I suppose it's important to clarify the required properties of the box:
1. Rigidity, to ensure the walls don't flex so much as to influence the air resonance of the box, degrading bass performance.
2. Damping, to prevent panel resonace which would color the sound and degrade imaging.
Stiff, thick panels meet the first criteria but push their panel resonances into the mid and treble areas. Thin-walled boxes have a low panel resonance frequency. It seems like the thin-box-decoupled-from-a-thick-outer-box approach would be a good compromise. The low panel resonance frequency of the inner box would not excite much resonance in the stiff outer panel.
The effectiveness of internal friction and damping of materials in controlling resonances is improved by lengthening the amount of time the vibration must spend traveling through the materials, I think. The two-box approach requires that acoustic energy in the box first set the inner box in motion, then the damping material, then the outer box. I would suppose that the time scale involved in this transfer chain is quite a bit longer than a simple box-with-damping-layer design, which is why it is more effective.
1. Rigidity, to ensure the walls don't flex so much as to influence the air resonance of the box, degrading bass performance.
2. Damping, to prevent panel resonace which would color the sound and degrade imaging.
Stiff, thick panels meet the first criteria but push their panel resonances into the mid and treble areas. Thin-walled boxes have a low panel resonance frequency. It seems like the thin-box-decoupled-from-a-thick-outer-box approach would be a good compromise. The low panel resonance frequency of the inner box would not excite much resonance in the stiff outer panel.
The effectiveness of internal friction and damping of materials in controlling resonances is improved by lengthening the amount of time the vibration must spend traveling through the materials, I think. The two-box approach requires that acoustic energy in the box first set the inner box in motion, then the damping material, then the outer box. I would suppose that the time scale involved in this transfer chain is quite a bit longer than a simple box-with-damping-layer design, which is why it is more effective.
Perhaps a bass cab needs bracing, a mid cab benefits most from total isolation, and a full-range cab has to compromise somewhere?
This creates a whole new semantics problem. Defining a sub is easy, but I would guess many woofers are asked to go to 300Hz or higher. This is probably plenty far into the "compromise" region...
This creates a whole new semantics problem. Defining a sub is easy, but I would guess many woofers are asked to go to 300Hz or higher. This is probably plenty far into the "compromise" region...
You raise some valid concerns about the constrained damping layer approach I mentioned. I (once) obtained an AES paper comparing the damping properties of a very similar approach to MDF and conventional plywood, that concluded that it was superior in that regard, and that the best results obtained from a centered damping layer (I seem to recall that not more than a couple millimeters thick was recommended). I don't recall (although my memory may be at fault) that the paper dealt much with the loss of panel stiffness due to using such an adhesive, although my impression of my results by using the contact cement (which hardened to a very stiff rubbery consistency) indicated that the composite laminate was much stiffer than either 3/8" piece of plywood alone. Probably there would be much less gain in stiffness if a softer material like a never-hardening caulk was used for the damping layer.
Vikash made a nice little tool that helps show where resonances reinforce each other:
http://www.vikash.info/audio/standing_wave_calc/
In a braced enclosure, there are probably additional subpanel modes, right?
http://www.vikash.info/audio/standing_wave_calc/
In a braced enclosure, there are probably additional subpanel modes, right?
Best in class
The best bang for the buck is a high mass/stiff baffle. It makes for a better midrange. The next best in my opinion is to create an optimally damped enclosure. Having done many a cabinet the best is a sand dampened cabinet. The enclosure shape is as important as its construction both in the terms of stifness and in terms of supresion of standind waves in the midrange region.
Mark
The best bang for the buck is a high mass/stiff baffle. It makes for a better midrange. The next best in my opinion is to create an optimally damped enclosure. Having done many a cabinet the best is a sand dampened cabinet. The enclosure shape is as important as its construction both in the terms of stifness and in terms of supresion of standind waves in the midrange region.
Mark
thoriated,
It's hard to say. I think he is pointing out that while it may be stronger, it may be only twice as strong as a single panel. A double thickness panel in contrast would be 2^3 = 8 times as strong, and the situation obviously becomes worse from there.
If the damping between layers is great enough, though, then it doesn't matter. That is what is hard to quantify.
It's hard to say. I think he is pointing out that while it may be stronger, it may be only twice as strong as a single panel. A double thickness panel in contrast would be 2^3 = 8 times as strong, and the situation obviously becomes worse from there.
If the damping between layers is great enough, though, then it doesn't matter. That is what is hard to quantify.
I used a constrained layer construction in Delta.
It has a 18mm outer MDF+ a 2.3mm barium-loaded PVC layer + 6mm inner MDF layer. The cabinets are also well braced. The bracing increases rigidity & the constrained layer damps any higher freq.
"Touch" testing the sides indicates it is very effective, but there might still be a hint of vibration here and there (it's hard to be sure) at high volumes. I'm satisfied with it, but next time I would use more closely spaced matrix bracing.
http://www.gattiweb.com/delta_construction.html
It has a 18mm outer MDF+ a 2.3mm barium-loaded PVC layer + 6mm inner MDF layer. The cabinets are also well braced. The bracing increases rigidity & the constrained layer damps any higher freq.
"Touch" testing the sides indicates it is very effective, but there might still be a hint of vibration here and there (it's hard to be sure) at high volumes. I'm satisfied with it, but next time I would use more closely spaced matrix bracing.
http://www.gattiweb.com/delta_construction.html
Your Delta was definitely an inspiration David. Thanks for documenting the construction so well!
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