At least 454 is on the right track, but he's still bringing up resonant frequency, which has precious little to do with how well sound transmits through a medium, in this case a solid wood baffle. This latest problem also has me rethinking whether I want to use concrete to make a massive and dead baffle. I think I had an argument with someone about that a while back, where I was wrong.
I may have to route out some concentric grooves in my planned rings, and fill the grooves with something with a very high modulus of elasticity in order to prevent the rings themselves from becoming sound generators.
One way or another I'll get this figured out in route to eventually building a perfect OB construction, massive and totally dead. The small steps in the meantime are exhausting with something new around every turn.
I may have to route out some concentric grooves in my planned rings, and fill the grooves with something with a very high modulus of elasticity in order to prevent the rings themselves from becoming sound generators.
One way or another I'll get this figured out in route to eventually building a perfect OB construction, massive and totally dead. The small steps in the meantime are exhausting with something new around every turn.
rick57 said:
As soon as you put a hole in the box the pressure is reduced at some frequencies...
wool felt, cotton "effective into the low frequencies"
Now even more intrigued. This is more effective than the bitumen pads used by the BBC?
No air space dampig (felt/wool/poly fluff/fiberglass is very effective at low frequencies.
Take a look at the attenuation data near the bottom of this page (some of the other information on the page is out to lunch and i really should add an editors note or 2)
http://www.t-linespeakers.org/projects/tlB/appendix/bradbury/index.html
The bitumen pads used by the BBC serve a completely different function, they are panel damping (like MJL21193's rubber pads (i wonder if there is a real name behind the moniker))
dave
That must be a solid little cab. I'm holding off for that kind of construction with something I believe will be permanent....bracing like mad, sand or sealed up lead for mass and damping, concrete for mass and strength. Still more to learn in what appears to be virgin territory with OB construction, so feeling like an explorer is far more rewarding than being a tedious perfectionist at the end. This sound entering at the backside of a well rounded cutout caught me by surprise though, and I burned up some pretty wood to make 3 pairs of this design. I haven't even bothered to mount the other 2 pairs of drivers yet.
MJL21193 said:
The mechanical vibration is coupled to as much of the mass of the box as possibel. That energy is dissipated by the natural constrained layer damping inherent in plywood. Most of the energy generated by the back of the driver bounces around inside the box until it is dissipated by the airspace damping material. Some no doubt is damped out by the plywood, but the idea is never to have enuff at just the right frequency to excite a panel resonance. The lowest frequency energy is usually dissipated much more quickly by being used to drive a port/terminus (and often this has a purposeful resistive element to it). Some energy gets back thru the cone, but we try to minimize that.
If only the energy we want to have escape from the cabinet escapes, we have achieved our job.
dave
Hallo, how would be MDF with carbom layers on one pr each side?
http://www.gms-composites.com/des4.html
http://www.gms-composites.com/des4.html
richard_majer said:
Much better than MDF ... this is a essentially the same ias suggested by Jonathan Carr -- he suggested fiberglass thou.
Panel characteristics would be changed significantly, so one can't say whether plywood or something else might be a better substrate.
I did use 1/2" HDF laminated both sides with formica many years ago,,,
dave
My views on the different types of damping:
First, fibreglass damping inside the box has very little direct effect on structural resonances in the box. The main purpuse of box stuffing is to attenuate the acoustical resonances in the air cavity. On the other hand the fibreglass may actually reduce the vibration of the walls indirectly, since damping the air cavity resonances reduces the sound pressure at these; this in turn leads to less vibration in the walls, at the cavity resonaces.
Second, people that I know that know structural dynamics tell me that the best way to apply damping to a mechanical structure is to make a sandwich construction. If the damping layer is in the middle of the wall (eg damping glue between two sheets of MDF) there will be strong shear forces on the damping glue, and so it will absorb a lot of energy.
Applying damping material (asphalt, carpets etc) on the inside of the box is of course not bad, but it is far less effective than a sandwich construction.
Third, bracing shifts the resonances upwards and makes them more resonant (higher Q). At these higher frequencies, the exciting forces are typically smaller, and this reduces the problem with panel vibration, in spite of the higher Q values.
First, fibreglass damping inside the box has very little direct effect on structural resonances in the box. The main purpuse of box stuffing is to attenuate the acoustical resonances in the air cavity. On the other hand the fibreglass may actually reduce the vibration of the walls indirectly, since damping the air cavity resonances reduces the sound pressure at these; this in turn leads to less vibration in the walls, at the cavity resonaces.
Second, people that I know that know structural dynamics tell me that the best way to apply damping to a mechanical structure is to make a sandwich construction. If the damping layer is in the middle of the wall (eg damping glue between two sheets of MDF) there will be strong shear forces on the damping glue, and so it will absorb a lot of energy.
Applying damping material (asphalt, carpets etc) on the inside of the box is of course not bad, but it is far less effective than a sandwich construction.
Third, bracing shifts the resonances upwards and makes them more resonant (higher Q). At these higher frequencies, the exciting forces are typically smaller, and this reduces the problem with panel vibration, in spite of the higher Q values.
planet10 said:
Blasphemy! You used MDF??? Doesn't that suck the life out of the music?
Getting back to the damping/resonance issue. I took the time to read through everything that was posted yesterday + what you have to say about energy dissapation in your enclosures and have come to this conclusion:
You use superior box construction and effective use of damping, I use superior box construction and effective use of damping. Aside from all of our differences, we are the same.
But that was a CRACKING good back-and-forth yesterday. Worth the price of admission.
Now, your prejudice against MDF is another thing.
Svante said:
It was never proposed by me that you could achieve critical damping in this way, just that it has a damping effect on panel vibrations. Personally, I never use a heavier material such as fiberglass for stuffing. I prefer to damp panels directly, and use BAF for stuffing (sparingly) when needed(as in a TL).I never use stuffing in a sealed box, not even to increase the apparent size of the box.
Even as the the panel damping I use (carpet) drives the resonance down (slightly), I prefer to damp the panels directly. I beleive it has a positive effect on sound quality.
Oh, and remember, this is on top of high quality box construction.
Svante said:
Effectiveness of CLD in undisputed, however the means to achieve it are foggy. Some say two layers glued together does it, some say two layers with flexable adhesive between. Others say unequal thickness for the layers, while others say equal.
I have been using the first method, in some cases gluing three layers together, not to make a constrained layer, but to form a stronger panel.
I did build one sub that had rubber underpad sandwiched tightly between an inner box and an outer box, with the only solid connection at the driver mounting hole. This worked well, but was fairly difficult to construct. May try this approach again someday.
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