huh? in what sense? RE: the papers bit, what I meant was Ive read it lol. Its quite easy for many DIYers here to agree with me that typical music has varying spectral compositions, and in this way vibration (from the exciter) in the typical home, does not match testing with say pink noise, i.e. we do not listen to boxes with pink noise for pleasure....i think 😀 So the point of stiffening to push resonances up (which is harder than to take them down with mass, I agree), is that there is perhaps 40% of the energy is at 1kHz and over, and 60% below this point. So less energy to excite the panels.
I should clarify what I meant in earlier posts. I was on my mobile, it doesnt make sense, I re-read it.....
What I am saying is that like electrical theory, the resonant 'reactances' you mention, are equal and opposite magnitude, in antiphase, +j or -j.
Correct?
Im agreeing with you here.
Like and electrical load, at resonance all that remains is the resistive component, the loss of the material.
Any excitation of resonance has to overcome this 'loss threshold' resistance, if it does not the energy is totally dissipated by heat, i.e. loss is 100%
I do not know for certain, but I assume the similarities/analogy with electrical theory go further.
I think of the loss parameter in tandem with the driving/excitation force/energy. I little like the loss in electrical wiring. I²R loss. Loss is dependant on excitation.
If this is the case then Mass will affect the amplitude of the resulting colouration.
It may simply be that a light (relatively) MDF could outperform a much thicker heavier one, as the added mass (if equal in both cases) would represent a larger increase in panel mass, relative to one made of twice the thickness MDF. Thus providing more mass damping relative to the panel starting mass.
Also it could mean that amplitude of colouration (OUTSIDE resonance) could be less in a very heavy cabinet. Q at resonance may be greater, or amplitude at resonance may be larger, everywhere else in the spectrum would require far greater energy to excite sympathetic vibration. Sounds like a good arguement for a heavy, well damped, but not necessarily stiff cabinet.
Please correct me if I am incorrect.
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I considered you a convert when you acknowledged that concrete would need significant damping additions to make a good cabinet material. That was, after all, the first comment I made in the thread several pages back (re. a special construction concrete with polymer additions so that it wouldn't "ring like a bell").
Not sure where you are going with coloration outside of resonance. The colorations are the resonances. At resonance the breakthrough is highest and the high Q (narrow band) nature guarantees that it will stretch out in time domain (i.e. ring). Between resonances the level is low and of no consequence.
Both Harwood and Sowter (see the Collums book) show the only apples and apples comparisons that I know of. They made two cabinets with the same exact outter dimmension, of the same material, but with one double the thickness of the other. If I remember correctly the mass doubles as you double material thickness (obviously) and the stiffness increases to 8 times (cube of the thickness). This means that the simple resonances will double in frequency or move up one Octave. Look at the curves and you will see that the breakthrough spectra look similar but shifted. There is no less energy between the resonances, the resonance peaks are not less due to the higher mass (or stiffness). In the relatively undamped case the increase in wall thickness (or stiffness, or mass) did not do anything that we might hope. It did not reduce breakthrough. Resonances were still there, they were just as strong, they were the same Q, just shifted up a little. Harwood then shows that damping was less effective on the thicker cabinet, since its mechanical impedance is inherently higher.
If anybody knows of any "modern" papers that contravene this, I'd be interested in seeing them.
David
p.s. I'll be going out for the weekend so if anyone wants to repost any of my previous responses every 5 or so posts, I'd appreciate it.
Not sure where you are going with coloration outside of resonance. The colorations are the resonances. At resonance the breakthrough is highest and the high Q (narrow band) nature guarantees that it will stretch out in time domain (i.e. ring). Between resonances the level is low and of no consequence.
Both Harwood and Sowter (see the Collums book) show the only apples and apples comparisons that I know of. They made two cabinets with the same exact outter dimmension, of the same material, but with one double the thickness of the other. If I remember correctly the mass doubles as you double material thickness (obviously) and the stiffness increases to 8 times (cube of the thickness). This means that the simple resonances will double in frequency or move up one Octave. Look at the curves and you will see that the breakthrough spectra look similar but shifted. There is no less energy between the resonances, the resonance peaks are not less due to the higher mass (or stiffness). In the relatively undamped case the increase in wall thickness (or stiffness, or mass) did not do anything that we might hope. It did not reduce breakthrough. Resonances were still there, they were just as strong, they were the same Q, just shifted up a little. Harwood then shows that damping was less effective on the thicker cabinet, since its mechanical impedance is inherently higher.
If anybody knows of any "modern" papers that contravene this, I'd be interested in seeing them.
David
p.s. I'll be going out for the weekend so if anyone wants to repost any of my previous responses every 5 or so posts, I'd appreciate it.
BINGO.
Trade offs are what it is all about.
From a materials science point of view, wood fiber is a fantastic material to work with.
OK everybody, what would you consider an acoustic "perfect" cabinet material for a ported enclosure of 20L? Ignore all other factors. I'll start: 1" cast lead. ROHS be damned. Very low frequency of resonance and very low Q.
I'd like to try a pair of 1/4" sheets of BB spaced around 1" and space filled with an expanding foam/urethane.
Having been in the const. trade, many of the principles Harwood outlines have been applied to demountable partitions. Rigid frames supporting damping materials of varying mass/surface resilience. Works well and is very practical in use ... unlike a couple large concrete cab's might be.
Having been in the const. trade, many of the principles Harwood outlines have been applied to demountable partitions. Rigid frames supporting damping materials of varying mass/surface resilience. Works well and is very practical in use ... unlike a couple large concrete cab's might be.
I would suggest to glue linoleum between layers instead of filling with polyurethane foam.
It seems like the the best reason for using concrete or some other pourable material is that you can cast it in odd shapes that are hard to replicate in plywood. Using a mould also means you might (depending on the type of mould) be able to quickly produce identical pairs or sets. I was looking at a thread on some other audio forum with someone using plaster to make some large horns. Really impressive and far more practical than turning a lathe.
KM
PS: I love this spirited discussion! But, someone build something already! ;-)
KM
PS: I love this spirited discussion! But, someone build something already! ;-)
Kyle my friend, when you were still a twinkle in your fathers eye, I did build concrete cabinets but in the last 30 or so sets of speakers I have not revisited the idea because I don't see the value in it.
As far as new stuff, well, let's just say my wife has imposed a moratorium. 😉
i couldnt say how that would work out. Generally im on the same page as Dave, much of my idea is my own mental image of the phenomena. I would propose that MY ideal 20l box would be made of graduated density and rigidity composite. It could be anything. A laminate of graduated density woodfibre? Perhaps balsa 'felted' fibre? The external surface being the opposite, perhaps HDF resin impregnated. Id imagind a graduated density would offer higher damping, maybe a nicer impedance transition? Im not sure how to justify my opinion lol
You know how it is with us young guys: everything old is new again! I just heard this great band called Led Zeppelin, I think they have a real future! 😀
This topic got me excited. I like the idea of being able to build some shapes that would be difficult for me even if I had my own shop or lathe.
KM
True. I was envisioning a 400lb lead bookshelf speaker. There's no reason it couldn't be a one ton floor stander.
KM
I'm working on a concrete variant of the Elsinore floorstander, and a matching pair of sub cabinets By my math, the Elsinore cabinets should be 350lb each before drivers if made with 1.5" thick walls and 2" front baffle. I personally like the idea of how well they should sit still. When its time to move them, i'll use a handtruck, so thats not so bad.
I have found that mixing up concrete to have fairly well damped qualities isnt so terribly difficult either, thats one of the things that makes concrete so cool.
Something to consider is that typical mixes of concrete have mass in the neighborhood of 2.7g/cc, the same as aluminum, as compared to ~0.7g/cc for baltic birch, MDF being slightly more, 0.75-0.8g/cc typically, and the 7.8 g/cc of steel.
I have found that mixing up concrete to have fairly well damped qualities isnt so terribly difficult either, thats one of the things that makes concrete so cool.
Something to consider is that typical mixes of concrete have mass in the neighborhood of 2.7g/cc, the same as aluminum, as compared to ~0.7g/cc for baltic birch, MDF being slightly more, 0.75-0.8g/cc typically, and the 7.8 g/cc of steel.
Great Band. The drummer (John Bonham) - his son Jason, also a drummer, is now 13 years older (45) than his dad was (32) when he died back in 1980. Yup, time flies.
Just to clarify - if mass and stifness are not a factor in resonance because they rule each other out (but that is just grossly oversimplified) then only thing left is damping and exciting force. If the amount of damping is a constant, then the only thing resonance depends is the amount of force applied to the system.
Dave has concentrated on purely elastic model of material, which is characterised by storage of energy and it's return to original state when the force (or load) is removed. What we are talking is a forced damped oscilator or it's derivatives.
In case of steady vibration, a very small viscous damping has large effects on system Q and amplitude. If we excite the system using pulse, that is another story, except if the damping is large (over 0.3).
Damping is also not panacea, because it can cause coupling between modes and aggravate the situation. And there is damping and damping (at least 3 types of hysteretic models, viscous, viscoelastic etc.)
Someone spoke about lead. It's coeff of viscous damping is cca 0.01. For example, concrete is 0.015 and wood is around 0.013. Not much difference in damping. In mass, yes.
I wanted to discuss this for general knowledge, but I can see that it caused additional confusion, because, let's face it, it is hard to comprehend when we go deeper into the subject.
In conclusion for the OP, Harwood style case is proven with experiments and produces repeatable result which not allways is like in that paper. The reasons are multiple, but the main one is oversimplification of conclusions. What can be said is, yes we can make a dead box using this technique, but it is allways going to have very low Qb.
My effort was to prove that concrete could be used as a material for speaker boxes. And in my opinion it can be, as any material as much as we pay attention to vibrational mechanics and acoustics. In that respect, it is not a user friendly material if we want to make a commercial succes because many process optimisations and experimenting should take place even before entering production. Batch to batch consistency is another problem (but not unsurmountable). Everything is doable with the right knowledge and the right tools.
Good night, Gentlemen.
Dave has concentrated on purely elastic model of material, which is characterised by storage of energy and it's return to original state when the force (or load) is removed. What we are talking is a forced damped oscilator or it's derivatives.
In case of steady vibration, a very small viscous damping has large effects on system Q and amplitude. If we excite the system using pulse, that is another story, except if the damping is large (over 0.3).
Damping is also not panacea, because it can cause coupling between modes and aggravate the situation. And there is damping and damping (at least 3 types of hysteretic models, viscous, viscoelastic etc.)
Someone spoke about lead. It's coeff of viscous damping is cca 0.01. For example, concrete is 0.015 and wood is around 0.013. Not much difference in damping. In mass, yes.
I wanted to discuss this for general knowledge, but I can see that it caused additional confusion, because, let's face it, it is hard to comprehend when we go deeper into the subject.
In conclusion for the OP, Harwood style case is proven with experiments and produces repeatable result which not allways is like in that paper. The reasons are multiple, but the main one is oversimplification of conclusions. What can be said is, yes we can make a dead box using this technique, but it is allways going to have very low Qb.
My effort was to prove that concrete could be used as a material for speaker boxes. And in my opinion it can be, as any material as much as we pay attention to vibrational mechanics and acoustics. In that respect, it is not a user friendly material if we want to make a commercial succes because many process optimisations and experimenting should take place even before entering production. Batch to batch consistency is another problem (but not unsurmountable). Everything is doable with the right knowledge and the right tools.
Good night, Gentlemen.
And it can bear proud label: "MADE IN USA" because it will be too expensive to ship it from China. 😀
I'll be starting on a pair in the morning. 3/4 MDF for the lower box, 1/2 for the upper. Maple and hardboard bracing. I get to try out my big honking 3/4" router bit now I made a new router table.
In school, one guy had a Leslie. Lived on the 4th floor. No fear it would be stolen!
In school, one guy had a Leslie. Lived on the 4th floor. No fear it would be stolen!
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