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Old 30th June 2007, 03:25 AM   #1401
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by johninCR
Too bad the scientists can't explain exactly what happens with edge diffraction, at least I haven't seen an explanation. Then we'd be armed better to attack them. Lynn refers to them as reflections, and though I don't think there's an actual reflection of sound at the edge, I think you get a better view of the picture.

Rudolf explained to me, in a way that made me see the light, that the pressure change at the edge causes a new sound source to occur there. In the case of a bipole with a sharp edged baffle, there is no pressure change and no sonic event would occur other than the 2 waves to combine and continue the same expansion. With a monopole there is a pressure change at the edge, because the wave has been constrained to half space expansion by the baffle, and at the edge there is a pressure drop because the wave suddenly has a larger space for expansion. With dipoles the pressure change at the edge is double, because the high pressure portion of the wave is meeting the low pressure portion coming from the other side. Since the pressure change is greater, the resulting sound created must be greater too.
Actually one can visualize the effects if an appropriate Finite Element Analysis tool is used. The technology is there, it's just a matter of whether the people that use it feel it worth the effort to go in that direction or not. Since it is already common knowlege that all efforts should be made to minimize diffraction effects, it's easier to just apply it based on the current simple tools available. These tools are already sufficient to make diffraction less dominant than driver performance.
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Old 30th June 2007, 03:40 AM   #1402
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JohninCR,

This is a clear if somewhat abbreviated and non mathematical summation of boundary layers and what affects them. I am sure that a translation of Prandtl's original paper would also help with the OB questions raised so far..... wanna bet there are even more energy functions occurring than those already argued over in such a friendly and sane fashion?

http://units.aps.org/units/dfd/prand...no12p42_48.pdf

Panomaniac,

Actually I think it would work, the hairy cones... but I do suspect that the efficiency would suffer a bit. But you might be able to see the waves in the grass and surely hawaiian dance music would get the stuff swaying...

Bud
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Old 30th June 2007, 03:40 AM   #1403
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Quote:
Originally posted by soongsc

Actually one can visualize the effects if an appropriate Finite Element Analysis tool is used. The technology is there, it's just a matter of whether the people that use it feel it worth the effort to go in that direction or not. Since it is already common knowlege that all efforts should be made to minimize diffraction effects, it's easier to just apply it based on the current simple tools available. These tools are already sufficient to make diffraction less dominant than driver performance.
I don't need a tool, though an animation might be nice. Just an explanation is fine, especially if it's down to the level where Huygen's principle leaves off. I'll figure my own way around it, and have little interest in simulating it only to get bogged down in trial and error simulations. Those got old long ago.
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Old 30th June 2007, 04:16 AM   #1404
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Quote:
Originally posted by BudP
JohninCR,

This is a clear if somewhat abbreviated and non mathematical summation of boundary layers and what affects them. I am sure that a translation of Prandtl's original paper would also help with the OB questions raised so far..... wanna bet there are even more energy functions occurring than those already argued over in such a friendly and sane fashion?

http://units.aps.org/units/dfd/prand...no12p42_48.pdf

Panomaniac,

Actually I think it would work, the hairy cones... but I do suspect that the efficiency would suffer a bit. But you might be able to see the waves in the grass and surely hawaiian dance music would get the stuff swaying...

Bud

Bud,

Interesting stuff. Now how about linkage with sound, since it propagates by back and forth movement instead of a flow. I'm seeing my shoe moving back and forth on a smooth floor that is covered lightly with sand, and the sand is my boundary layer. Also assuming there's linkage to sound, is it significant enough to become audible? I was visualizing your treatment's effect on driver cones to be something different, instead diffusing the waves propagating within the cone itself as opposed to something going on in a boundary layer.

Regarding the hairy cones, with enough micro fibers they may even increase sensitivity due to the greater surface area offering a better impedance match with air.
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Old 30th June 2007, 04:59 AM   #1405
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JohninCR,

Now move your foot really fast and really far, about 13k inches in a second.

The voice coil is a pump, the emitter surface is a passive device that is being acted upon by lateral energy displacement and this lateral displacement creates a compression wave in the adjacent air, that is attached to the emitter at the moving peak of the lateral energy displacement.

This is the classic Walsh description of a Bending Wave transducer. All transducers act like this and as Beranek and Corrington showed us 50 years ago or so, the piston model functions only at one frequency, all other frequencies use the wave ripple model, both above and below that perfect piston point. The wave ripple model uses the boundary layer to transform energy from a lateral wave to a compression wave.

That boundary layer is extremely fragile and can be disturbed by very minute events, like paint that alters the surface height or Reynolds number, or both.

Once the emitter has been passed by, the compression wave and that lateral connection to the boundary layer of the next surface, move across that surface. When the surface terminates, a multitude of events occur. All have been described in this argument to date.

One of these events is, that what energy cannot leave that boundary layer, reflects, creating a new pressure wave as it re traverses the surface, attached to the boundary layer. Control of that boundary layer reflection is what this EnABL thing is all about.

It works because the boundary layer interface is fragile and the blocks of paint are just enough to raise the impedance to that back wave reflection, to cause that energy to exit in one of the other modes available. The least expensive being in phase with the original compression wave.

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Old 30th June 2007, 05:37 AM   #1406
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Quote:
Originally posted by johninCR



Bud,

Interesting stuff. Now how about linkage with sound, since it propagates by back and forth movement instead of a flow. I'm seeing my shoe moving back and forth on a smooth floor that is covered lightly with sand, and the sand is my boundary layer. Also assuming there's linkage to sound, is it significant enough to become audible? I was visualizing your treatment's effect on driver cones to be something different, instead diffusing the waves propagating within the cone itself as opposed to something going on in a boundary layer.

Regarding the hairy cones, with enough micro fibers they may even increase sensitivity due to the greater surface area offering a better impedance match with air.
There are two things that take place with the pattern.

1. When used on structures, It stimulates a turbulant boundary layer, and delays the boundary layer separation, due to this delay in separation, the diffraction effects are reduced in magnitude. I believe you will can measure the differences. Effects can be visualized via analysis. The theory is similar to why golf balls have those dimples.

2. When used on thin membranes such as cones, the patterns will breakup the vribration wave. Wavelength of effectiveness depends on material size, material properties, and pattern layout and pattern material.
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Old 30th June 2007, 06:15 AM   #1407
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Across a structure, there isn't a flow, just very small back and forth movement of the same molecules, so my shoe would make very small fast movements. That's why I asked for the linkage that a similar analysis applies to sound. Due to the type of movement involved, boundary separation must be a very different thing.

With the cone treatment, I'm having real trouble getting past that EnABL isn't a strategic geometric damping/diffusion of waves in the cone itself, just like the blocks in the water tank (which doesn't look like something related boundary layers to me).

I guess I'll just have to try blocks of paint on one baffle and something thicker like a foam tape on another, and see what happens.
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Old 30th June 2007, 06:50 AM   #1408
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Default Can't resist, sorry.

All ... Please keep up this endless BS of diffraction academics and allow the actual building of many more important aspects of building an OB slowly die.
At my age I will expire before anything gets accomplished. Forgive my impatience, and I do expect a little wrath over my butting in to this purely classroom boring rhetoric.
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Old 30th June 2007, 07:34 AM   #1409
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Possibly this thread has strayed away from its original direction.

I have not heard any more from Lynn Olson since this diffraction discussion began.

Maybe a new thread about diffration and its effects on an OB should be started.

I am interested in the latest OB design imformation that Lynn has
thought of.

Anything new Lynn?
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Old 30th June 2007, 08:06 AM   #1410
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By all means let's go back to the endless banter about drivers and XO's.
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