7V said:
Hmmm. An interesting thought, and one that I had not previously considered. But! I've just had another. For the sound to be transmitted through a wall to another room, the wall must flex and re-excite the air on the other side. So, although I agree that flat panels of the foam/aluminium composite probably would become transparent at bass frequencies, I think the shape will rigidise it so that doesn't occur.
Please may I engage smug mode?
RobWells said:How about putting a vacuum between the layers
I'm afraid not. The composite works because when you attempt to bend it, you force two different radii and have to stretch the outer layer compared to the inner, so the two must be connected together. Essentially, that's all that the honeycomb in Aerolam does - it connects the two spaced layers.
I'm missing something here
If you look at post #60 there is an outer skin and an inner skin. Nothing inbetween. If you seal the top and bottom (like in double glazing), then you could vacuum the air out.
Maybe the sealing piece would short circuit the transmission of sound, ruining the concept.
Cheers
Rob
** I've just re-read your post and I understand what you mean now
**
If you look at post #60 there is an outer skin and an inner skin. Nothing inbetween. If you seal the top and bottom (like in double glazing), then you could vacuum the air out.
Maybe the sealing piece would short circuit the transmission of sound, ruining the concept.
Cheers
Rob
** I've just re-read your post and I understand what you mean now
**
I think you are all under-estimating the stiffness of composite sheets, especially since I called them flexible.
Once upon a time, when I weighed only 140lbs, I stood on two 5'x2' sheets of 3/8" fibreglass-honeycomb sandwich held up at the ends by a few wood studs (just to see what would happen). It maybe flexed a whole 1/4" and I was really disappointed by the lack of breakage. This was the cheap stuff and the sheets weren't even bonded together. Imagine if the fibre outer layers of the sheets were carbon, Kevlar, Spectra or even aluminum.
My understanding is that if you want no sound emanating from the "box" then you want the box as rigid as possible so that the internal sound pressure doesn't cause the walls to flex and then cause the air on the outside to resonate.
If you use some exotic composite combinations, you get ridiculous amounts of strength and rigidity. In the HPV world, it is not unusual for a monocoque bike to be made of an inner layer of carbon fibre and an outer layer of Kevlar. The major drawbacks of composites are the cost of materials and labour.
For the design sketch in question, hand-laid layers of composite fabric that is vacuum-formed to a negative mould would be the way to go. You could even mould the inside wall with a calculated roughness that decreases the direct internal reflections. I'll let Chris describe the process better as I haven't done it in a long time.
ensen.
Once upon a time, when I weighed only 140lbs, I stood on two 5'x2' sheets of 3/8" fibreglass-honeycomb sandwich held up at the ends by a few wood studs (just to see what would happen). It maybe flexed a whole 1/4" and I was really disappointed by the lack of breakage. This was the cheap stuff and the sheets weren't even bonded together. Imagine if the fibre outer layers of the sheets were carbon, Kevlar, Spectra or even aluminum.
My understanding is that if you want no sound emanating from the "box" then you want the box as rigid as possible so that the internal sound pressure doesn't cause the walls to flex and then cause the air on the outside to resonate.
If you use some exotic composite combinations, you get ridiculous amounts of strength and rigidity. In the HPV world, it is not unusual for a monocoque bike to be made of an inner layer of carbon fibre and an outer layer of Kevlar. The major drawbacks of composites are the cost of materials and labour.
For the design sketch in question, hand-laid layers of composite fabric that is vacuum-formed to a negative mould would be the way to go. You could even mould the inside wall with a calculated roughness that decreases the direct internal reflections. I'll let Chris describe the process better as I haven't done it in a long time.
ensen.
Hmmm, I think the vaccuum idea is sound in principle but I wouldn't want to try to make it work. If I'm not mistaken, (and I often am) using double glaze for sound requires larger separation because they are separated by argon, not a vaccuum. Were it a vaccuuum, the gap could be very small. For the vaccuum to work however, in our case the layers would have to be mechanically held apart and sealed by something strong and flexible enough to take the strain of the vaccuum, accousticly inert enough not to transmit the sound and perfectly bonded so as not to leak. A pretty big engineering task for sure.
As for how to build the design in question using vaccuum bagged composite sandwich, there are a few things to consider. Firstly I wouldn't bother with Kevlar(aramid) or carbon for several reasons. Kevlar isn't really all that stiff. Most applications use it because it takes a lot to make it fail. It is a freakishly tough material and a real bitch to work with. I only use it in combination weaves for the skin of the board where impact resistance is required. The fan cowls on jet engines use it to contain fragments in case of a blade failure.
Carbon's main advantage is the ability to mould compound curves where aluminum would be very difficult. Beleive it or not, aluminum actually has a better stength to weight ratio. I can only suppose that carbon is used in the floor panels of aircraft due to bonding problems between aluminum and the honeycomb. I'm sure that there are some exceptions, but anywhere I have seen these types of panels made with aluminum skin, they have always used balsa core, which alows a much larger bonding area. I have never had much success bonding aluminum to anything that will flex much, and I have gone through the whole etching proceess with only medeocre results.
The two reasons to use vaccuum bagging are the ability to clamp the entire assembly, which is very usefull when making fabric or core material conform to tight curves, and it allows a better fibre to resin ratio, which in turn increases strength to weight.
The final point to consider is that of core thickness. If you double the distance between the inner and outer skins you square the strengh. Thus, the only way to really take advantage of this method would be to either build 2 sandwiches (glass fiber/core-cell/glass fiber) and then separate them by and bond them to a thick core, or build a sandwich which has several layers of core-cell bonded to each other. You can get core materials much thicker, but you'd never bend them around the curve we're talking about.
All in all, I guess all the trouble may be worth it if you can push the resonant freq out of the audible range, but I can't comment on that.
For overall stiffness, I think it's pretty hard to beat poured plaster or concrete or Steve's stacked MDF.
I think one of the most promising cabinet materials are the inert poured resins being used in some of the flagship speakers. I can't recall which ones off hand though.( Watt maybe) The guy on this forum who seems to have the best handle on polymers and plastics is SY. I'm kind of curious what the acoustic properties of the synthetic wine corks are. Would it be a suitable core, and could it be used by the hobbyist?
I guess that was more than 2 cents worth eh?
Chris
As for how to build the design in question using vaccuum bagged composite sandwich, there are a few things to consider. Firstly I wouldn't bother with Kevlar(aramid) or carbon for several reasons. Kevlar isn't really all that stiff. Most applications use it because it takes a lot to make it fail. It is a freakishly tough material and a real bitch to work with. I only use it in combination weaves for the skin of the board where impact resistance is required. The fan cowls on jet engines use it to contain fragments in case of a blade failure.
Carbon's main advantage is the ability to mould compound curves where aluminum would be very difficult. Beleive it or not, aluminum actually has a better stength to weight ratio. I can only suppose that carbon is used in the floor panels of aircraft due to bonding problems between aluminum and the honeycomb. I'm sure that there are some exceptions, but anywhere I have seen these types of panels made with aluminum skin, they have always used balsa core, which alows a much larger bonding area. I have never had much success bonding aluminum to anything that will flex much, and I have gone through the whole etching proceess with only medeocre results.
The two reasons to use vaccuum bagging are the ability to clamp the entire assembly, which is very usefull when making fabric or core material conform to tight curves, and it allows a better fibre to resin ratio, which in turn increases strength to weight.
The final point to consider is that of core thickness. If you double the distance between the inner and outer skins you square the strengh. Thus, the only way to really take advantage of this method would be to either build 2 sandwiches (glass fiber/core-cell/glass fiber) and then separate them by and bond them to a thick core, or build a sandwich which has several layers of core-cell bonded to each other. You can get core materials much thicker, but you'd never bend them around the curve we're talking about.
All in all, I guess all the trouble may be worth it if you can push the resonant freq out of the audible range, but I can't comment on that.
For overall stiffness, I think it's pretty hard to beat poured plaster or concrete or Steve's stacked MDF.
I think one of the most promising cabinet materials are the inert poured resins being used in some of the flagship speakers. I can't recall which ones off hand though.( Watt maybe) The guy on this forum who seems to have the best handle on polymers and plastics is SY. I'm kind of curious what the acoustic properties of the synthetic wine corks are. Would it be a suitable core, and could it be used by the hobbyist?
I guess that was more than 2 cents worth eh?
Chris
How about using GLARE, sandwich construction of aluminum (0,4 mm) and glassfiber.
The new airbus A380 is build from it, it's outerhull is only 3-4 mm thick !!!!
The process is like this: make a mould, put layers of aluminum (very clean with primer for gleu) and fiberglass on top of eachother with layer of epoxy in between en compress. For the best result compression should be under high temperatur where he epoxy gets very liquid and forces any airbubles out of the construction. Voila
The new airbus A380 is build from it, it's outerhull is only 3-4 mm thick !!!!
The process is like this: make a mould, put layers of aluminum (very clean with primer for gleu) and fiberglass on top of eachother with layer of epoxy in between en compress. For the best result compression should be under high temperatur where he epoxy gets very liquid and forces any airbubles out of the construction. Voila
I'd never heard of this until your post, but I suspected that there were other reasons for using this as it would be no stronger or lighter than a single sheet of aluminum the same thickness as the GLARE laminate. I did a quick google search, and the reason for GLARE is that it seems to be highly superior in the fatigue department. For our needs, not a concern.
But I'm diggin' everyone coming up with all these unique ideas, maybe we'll hit a good one yet. Something that occured to my a while ago (mostly for rapid prototyping) would be double layered drywall. It's cheap, easy to work with using simple tools, and quite rigid. As a real cabinet, you could even build an inner and outer cabinet and separate them with some kind of damping foam.
How's that?
BTW don't ever try heating room temp cure epoxy to make it flow better. It'll cure in the pot right before your eyes and start to smoke. Bad thing. It also will kill the strength of the epoxy. You can heat a finsished laminate to get a higher strength finished product, but you really have to know what you're doing. I've done it by accident while working wet epoxy on carbon fiber in bright sunlight, I can't say if it was any stronger than normal, all my boards break eventually, I build them very light and sail them like they're made of rock.
Chris
But I'm diggin' everyone coming up with all these unique ideas, maybe we'll hit a good one yet. Something that occured to my a while ago (mostly for rapid prototyping) would be double layered drywall. It's cheap, easy to work with using simple tools, and quite rigid. As a real cabinet, you could even build an inner and outer cabinet and separate them with some kind of damping foam.
How's that?
BTW don't ever try heating room temp cure epoxy to make it flow better. It'll cure in the pot right before your eyes and start to smoke. Bad thing. It also will kill the strength of the epoxy. You can heat a finsished laminate to get a higher strength finished product, but you really have to know what you're doing. I've done it by accident while working wet epoxy on carbon fiber in bright sunlight, I can't say if it was any stronger than normal, all my boards break eventually, I build them very light and sail them like they're made of rock.
Chris
Christopher said:
Yes, in recumbent bikes, Kevlar is used as the outside layer to protect the rider from objects puncturing into the fairing during crashes. The carbon is the actual structural material. Still, it would make for a dent-proof enclosure, if you could get past the hideous cost.
I like the idea of the synth cork. Utilizing the natural damping characteristics is interesting. What about other damping materials as a core?
Sand. Not a lot, but enough to separate inside and outside with energy absorption. You'd have to make a box in a box and fill the gap with sand. Vibrate it with the woofer as low as possible to settle it while filling completely.
Silicone in the gap. Or heavy machine grease. Or that expanding foam stuff they using plumbing.
What if you just patterned the interior after an anechoic chamber?
ensen.Fearful that I've gone off the deep end of an empty pool...
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