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Old 24th August 2006, 05:03 AM   #61
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Quote:
Originally posted by inertial
Hi Critofu,


Yes I know thermos.
Excuse me, but do you know structural-vibrations propagations?
No offence , perdone my basic language .

Cheers,
Inertial
Sound doesn't travel through a vacuum. If you constuct an air-tight two walled speaker cabinet and between the walls is vacuum, then it will be a very excellent enclosure.
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Old 24th August 2006, 08:48 AM   #62
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Dear Critofur,

You can see a point of contact from the two structures/boxes.
At that point , the wacuum is "bypassed".

Cheers,
Inertial
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Old 24th August 2006, 03:03 PM   #63
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Quote:
Originally posted by inertial
Dear Critofur,

You can see a point of contact from the two structures/boxes.
At that point , the wacuum is "bypassed".

Cheers,
Inertial
But that is very insignificant compared to both transmitted sound and resonances induced by sound in/through the walls of a traditional speaker enclosure.

Additionally, that small point of contact is not difficult to isolate with low-Q damping material.
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Old 24th August 2006, 04:04 PM   #64
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Having just read through all the posts here it is very intersesting. I just want to throw in my 20 cents worth.

About 12 months ago I built a pair of speakers that used some very cheap drivers that originally came in some moderate bookshelf sized cabs. The complete speakers cost me just $30.00 per pair (two 6.5s ad two tweeters).

The long and short was I built these into cabinets that had a front baffle 45mm thick, and side walls comprised of 3 thicknesses of MDF with non slip rubber matting between the layers, and concrete slabs top and bottom of the cabs. Basically they are meant for my surround sound home theatre. Each speaker weighs at least 80kg probably more, it takes two to lift them anyway. There were many other measure taken to address much of what has been mentioned here as well.

End result virtually no cabinet resonance and as far as I can tell very little colouration, compared to other speakers I have they sounded very live. pure and detailed and far far better than such a cheap set of drivers should..... a comparison. Not dead and lifeless as some people seem to think they would.

Well I have some B and W 602S3s in my office and by comparison they sound very very coloured and dead and most folk think they are quite OK speakers, including me until I built the incredible bulks.

I have since moved on the full range driver setups but that another story all together.

In my opinion far too little attention is paid to the issues of this thread, probably because to do so commercially would be difficult and expensive........ but that doesn't dimish the importance of these issues for good sound. I think it is very odd that DIYs often spend massive amounts of money on very pricey drivers and then put them into cabs that are actually very compromised. I am very much of the opinion that a average or even cheap drivers in a really solid well built cab can kill an expensive drivers in an average box.
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Old 24th August 2006, 04:37 PM   #65
sletol is offline sletol  Norway
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After a lot of experimenting, I have come to the conclusion that most of the sound (=unwanted coloration) from the walls of the typical box has its origin in the mechanical transfer of vibration from the drive units. Perhaps this is the second most overseen factor in loudspeaker construction.

The MOST overseen factor has probably to do with sound leakage (from the -sometimes enormous- sound levels which are reached inside the enclosure), and its leakage through the weakest spot of the speaker cabinet, seen as a whole. What is this spot? The weak spot is not the walls, but the thin membrane + surround of the typical mid/bass drive. The leakage here does not stop because the membrane itself is moving and producing the sound in the first place. The reflected sound from the inside does not care about this. The result would be the same if one cut out a big round circle in the middle of a side panel and covered it with a thin membrane of paper or plastic, or whatever.

There is a reason why the speakers which are constructed to absorb a maximum of the sound energy –not only at bass frequencies- from the rear side of the membrane often has such a clean sound. (say, TL cabinets with lots of absorbing stuff along the internal walls).
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Old 24th August 2006, 06:10 PM   #66
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Critofur,

"very insignificant .." what ? Excuse me but this is only your opinion.

and about the " small point of contact.." , please post a serious drawing ( if you want).
I am interested !

cheers,
Inertial
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Old 24th August 2006, 11:40 PM   #67
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Quote:
"very insignificant .." what ? Excuse me but this is only your opinion.
Quote:
At that point , the wacuum is "bypassed".
I agree with his opinion, a small ring joining the inner and outer boxes does not have enough area to be significant. Factoring in losses and the inability to transmit vibes perfectly plus the fact that he would design it with a well damped material, I doubt very much would get through. Plus, most acoustic energy from the enclosure is due to large surface area panel vibration as he stated, and the comparing the two values I would say that the joint is insignificant. Especially with a well damped and thick baffle, the one face that will radiate most of the transmitted energy.

As to the math that pinkmouse and poobah did awhile back I, for some reason looked over this whole post again, noticed that I disagree with the methods used. Mainly conservation of momentum which I believe should only be used for unconstrained objects. Since the box is constrained by the floor I decided to try a force balance and determined that the base wont move(using your numbers guys) and tipping will have to occur, see attachment one, or with spikes on carpet I did a loose analysis of carpet compression that could result in angular rotation of the box about the base, see attachment 2 (next post).
Anyways, unless I missed something I believe these number to actually represent a real world scenario and feel free to question any part of it
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File Type: jpg box sliding.jpg (62.4 KB, 1824 views)
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Old 24th August 2006, 11:41 PM   #68
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The rest of it...
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File Type: jpg box tipping.jpg (56.5 KB, 1747 views)
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Old 24th August 2006, 11:50 PM   #69
TNT is online now TNT  Sweden
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Click the image to open in full size.

and

Click the image to open in full size.

and

Click the image to open in full size.

and ...

no vibration what so ever. Even at near xmax, one can't feel any vibration in the cabinet.
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Old 25th August 2006, 12:17 AM   #70
owdi is offline owdi  United States
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Quote:
Originally posted by pinkmouse
Simple Newtonian physics.

Lets say a cone weighs 50 grammes, and the enclosure weighs 10 kilos, a not unreasonable assumption I feel. If the cone is moving, by Newton's conservation of momentum the forces must balance. The ratio of cone movement to to enclosure movement is therefore 50/10 000, which gives us a maximum possible movement in the box of 0.005% of the movement of the cone. And that's with a box that's not coupled to anything, and is completely free to move. I'm not that worried.

Quote:
Originally posted by Cal Weldon
I believe his arithmetic is correct. The mass in motion ratio is 50 to 10000 or .005. Where there might be concern is that is was listed as .005% rather than .005 to 1. It might read 0.5%

Quote:
Originally posted by pinkmouse
Quite right Cal. I blame Bill Gates.

But as I say, that is for a box that is completely free to move, say, resting on a frictionless surface. Any kind of restraint will reduce that by a significant degree
.5% is equivalent to -23db.

Things get more interesting when you consider the swept volume of the vibrating enclosure compared to the diaphram. Since the front and back of the enclosure can easily be 10x the area of the diaphram, the volume would be 5%, or -13db. For low frequencies it doesn't matter, because the front and back are out of phase. For frequencies where the quarter wavelength is less than the enclosure depth it starts to matter.

Since the box is not completely rigid and free to move, the real effect is much lower. Now the question to ask yourself, is it less than -20db? -30db? When is it ok to start ignoring it? I don't have the answers, but I'm definately interested in further discussion on the subject.

Dan

Edit: TNT - fantastic! I want to try something similar, but using aluminum rods and sonotube. Any chance you've taken nearfield distortion measurements with one vs. both drivers active?
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