tvrgeek-i am in agreement with you and i understand the physics, i have been involved in building a studio...since this is a chamber built to satisfy milspec noise tests and the noisefloor was measured and satisfied the BS and EN standards, i have no doubt it attenuates a good 40-50dB. If you want perfect you require 2ft thick double skin sand filled walls, and suitable reflective damping, like used in the studio i mentioned. Of course its absorption is not perfect, and as the noise wavelength increases so must the absorbent thickness, but it works, and it satirfies a 32hz to 8khz sound power measurement criteria for tests
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Sorry to upset your ego. LMAO. Concrete is week in tension and requires reinforcing by steel to prevent crack propagation either by keeping the concrete in compression or reducing the level of tensile forces on the concrete. Sorry to disagree with you
EDIT: By the way I think people are fed up with this anal waffle. Let's move on
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"Rings like a bell" may be a slight exaggeration but Concrete is a relatively high Q material. That means that at every panel resonance the cabinet will be totally transparent. The only way around that is to add damping and the high mass of concrete will fight against that.
It looks like structural engineers have tackled the problem with a polymer concrete mix.
http://www.colingordon.com/pdf/HA_Concrete_Damping_FabTech2005.pdf
David
It looks like structural engineers have tackled the problem with a polymer concrete mix.
http://www.colingordon.com/pdf/HA_Concrete_Damping_FabTech2005.pdf
David
We agree completely that concrete is only strong in compression. I only said it was not rebar that makes it ring. I have been advising the OP to move on from my first post.
Cracking can be reduced or complete removed using plastic, glass & steel fibres. The properties of any composite material depend upon mixture of materials and porosity. By ensuring homogeneity repeatable Young's modulus is ensured, and that ensures precision in vibration response. Thus, a precise and very effective damping solution can be applied.
The other solution is to use a matrix with damping properties which is usually (but not always) followed by a reduction in overall stiffness.
I once pondered upon using concrete as a cheap alternative to wood or other materials, but as it always is - you get what you pay for. What is gained in price is easily destroyed in finishing options, transportation difficulties, brittleness, etc. The question is not what material are we trying to use but what problem are we solving using this particular material. There is a lot of other, less 'cool' solutions.
The other solution is to use a matrix with damping properties which is usually (but not always) followed by a reduction in overall stiffness.
I once pondered upon using concrete as a cheap alternative to wood or other materials, but as it always is - you get what you pay for. What is gained in price is easily destroyed in finishing options, transportation difficulties, brittleness, etc. The question is not what material are we trying to use but what problem are we solving using this particular material. There is a lot of other, less 'cool' solutions.
Trust me on this. The baffles I make of concrete for the NaO II did indeed ring like a bell. I would estimate the ringing at around 1k or 2k Hz. Very high Q. Not associated with the steel reinforcing rods because I had the same result without the rods. But being high frequency it was easy to damp. The application of loaded vinyl made them dead as can be.
The cracking was because the forces applied to the baffles result in a bending moment. Thus flexing the baffle. As a result alternately one side is under tension and the other in compression. These forces oscillate and over time cracks develop.
Obviously the problems with open, flat baffles of concrete will be different than a complete enclosure.
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The problem created those who did not know why all masons long before them used gravel with stones of random sizes. Now they takled the problem they themselves created. Do you think that builders of antiquity did not know how to make flat walls of even size from concrete with even particles?
As I wrote few pages before, all that "ringing", "hi Q", and other theoretical musings are imaginable. Why? Because concrete is flexible material. No need to imitate boring boxes made from flat MDF panels when you can create enclosures with randomized thickness of walls without parallel surfaces. And my experiments demonstrated that.
May be it is the time to do something using own hands, instead of using tonques on forums forever for silly theoretical discussions?
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Ah, the myth of randomized thickness and non parallel walls. With a high Q material you will still have strong resonances even with those features, just harder to predict the frequencies. For example, it doesn't matter how complex a car body shape is, the structure, made of steel, still resonates.
Give up our silly theoretical discussion. Why?
You are absolutely right, Dave. I conjured some graphs this morning to explain just that.
I made three sims depending on the curvature of the material. For all three cases the same boundary conditions are present and the dimensions and the material are the same (glass reinforced concrete of low quality). First model is a plain old plate, second is the plate bent on the short length, and lastly, third is the plate bent in both lenghts.
First:
Second:
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I made three sims depending on the curvature of the material. For all three cases the same boundary conditions are present and the dimensions and the material are the same (glass reinforced concrete of low quality). First model is a plain old plate, second is the plate bent on the short length, and lastly, third is the plate bent in both lenghts.
First:
Second:
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So it's 2012.
Do we really not know how to make an ideal enclosure by now?
Please dont tell me to use "1-inch MDF with bracing" ...
Sure we know, ask any audiophile: "Make it as rigid and heavy as possible, don't worry about damping"
Of course they would be wrong.
David
That's what i'm getting at. Ive been down the "rigid and heavy" road and know it's a fail.
How about the Harbeth approach?
To be more specific, do we aim for high Q or low Q resonance and at what frequency? What tests have been done to determine audibility of either?
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If you mean the thin wall, heavy damping approach (as first described by Harwood), I am a firm believer.
"Rigid and heavy" pushes the resonances up a bit but never deals with their Q. Proportionately lighter walls covered with heavy damping does deal with resonances.
Google "wall TL" to see how architects deal with reeducing sound between you and your neighbors. Flexible mass is one solution while air spaces between detached layers is the key to highest issolation.
David
... but the increase in lossiness is translated in the rest of the system and more energy is spent for achieving the same SPL in low freq's.
Yes, from the BBC research papers: http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf
But is there a point where the resonance is high up enough such that it's no longer significantly audible? And high enough in Q that it is rarely excited?
Resonance is lower in frequency in this case, as is Q. Any possible issues with that? Less accurate bass response?
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From what I've seen the Q is constant as you push resonances higher so I don't see how it becomes less excited. Also, dramatically thicker walls don't push the resonances up that high. Moving them an Octave costs a lot in material. Harwood found that they would be more audible higher in the midrange than if you left them lower.
Lower in Q in this case means lower transmission level (at resonance), exactly what you want. As to accurate bass, the primary TL determinant for bass is mass, the ideal is to provide damping mass. Stiffness does little but determine resonance frequency. Acousticians describe the ideal sound barrier as a "limp mass" with weight and damping but no stiffness, such as a heavy rubber curtain.
In the end it is not the amount of transmission through cabinet walls that matters so much as the nonlinear, resonant nature of it.
David S.
Lower in Q in this case means lower transmission level (at resonance), exactly what you want. As to accurate bass, the primary TL determinant for bass is mass, the ideal is to provide damping mass. Stiffness does little but determine resonance frequency. Acousticians describe the ideal sound barrier as a "limp mass" with weight and damping but no stiffness, such as a heavy rubber curtain.
In the end it is not the amount of transmission through cabinet walls that matters so much as the nonlinear, resonant nature of it.
David S.
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