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Old 11th April 2016, 11:43 AM   #1
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Default Question about Speakers vibrations

Dear all!

In first place, i want to say hi to all the comunnity. You have the best forum in the world about diy audio and its a gorgerous resurce for absolut dummies because your rigurosity. Im from Malaga, south Spain, and its very difficult found resources in spanish with rigurosity and good quality info.

However and before of entry in the matter of this post, i want to say sorry for my english, its not my mother tongue.


Few months ago, i build four scoops thats lather i converted in two 2x15. I dont follow any planes and started from scracht. I committed a lot of errors in that desing, but i dont care, i learn a lot about that errors (with help of your forum and anothers) and i solve the major part with sucesive mods.

At day of today, im very happy with that subwofers. Has good "wooob woob" and are so louder for 2x15' 50 300w speakers.

But for finish them, i have this problem: I use MDF of 16mm and the wood has loads of vibrations. I put a lot of screws. I want to ask you posible solutions about this problem.

I focused in this options, but i dont know if its better doing one, some or all the options and i want your advice.

1) Break the linearity of the flat bottom with a curve wood
2) Put foam or other absorbent material inside
3) Reinforce all the walls with another layer of wood (this will make heavy for the low power of that boxes)
4) Seal all the unions of the wood with Sicaflex, silicon or something similar

Sorry for the looong text!!

Very big hug and gratitude for all!

Click the image to open in full size.

PD: Are the bottom subwoofers (black and red)
PD2: Another day, i want to publish my other desing, the 2x 6x9 speakers. And although not believe me, they sound great for plywood boxes only 75w and 15.
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Old 11th April 2016, 02:40 PM   #2
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well MDF is not the right material for sub boxes that are going to be worked hard in a DJ rig.

baltic birch (furniture or marine grade) with multiple plies is far superior(but costs more sorry in this case you get what you pay for) and sealing all adjoining pieces is a must (leaks can severly reduce output)
as far as "rounding" over fold areas it will increase higher frequency output ( not really what you want coming from a sub) stuffing or damping is useful but is not a cure all.
internal bracing such as 1x2 glued and scewed at odd angles can help reduce the vibration of large panels.

Last edited by turk 182; 11th April 2016 at 02:46 PM.
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Old 11th April 2016, 03:06 PM   #3
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Another chance: dipol.
The backward sound is not cased and does not pump the casing. And you would not need big casings. And, pa, you could tune by filling with any stuff - all different circumstances.
To stiffen "closed" mdf-casings is very elaborate. You have to connect the opposite planks all 8 - 10 cm!
"Closed" casings I would fill up - not pressed, but full - with textile, like t-shirts, stockings,-) No wool, no foam, no expensive speaker-damping. Everything else and unfilled, or sparsely filled does rumble and roar (the inner sound comes through the membranes, modulates the correct-phasing sound, as example).
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Old 11th April 2016, 10:12 PM   #4
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Quote:
Originally Posted by echelion View Post
1) Break the linearity of the flat bottom with a curve wood
2) Put foam or other absorbent material inside
3) Reinforce all the walls with another layer of wood (this will make heavy for the low power of that boxes)
4) Seal all the unions of the wood with Sicaflex, silicon or something similar
None of the above. The boxes likely need to be cross-braced internally.
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Old 12th April 2016, 04:07 PM   #5
DrBoar is offline DrBoar  Sweden
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A dipole is as pressure stressed as any bass reflex cabinet, the fact that is open does not change this ṕhenomenon. Try an open baffle and feel the vibrations. The whole point of baffle is to separate the front and backwave (of oppoiste pressure) to cancel out each other.

Imagine that the cone is moving forward. A positive pressure wave will propagate radially in front of the driver, and an equal negative pressure wave from the back of the cone. The pressure waves are separated by a wall be it in a box or baffle. Both are subjected to Newtons third law with respect to the cone movement and to the pressure differerenses.
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Old 12th April 2016, 04:52 PM   #6
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Originally Posted by DrBoar View Post
A dipole is as pressure stressed as any bass reflex cabinet....
Strange physics!

A moment's thought will reveal that the ghostly effects of phase cancellation around the back (which monumentally disturbs members of this forum) are nonsense. Yes, for steady-state sine waves there will exist certain points, lines, and contours in space where complete cancellation will exist in an anechoic chamber. But instead, think of a music bouncing off six surfaces.....

Using dipoles for decades over 140 Hz, I've never observed or measured any such cancellation. Likewise, when I had a gigantic open baffle sub with a 20 Hz driver, the room eigentones swamped any cancellations too.

Ben
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Old 12th April 2016, 05:19 PM   #7
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Theory and experience,-)
FULLRANGE! 12 x 12". Even in front of a wall they play subs - but ugly in general, cause all reflexions and resonances of the (big) casing are reflect. But with some, or a lot, place, a dream. Clean! Clean! Clean! - relative to "closed" big casings. A dream. Never I would use separate subs, big horns or br or bp for big performances again.
And cheap,-!
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Old 12th April 2016, 05:53 PM   #8
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Quote:
Originally Posted by cumbb View Post
Another chance: dipol.
The backward sound is not cased and does not pump the casing. And you would not need big casings. And, pa, you could tune by filling with any stuff - all different circumstances.
To stiffen "closed" mdf-casings is very elaborate. You have to connect the opposite planks all 8 - 10 cm!
"Closed" casings I would fill up - not pressed, but full - with textile, like t-shirts, stockings,-) No wool, no foam, no expensive speaker-damping. Everything else and unfilled, or sparsely filled does rumble and roar (the inner sound comes through the membranes, modulates the correct-phasing sound, as example).
...
Quote:
Originally Posted by cumbb View Post
Theory and experience,-)
FULLRANGE! 12 x 12". Even in front of a wall they play subs - but ugly in general, cause all reflexions and resonances of the (big) casing are reflect. But with some, or a lot, place, a dream. Clean! Clean! Clean! - relative to "closed" big casings. A dream. Never I would use separate subs, big horns or br or bp for big performances again.
And cheap,-!
This is a pro sound application. Dipole losses are very real and suggesting a dipole subwoofer for a prosound application is ridiculous, it won't work.

Looking at your pictures, it appears that you have 12 drivers that are at least 12 inch, maybe 15 inch drivers in a very small room. THIS is exactly why a dipole subwoofer for a prosound application simply isn't going to work.

The lean and clean sound of dipole comes at the very real cost of dipole cancellation. In a small room dipoles can do well if designed properly but only above the Schroeder frequency. Below the room's modal region it's impractical to use dipoles, as dipoles don't produce any room gain.

In an open space prosound environment where there is no Schoeder frequency, no room modes, no reflections, dipole simply makes no sense. You would need to spend a fortune on drivers and it still would never be loud enough. You could fill the entire stage with dipole subs and it would never be loud enough.
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Old 12th April 2016, 06:05 PM   #9
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Quote:
Originally Posted by bentoronto View Post
Strange physics!

A moment's thought will reveal that the ghostly effects of phase cancellation around the back (which monumentally disturbs members of this forum) are nonsense. Yes, for steady-state sine waves there will exist certain points, lines, and contours in space where complete cancellation will exist in an anechoic chamber. But instead, think of a music bouncing off six surfaces.....

Using dipoles for decades over 140 Hz, I've never observed or measured any such cancellation. Likewise, when I had a gigantic open baffle sub with a 20 Hz driver, the room eigentones swamped any cancellations too.

Ben
The dipole cancellation phenomena is well understood, well documented, and very real. Anyone who has actually used a dipole speaker or subwoofer should know very well that they are output limited below the baffle step frequency due to cancellation. Using them in a small room will obviously net some benefit above the Schroeder frequency due to reflections and modes but dipole cancellation is very real and it does take a huge toll below the baffle step frequency.

With regular normal placement in a room the dipole cancellations will happen right at the edge of the baffle, long before the waves ever get to the boundary reflection points. So whatever is left after the cancellation can be reflected and can cause room modes but in any normal situation the cancellation occurs long before the waves ever reach the walls of the room.

It may be an inconvenient fact for you, but measurements do match the math on this subject and it can all be easily simulated. I have a wealth of experience with dipole speakers and subs, the measurements don't lie, they directly support the theory.

Your 1 meter square dipole panels are no exception. Due to their huge size the baffle step frequency is quite low. This is a baffle diffraction profile of a 1 meter square baffle with the entire surface area radiation source, like your ESL panels.

Click the image to open in full size.

As you can very clearly see there ARE no losses until you get lower than 140 hz. THIS is why you didn't notice or measure any losses in the panel's passband when only used above 140 hz. The theory isn't wrong, you just don't understand it. The losses below 140 hz, below the baffle step frequency are significant.

Maybe ask yourself why you chose 140 hz as a lower limit to use these panels. Was it a manufacturer's recommendation? Was it because they don't do well below 140 hz? It seems strangely convenient that you chose 140 hz to use as a crossover point when the theory indicates that 140 hz is the lower limit to the diffraction profile's flat response and below 140 hz the diffraction profile takes a nosedive into very low spl territory. This can't be coincidence. This is dipole cancellation theory in full effect providing real world measurable results that match the theory.

As for your dipole subwoofer, it's a unique case of a massive baffle size located parallel and only a few inches from a wall. It's not a true dipole in that sense, it's actually just as similar to a bandpass as a dipole because it does have an acoustic load on the cone provided by the air between the massive baffle and the wall.

The theory is not wrong, the measurements match the theory and the sims match the measurements. The problem is that you don't understand the theory.

Last edited by just a guy; 12th April 2016 at 06:24 PM.
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Old 12th April 2016, 08:17 PM   #10
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I would furthermore recommend:
Your sum: In rooms no problems. Further: Including a lot of stuff and people ... One drawback: Rumbling and roaring not everywhere. Dipols play a bit focussed.
Open air: A chance for the neighbourhood not to get everything and not to play subsonics. Nobody wants +/- 0 dB, 20 or 40 Hz. Your mesurement under "ideal"-) circumstances shows - 3 dB, 100 Hz, -6 dB, 80 Hz. Square 1 meter. 15" drivers end in 1,50 meter or more. And it is simple to extend locally. Or by filling a bit (as described) to get an other relation front - back - sonic. And including a lot of stuff and people around...
Than: The most never listened clean subs. They listen per Dynaudio or Sonus Faber, per Marshall or HK. Clean subs, clean tones at all, require stone or any synthetics. No chance to get per wooden-material. The most misunderstand rumbling and roaring casings as subs. A little chance are dipols. Required decoupled and clamped drivers, as every driver has to be, and tuned ...
I would furthermore recommend.

But: cause the most never listened clean subs, may be they would miss rumbling and roaring. A problem that I know. And: The most want bawl, bellow, cry, not listen good music well or get real rhythm to dance,-)
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