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Old 23rd May 2002, 12:36 AM   #1
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Default DIY aerolaminate for speaker box

I have a reasonably large quantity of aluminum honeycomb core material and was thinking about using it to make some super rigid panels to make a cabinet for a subwoofer. Anyone have any experience with this sort of thing? I am specifically wondering what materials to use for the skin, what adhesives to use (epoxy?) and finally, how to join the cut panels to each other.

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MR
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Old 23rd May 2002, 01:26 AM   #2
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I would suggest using an inner and outer skin of 12mm mdf to make panels using high quality epoxy or contact adhesive held together with pressure whilst the glue sets, (you could use bricks or sandbags for this to get an even pressure).

If you have a large quantity it might also be worth trying filling the spaces in the lattice with very dry silver sand to stop the aluminium resonating, and comparing the two enclosures to see which sounds best.

As for machining the finished panels, if you're careful and the aluminium is not too thick, ordinary woodworking tools with fine cut blades should do the trick (the sand will possibly screw up power tools though!!!)

I would be tempted to build an inner box to your required dimensions, then apply the lattice, then build an outer box to fit around the whole thing with maybe some nice veneered ply, thus ordinary woodworking tools and techniques should apply

Let us know how it turns out...

Al
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Old 23rd May 2002, 01:53 AM   #3
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This is going to be very interesting. I know the stuff will be rigid since a 2nd cousin of mine invented paper honeycomb houses that you could land a helicopter on back in the 70s. The problem here is where to put it since I don't think using it as the center of laminates will be more beneficial than a lot of other materials. Being metal it will be a great conductor of energy but not absorb any of it. I don't know how thick it is or the guage of the metal so I will say that if it is 1/2 inch thick or less that it probably would not be benificial at all. If it is 1" or more a structure built out of it and filled with plaster in the honeycomb and the inside covered with sound deadoning aught to work out. Be lighter than cast concrete but could work like it. Cover it with anything you want for looks and use MDF for the baffle. Other than inert gas welding an epoxy like JB Weld would work for construction. Attach the outer shell with silicone to isolate it. Sorthobane ? You probably could make some great shelving units with it.
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Old 23rd May 2002, 02:34 AM   #4
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But surely if it's for sub bass use energy transmission should not be a problem, as the resonant frequency of the metal should be way higher than any sub bass frequencies, and thus all you need is rigidity at right angles to the box panel to prevent the walls moving with the driver?

Al
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Old 23rd May 2002, 03:15 AM   #5
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All the vibration would be tranmitted to what ever it was touching. While it would make a great structure as far as being rigid any vibration that get through to the outer covering will turn the covering into a transducer. The fact that it is metal creates as many problems as it solves. You don't want to listen to your cabinets, just the drivers. And it is energy. There are even sound wave storage devices. Go figure why someone would want to bottle up sound energy, but its got to be good for something I guess.
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Old 23rd May 2002, 04:26 PM   #6
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Default high mass > low resonance freq.

I was under the impression that an extremely rigid cabinet was desirable. If it is rigid, it won't easily start vibrating. The traditional method of achieving rigidity is to use very thick wood, concrete, etc. all of which have high mass. The high mass means that the panels will have low resonant frequencies. This is why the panels are braced inside the speaker- to give them extra rigidity to prevent them from vibrating. You are using low resonance frequency panels to build a low resonsnce frequency speaker. That sounds like you're just begging the cabinet to vibrate.

By using low mass, high rigid panels, the resonance of the panels is moved up higher so they are less likely to be excited by the low frequency vibration generated by the speaker.

Correct me if I'm wrong.

MR
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Old 23rd May 2002, 04:51 PM   #7
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There is a big argument/confusion over rigidity and/or damping... I'm just getting my reasoning together, and then I'll give a full reply.
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Old 23rd May 2002, 05:16 PM   #8
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Rigid and inert. Rigidity does not mean that it isn't a good conductor of vibration and it does not have to be a sympathetic vibration. I just can't see any advantage to be obtained while I can see how what ever is attached to it could come loose and create lots of problems. Having a big powerfull sub going is going to give any kind of enclosure a beating and while this would be very rigid it would not be completly rigid. Adhesives will sooner or later break free somewhere and it will buzz at what ever frequency it can. I guess you could use bolts to clamp laminates to it along with the adhesive and that would help, but next time a kid with a couple of subs in his car drives by and rattles your house think of what that car is made of. The problem with it from my point of view is not how rigid it is or mass and vibration but how the material itself is such a good conducter of vibration at any level. I am not saying its not worth a shot, but I do believe you are going to have to do so much to avoid the problems inherent to metal itself that it could be a waste of time. That I still don't know how thick it is and how large the cavities are has still got me making assumptions, and I am just guessing at what I think would happen. And just out of curiosoty how would the mass of a MDF box affect the vibrations? The bracing is to keep the box from flexing. The Hz are low enough that I would think that the waves would be too long to start an occilation inside of a box that small. I have yet to build a sub enclosure and the speakers I have built so far have been TLs that I have attempted to tune to quarter waves and are not sealed or have big powerfull drivers. The exception being a back loaded folded horn that goes down to 30. Still pretty wide open compared to even a ported sub. Thinking about getting a Shiva or 2 though.
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Old 23rd May 2002, 05:53 PM   #9
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Default laminate construction

I once built a set of isobaric subs, using a kevlar aramid honeycomb core. I used 'sandwhich' construction, with 3/8" baltic birch plywood for the upper & lower layers, with 1/2" honeycomb in the middle. For the front baffle, I repeated the laminations until I had an 8" thick front baffle, which housed the isobaric tunnel. Needless to say, the result was a very stiff, very heavy enclosure. It was also very time consuming to build.

About 10 years ago, Celestion released a high-end monitor type speaker, which was built of aluminium aerolam. I listened to them on several occasions, and they were excellent - pinpoint imaging, very good clarity. Very free from any 'box' type colorations. The cabinets were seamless, and it was not evident how the panels were joined.

This type of material is widely used in the aerospace industry, where the preferred method of attachment is via mechanical fastener. Oversize holes are drilled though the sandwich panel, and hollow cylindrical plugs ('inserts') are inserted into the holes and bonded with epoxy. This is to prevent the fastener from crushing the sandwich. Bolts (or blind rivets) are then used to attach the panels to beams, or angles. This method is labour intensive, but effective. You would need to fill the exposed edges with epoxy resin as well. Problem is that this results in exposed fasteners.

You could try to bond the panels to right angle sections to make up the box, as you do not need high ultimate strenght, just a stiff bond. Bonding alone might be adequate. 3M provide a good range of epoxy resins to industry - you could try a marine (fiberglass) supplier, or an automotive body shop supplier.

I would cut the material on a table saw. I successfuly cut up to 1/2" thick aluminum sheet on my table saw (Delta contractors saw), using an ordinary (and cheap) carbide tipped blade. The key is to apply gentle, steady pressure during the cut. It works like magic.

Any panel resonance could be dealt with by properly treating the inner surfaces of the box with damping materials. Good damping materials are available to the automotive industry, for damping sheet metal. Auto audio shops (ugh!) usually carry it.

Good luck on your project!
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Old 23rd May 2002, 06:01 PM   #10
Edgar is offline Edgar  Norway
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I don't think you are wrong. If you are making a true subwoofer rigidly is the most important factor. Some people are making their sub's with >30 mm MDF, making them too heavy and expensive. I prefer using 19mm or so, with extensive bracing, this way the sub performs better and is lighter.

In your case, you are going to the extremes, very stiff, and very light, that can also be a problem. Because of the light enclosure, it may rock, not being a good enough anchor for your driver. Also, you still have to use extensive bracing of course, even if the honeycomb is very stiff.

If you are crossing over at say >80-100 hz , or if you are using a low order slope (6,12dB), you need damping (i think i would use some damping anyway). A very exciting project, but lots of practical building problems, good luck!

Edgar
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