW13P-8 in a bass reflex alignment tuned to around 70 Hz. Given relatively high tuning for this driver and some 5-5,5l net internal volume, simulations showed that I can use 2nd order BW highpass at some 130 Hz to land at -6 db at 80-85 Hz, where sub will take over.

It would a be a floorstanding design simply because my wife doesn't approve speaker stands. It will be 108 cm tall by 22 cm wide by 13 cm deep, but speakers will only use top 33 cm, so this subenclosure will also act as bracing at around 1/3 panel height. I plan to use 8 mm of alubutyl (2x 4 mm Silent Coat mat equivalent) damping on the inside to lower the amplitude of panel resonances. Front and back baffles would be mounted by screws, along BBC's design principles.

Your input would be much appreciated as I wanted to have panels cut this week to work on them during Christmas brake 🙂
 
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#2
Speaker building using resonant wood is not a simple affair. Its like building a good tuned cabinet, equally to a good musical instrument if you want the wood to resonate in a controlled way. I've never heard of a BBC approach in this field, but many companies have done it this way before small bookshelf speakers, totally damped and inefficient came in fashion. My speakers use 14mm spruce ply, but for smaller designs you can go smaller diameter. You want to use spruce, not pine wood. The best is without any limbs, those destroy the homogenity of the wood to resonate in a controlled fashion.
Experiment with very few damping material, and just in some areas where its necessary.

P. S. Nobody needs Alubytol mats on the inside of a good speaker. Makes no sense to me. If you want it stiff, then use MDF, otherwise its a resonant design with its own strengths that no stiff cabinet ever will have.
 
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#3
The thinner the material , the more important bracing & damping are. Consider gluing two layers of the 7mm ply together for the front and back (or use MDF for the back), and heavily bracing and damping the sides. I've often used 12mm but prefer 15. Anything is possible, but some routes take more effort 🙂
For a floorstander, I would be inclined to use heavier materials
 
#4
PeteMcK said:
The thinner the material , the more important bracing & damping are.
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This depends on what kind of cabinet should be achieved- a non resonant or a resonant one.
If you know guitar amps, than you might know that most of them are of the resonant type of cabinet.
If you want a non resonant cab, then we wouldn't have to discuss thin plywood enclosures at all. Use a stiff MDF cab and your done.
But if we talk about a resonant design, than it would become more complex.
Thin plywood in combination with heavy damping makes no sense at all, go for MDF.
But thin plywood and resonances makes some sense, and in this case, very little damping is the goal. The sidewalls should be damped by insertion of an inner structure like it is being done in every music instrument.
This is no easy job choosing materials and design the speaker, you should know what to do and how to. This can only be achieved by educating yourself.
Not with one question on an audio forum on a rainy afternoon.
 
#7
Right, for up to ~300-500 Hz [depending on who you ask] we want to push the cab's resonance at least an octave up and for the HF push the resonance down ~ an octave.

Without my Excel calculators I can't give specifics, just that stiffness increases at the cube of thickness as a rough guide, so find the material's specs you're interested in and use this calculator to find its modulus of elasticity [MOE] with the higher the number, the higher its stiffness, hence resonance: Composite Sandwich Panel Bending Stiffness & Deflection Calculator - Simple Supported

GM
 
#10
@xrk971 what You did follows the same lossy enclosure principle that I plan to use. How was bass from those foam core speakers? I've seen claims that thin wall boxes simply flex too much at bass notes, so was it slightly boomy, or with long decay, compared to plywood version or some more traditional speakers?

Thin damped walls are what I am aiming for, thin walls push the resonance lower in frequency, where its less audible, and damping pushes it lower in amplitude, resulting in a resonance-free sound. At least thats the theory. So damping is crucial. And seeing this foam core project it seems that its even more important than wall material itself.
 
#11
It can be, if you go for this particular approach. It's not exactly the most straightforward to design though, as for optimal results, you'll ususally need to spend some time on empirical testing for structural resonance and their amplitude, then determining how much bitumen you need. As I recall the BBC for example ended up specifying (literally: they were very specific) beech battons for some models; other woods for those particular applications had undesirable resonant modes.
 
#13
Thanks Lojzek, I did read it, that was the reason I choose plywood amd got the idea that I need to use damping of similar thickness and box panels but tbh I havent seen that much bitumen in Harbeths for example, rather a single layer. Fortunately in case of panel damping there is no penalty for using too much, and I have included extra volume for 8 mm damping on the inside. I also get that 7 mm ply would be more elastic (or less rigid), but still I don't know if it won't be too thin.

From my understanding 8-9 mm was also chosen for structural rigidity reasons, and I dont plan to move my speakers anywhere 🙂 Also, BBC had to specify wood types for battens because of differences in mechanical impedance, that is, as far as I understand, hardwoods were too stiff to conform to the lossy enclosure idea. I plan to decouple front and back baffles by using 2 mm butyl tape between baffle and fillets, and screw it together. After writing all of this I can see that I'm making quite a few assumptions, thats also why I wanted to ask for opinion on this forum 🙂
 
#14
Milus said:
Thin damped walls are what I am aiming for, thin walls push the resonance lower in frequency, where its less audible, and damping pushes it lower in amplitude, resulting in a resonance-free sound. At least thats the theory.
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Gray, dear friend, is all theory and green of life's golden tree.
Faust 1, Studierzimmer. (Mephistopheles)

What the BBC did is absolute bollocks in terms of good sound.
BBC is a broadcasting company, that doesn't mean they are specialists
or even experts on the best home audio sound.
Being there, done that. And it was worth nothing.
40 years of experience in loudspeaker cab building practise, not theory, tells me.
But everyone has to make his own experiences, so go on, try that design.
Doesn't matter if your young and could spend money.
But don't forget to tell the sonic results, if you really could compare to better designs.
Good luck!
 
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#15
From my understanding 8-9 mm was also chosen for structural rigidity reasons, and I dont plan to move my speakers anywhere Also, BBC had to specify wood types for battens because of differences in mechanical impedance, that is, as far as I understand, hardwoods were too stiff to conform to the lossy enclosure idea. I plan to decouple front and back baffles by using 2 mm butyl tape between baffle and fillets, and screw it together. After writing all of this I can see that I'm making quite a few assumptions, thats also why I wanted to ask for opinion on this forum
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I used white oak battens (rescued from a Civil War era barn demolition) and had them glued only at the 8 corner joints with PL Premium, and along the length with latex caulk for vibration damping. 9mm BB on all walls except 12mm on front. Extra ring of wood added to woofer mount backside for screws since flush rebate made it too thin. If I did again I would just use 18mm for front. All walls except front have butyl sheets and eggcrate foam.

How was bass from those foam core speakers? I've seen claims that thin wall boxes simply flex too much at bass notes, so was it slightly boomy, or with long decay, compared to plywood version or some more traditional speakers?
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If bracing of foam core is done right, bass is the same as with wooden cabinet. Probably wooden cabinet has some advantage for loud SPL as less energy lost to flex, however re radiation like a passive radiator at bass frequencies is sometimes a good thing. Both cabinets sound excellent. I think putting thin plywood (5mm) veneer in the foam cabinet might be best of both worlds. The foam core really does absorb the mid range internal energy better than a hard wood cabinet.
 
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#16
I did build a cabinet with 9mm birch ply for a fullrange driver, except the front and back that is 18mm birch ply, and used an internal frame as only bracing, but damped the thin walls with 4mm thick bitumen. And it's by far the best sounding cabinet i build for mid high (where i use it for). It's not resonating that i can hear or feel, even when it's standing on a bigger woofer cabinet of 18mm plywood. I'm planning to build a new 2 way that way also (but good plywood is hard to find now due to coivd down here). I think those thin wall mass damped cabinets are good for mid and top, and the classic thick wall heavy braced cabinets are good for bass and sub-bass. And that is how i intend to use it in the future for some of my builds (at least when the plywood stocks are back up down here...)
 
#17
Speaking for myself, I prefer high rigidity enclosures. However, stating the BBC's approach was 'absolute bollocks' I'm afraid just indicates a lack of knowledge of their research and development department & its work. It was certainly not 'absolute bollocks' and they had the data to back that up, as set out in

Harwood, H.D. & Matthews R., BBC RD 1977/3 Factors in the design of loudspeaker cabinets (British Broadcasting Corporation, Research Department, Engineering Division) January 1977.

It can be freely downloaded from the BBC's website here: http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf

As noted, I personally favour alternative approaches, but the damped thin-wall construction was not pulled out of thin air, and in engineering terms is a valid solution given the priorities it set out to address.
 
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