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Multi-Way Conventional loudspeakers with crossovers

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Old 12th November 2004, 08:59 PM   #31
VvvvvV is offline VvvvvV  United Kingdom
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People say that concrete doesn't have the right acoustic properties, it makes a high-pitched sound when you tap it, I think the best speaker material probably makes a dull thud kind of sound.

Here's the plan:
1: make a cardboard tube of 12 inches diameter, set it on a motor say that it turns like a roller
2: get lots of expanded polystyrene balls and mix them in with some casting resin just so they are sticky enough, and then pour one inch of the mixture on the roller as it turns
3: use the rest of the casting resin to create another inch of pure plastic to form the main part of the casing et voila
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Old 13th November 2004, 03:31 AM   #32
Variac is offline Variac  United States
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In thin layers concrete is pretty flexible. With steel reinforcement even springier. I suspect about an inch thick with granular admixture of say marble chips and NO steel, it would be fine.
Inch think would be plenty heavy though!!
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Old 13th November 2004, 04:22 AM   #33
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With respect to the idea of the rotating motor. This idea isn't as wild as it may sound - when I was investigating my resin based design I considered several approaches not just the moulded one. I called several industrial suppliers to see what I could get made for me and the cost.

The second place choice which I think would have been excellent was spun fiberglass/kevlar/carbon fiber. The deal with this approach (as I understand it) is they get prepreg fiber materials , make a mould and spin the mould to lay on the layers of matting/cloth. using this method you can get a cabinet of any reasonable wall thickness you could want.

This approach brings up an issue I should mention: you have to consider how you are going to mould the top and bottom without using a two part process (two parts moulded separately, then glued together to make a single piece), it isn't practical to mould all sides in one pour.

I chose to build the mould in such a way that I poured the top, and side walls all in one pour, and then used a heavy piece of oak for the base to cover the area of the last panel that wasn't mouded. To make assembly easier, during the moulding process I put bolts into the uncured resin so that the thread portion of the bolt protruded through my base board. This left the heads of the bolts in the resin. No T-nuts needed no inserts. The bolts are part of the cured plastic now, and they aren't going anywhere. Also, because they went through the actual holes in the actual board used to seal the base they were in perfect alignment for final assembly.

If you aren't going to do something like I did you should put some serious thought into how you are building your top and bottom panels. My experience shows these panels have the most vibration after the baffle. So extra care here is in order.
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Old 13th November 2004, 04:26 AM   #34
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I have to agree with RDF, if you aren't doing dedicated cabinets for each frequency range, a heavy cabinet is going to perform better over a wider range of frequencies. I loaded my polyester resin with dry, clean playground sand and they were heavy but manageable for a single person. Aside from the baffle (sigh!) my cabinet walls show very little vibration.
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Old 13th November 2004, 09:24 PM   #35
Ron E is offline Ron E  United States
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I like the polystyrene idea, a few 1" sheets of this with skins of carbon fiber or fiberglass on each side wouold make very stiff panels that weigh almost nothing. Think surfboard, dude THe only problem is then in joining them.
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Old 13th November 2004, 10:41 PM   #36
rdf is offline rdf  Canada
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Here's where I stumble on the light-weight wall concept. Picture a completely inert room - say 20' concrete on five sides - with one wall constructed of any of the proposed methods. Put a listener on one side of the test wall and wide band noise source with substanital low frequency content on the opposite. A truck for example. Which would allow the absolute minimum pass through? I can't help but think the truck would be clearly audible through a foam-type wall, all other variables (stiffness, etc.) being equal. There's obviously much more to consider than just acoustic pass through but it's hard to see how a light cabinet could ace this test.
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Old 14th November 2004, 01:50 AM   #37
Ron E is offline Ron E  United States
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Celestion made some speakers out of HexCell(tm) type material (They called it AeroLam) and they were critically acclaimed, but they were small monitors and had lightweight bass to begin with..

Acoustic transmission through building walls is an area where mass is an advantage. The wall is like a very big membrane and is analysed as such - a membrane without stiffness but with mass. In the area above resonances, the mass law is a very effective predictor of transmission through walls TLo=20*Log(ws*f)-28dB where ws is the weight per square foot and f is frequency

In a speaker, the wall is much smaller (so proportionally much stiffer) and the resonant frequencies of said membrane is in the hundreds of Hz - beyond the frequency range of a subwoofer, In the area below resonance, the mass law does not hold.

If it is so important to have massive walls, what about sound that is reradiated through the cone?
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Old 14th November 2004, 02:47 AM   #38
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Martin Collums' book High Performance Loudspeakers addressed this issue. I haven't read it in a while, so I am paraphrasing here:

Below a certain frequency - say 40 hz stiffness is the most effective method of controlling vibration. Above this frequency resistance/damping is most effective until a second break point - say 100 hz, and above that frequency range it's mass that is most effective.

I don't have the book on hand, or I would be more specific (and accurate please forgive my vagueness.

So true subs - cabinets that play infra bass, and the bottom octave are best served with ultra stiff cabinet walls. Woofers are best served with stiff well damped walls, and mid woofers should have heavy, well damped stiff walls.

I don't know if the break points I mentioned above are the correct points, but if the description above is correct I think it's fair to say that the mix of frequencies present in the bass cabinet will indicate the most efficient approach to removing vibration from the cabinet walls. VvvvV, you didn't indicate the type of cabinets you wanted to build, were you thinking of a two-way bookshelf speaker? a sub woofer? knowing what you had in mind would help direct the discussion.
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Old 14th November 2004, 08:37 AM   #39
Nuuk is offline Nuuk  United Kingdom
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It may be worth pointing out that while a number of theories have been put forward in this thread, my suggestion to use polystyrene is based on (my) experience!

If practical, I prefer to 'do' rather than hypothesize. It saves quite a lot of time!

The idea that you must stop sound escaping from a loudspeaker cabinet is almost ludicrous, and is pratically unachievable. Think about it, if you had a cone that sound-proofed the speaker cabinet, it would be too heavy to move!

Agreed, there is a need to prevent the rear wave of the cone meeting the front wave and causing bass cancellation, but I know of no other reason to prevent all the sound from inside the box getting out. In fact, it may even be beneficial to have some 'escape'!
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Old 14th November 2004, 04:41 PM   #40
rdf is offline rdf  Canada
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Some factors are being overlooked. My Jordans are in 24" x 8" square pine MLTL cabinets - thrown together for break in at a whopping $15. The cone's surface area comprises roughly 1.3% of the total system surface area. In its pistionic range the amplifier's feedback loop acts to attenuate cabinet sound coming back through the cone. Above that range, for probably all but subwoofers or atypical designs, internal stuffing has attenuated most of the rear wave. In all acoustic return through the driver in my example will be well under 1% of total system maximum potential output due to pass though. It's true that some internal cabinet sound will find its way back though the cone but I don't see how it's valid to write off the remaining potential 99% plus.

In any case, going way back to the begining of the thread, I'm in a boat on the same lake as Nuuk but rowing in the opposite direction. It was my experience with similarly alignmed cabinets of constrained layer damped 1/4" steel (TB W4-657s) against regular 3/4" pine (Jordans JX92s) which leads me to believe acoustic pass through is an important component of a cabinet's design, along with stiffness and a low resonance signature.

OT - Hey RonE, thanks for attributing that quote in your sig. It's what we said of our Engineering profs but I never knew its origins.
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