Ameliorating the diffraction problem completely, home audio

[thadman]

I was contemplating how to make the perfect baffle (ie no diffraction effects) and began reading the information on Linkwitz' website and found this quote.

Originally posted by Linkwitz

The benefits of edge rounding come into play only when the radius is greater than 1/8th wavelength. Thus a typical 1/2 inch radius begins to diffuse the diffracted wave at frequencies above 3.4 kHz, but will decrease in relevance at higher frequencies, when the driver illuminates less of the edge due to its increasing directivity. With most speaker cabinets the radius or chamfer is acoustically too small and is primarily cosmetic.

How effective is a baffle with no edges (sides completely rounded over) at removing diffraction artifacts? I want to incorporate front and rear firing ribbon tweeters and dome midranges similar to what Linkwitz has done with his Orion setup and the front/rear firing Seas Milleniums. For reference, the Aurum Cantus G2si/NeoCD3 are <3.5" deep and the Dayton RS52 is 5.125" wide, so for simplicities sake lets entertain the idea of a baffle with a depth of 8 inches and effective "flat" width of 6 inches. For the roundover to completely consume that edge, a radius of 4" and a 4" roundover bit would be required. We now have an effective 14" wide by 8" deep baffle (4" radius + 6" baffle front + 4" radius=14")

radius=1/8 lambda, 4=1/8 lambda, lambda = 32". 13,397in/s divided by 32in=418.65hz. Thus the 4" roundover will begin to diffuse diffraction at 418.65hz.

Baffle width=14", 13,397in/s divided by 14"=956.93hz. Thus the panel will switch from 2pi to 4pi radiation ~956.93hz, ~1.3 octaves above where the effects of diffraction are being diffused by the roundover.

Here is a cross-section of the baffle (top view)


Opinions?

(I'm aware that a 4" roundover bit is impractical and I've already thought beyond that. I could use a jigsaw to cut the specific baffle shape several times...ok maybe more than several times and layer them so that they ascend to a certain height)

[DSP_Geek]

Would this help?

http://www.rockler.com/product.cfm?O...Select=Details

[Aengus]

thadman said:

I'm aware that a 4" roundover bit is impractical

If you are going to stack-laminate it ("layer them so that they ascend to a certain height") then cut it very carefully once, then:
- cut the next layer roughly to shape and slightly oversize (~1/8")
- glue or otherwise fasten it on
- use a router with flush-trim bit to cut it to the shape of the template (i.e., the first carefully-cut one).

This technique works well for stack-laminated plywood or MDF.

Regards.

Aengus

[Andy G]

http://members.optusnet.com.au/~grad...GOS/Stacks.htm

[liasom]

I saw Norm do a thing on New Yankee Workshop to turn two identical half-round ornaments on a lathe to frame a mirror. It involves gluing up a wood/paper/wood sandwich. However I'm completely uncertain if it will scale up to what you need to accomplish.

If you have access to a wood lathe that will handle the large diameter you might could glue 2 rectangular , e.g. 4.05" x 8.1", cross-section pieces together to make a 8.1" square cross section piece. Use regular yellow wood glue and some heavy kraft paper sandwiched all in between them down the length dimension (heightwise in baffle terms) of the piece. Then, after easing over the corners with a table saw, use the lathe to turn an 8" diameter cylinder. After turning, the two halves could be separated by prying them apart, thus tearing the paper, and you'd have two half-round cross section pieces with 4" radius and whatever length the lathe could turn. Sand the paper off before gluing them to either side of your rectangular baffle.

[CeramicMan]

Hi Thadman,

Yeah I think it's a great idea! I had a similar thing in mind here, whereby stacked layers enable you to create an intricate internal layout as well.

[edit: the image didn't want to paste, but it's that link about halfway down the page.

In terms of timber consumption it would be quite wasteful and time consuming - especially if you want to give the box a compound curve rather than just a cylindrical shape. But since the design isn't constrained to simple straight lines, you might as well make something more complicated.

[croat47]

Or...

http://aitwood.com/StoreFront.Asp?Wo...20Cylinders%20(180%20degrees)

I think there is a similar idea for a large radius edge on an OB floated in the "Beyond the Ariel" Thread...can't find it just now.

[Andy G]

http://members.optusnet.com.au/~grad...urved_loud.htm

[Svante]

Rounding the edges of a baffle does not eliminate the reflection at the baffle edge. A rounded edge can be seen as very many smaller edges, and each of these will result in a small reflection. The sum of these reflections is equivalent to the single reflection of a single edge, but it is smeared in time (which is good).

The only way to eliminate baffle diffraction is to put the loudspeaker "in-wall". Or to put it in vacuum...

[planet10]

Originally posted by thadman
affle width=14", 13,397in/s divided by 14"=956.93hz. Thus the panel will switch from 2pi to 4pi radiation ~956.93hz, ~1.3 octaves above where the effects of diffraction are being diffused by the roundover.

That may be where BS starts but we usually consider the -3dB point, 4560/14= 326 Hz.

dave