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Old 1st September 2009, 01:16 AM   #1
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Default Elliptical curve better than simple radius for cabinet edges?

I haven't seen any research as to whether an elliptical curve might be better than a simple radius to break cabinet edges to reduce diffraction. I mean the sort of shape generated by a French curve template, with the more gradual slope tangent to the baffle front, then falling away faster as it turns toward 90 degrees. If this is large enough, would it not be better at reducing diffraction? We know a larger radius is better than small, which really has little effect. So a really large radius facing the listener, smoothly increasing to a smaller one facing away from the listener, might prove even more effective.

I know this shape would be more difficult to produce with power tools, but could be shaped by hand. Would it be worth it?

I am interested in executing unconventional midrange/tweeter cabinet shapes.

I also intend to make the rear of the cabinet triangular, so the sides meet at a point instead of a flat back in order to reduce internal reflections. The walls of the triangle would be damped, and the point of the triangle very heavily damped with non-woven cotton batting. Have you ever looked at the absorption co-efficients of cotton?

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Tom E
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Old 1st September 2009, 02:03 AM   #2
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This is the kind of shape Terry Cain used on his supraBaffles...

dave
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Old 1st September 2009, 02:17 AM   #3
dlr is offline dlr  United States
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Quote:
Originally Posted by madisonears View Post
I haven't seen any research as to whether an elliptical curve might be better than a simple radius to break cabinet edges to reduce diffraction. I mean the sort of shape generated by a French curve template, with the more gradual slope tangent to the baffle front, then falling away faster as it turns toward 90 degrees. If this is large enough, would it not be better at reducing diffraction? We know a larger radius is better than small, which really has little effect. So a really large radius facing the listener, smoothly increasing to a smaller one facing away from the listener, might prove even more effective.

I know this shape would be more difficult to produce with power tools, but could be shaped by hand. Would it be worth it?
It will be different at altering diffraction. It would require measurements to see if any particular profile is better.

However, a round profile is essentially elliptical with regard to the axes from driver center to baffle edge at all angles with the exception of those angles that are normal to the roundover axis. As an example, a square has 4 and only 4 angles that are actually quarter rounds to the ray that passes over it. At all other angles the effective profile is an ellipse. Just cut a tube at any angle that is not straight across. The resultant profile will be an ellipse.

It's not worth the effort unless the shape alters the diffraction in a way that happens to work with a particular driver (primarily tweeters)and it's directionality and/or inherent raw response.

Dave
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Old 1st September 2009, 02:34 AM   #4
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Planet 10:
Thanks for pointing that out, but I'm not sure what your purpose was in doing so. I wasn't suggesting it hadn't been tried before. I am wondering if it might be significant, or at least worthwhile, or even slightly preferable. Any idea what the effects might be? Do you believe it was a factor in the sound of those speakers? I've heard of them, but never heard them. I believe they enjoy a favorable reputation, but not commercial success. It might be that attention to such details contributes to a level of excellence, or it could just be wasted effort in the desire to be unique. It seems that his designs are carefully considered, but might be termed eccentric, or certainly unusual, in some regards. The cables also appear to be of a unique design, again, possibly to the point of eccentricity, which is a wonderful quality when it makes something better as opposed to merely different.

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Tom E
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Old 1st September 2009, 02:41 AM   #5
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There is a standard router bit with that sort of profile, I think it is called a "Balustrade bit " I looked at one but my router is too small. I do recall that they were very expensive.
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Old 1st September 2009, 02:45 AM   #6
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I have a feeling it might get you a small improvement, but given that a champher works as well as a round over from a practical POV, the major benefit may well be aesthetic.

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Old 1st September 2009, 02:47 AM   #7
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Quote:
Originally Posted by Moondog55 View Post
There is a standard router bit with that sort of profile, I think it is called a "Balustrade bit " I looked at one but my router is too small. I do recall that they were very expensive.
They are pricy... Chris bought one for doing supraBaffles.

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Old 1st September 2009, 02:58 AM   #8
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DLR:
I see your point about sectioning a roundover at any point other than the four 90 degree quadrants of the driver's center being an ellipse. However, any elliptical "roundover" will be even more elliptical than that simple radius at any point.

However, it's not the elliptical shape that I think might be significant, but the large size of radius tangent to the baffle front, getting smaller only as it "turns away" from the listener.

Changing diffraction--more specifically, reducing it--is precisely what I'm trying to do. Do you think it would be driver dependent? Say, a 1" dome tweeter with a 4" flange and very good dispersion, mounted on a baffle with a flat surface no wider than the driver itself? This would be close to the egg-shaped ideal. And I'm don't mean a flat surface sliced onto an egg, as so many are improperly implemented, but a radius tangent to the baffle (or tweeter flange), similar to B&W Nautilus design, or the Cain & Cain design.

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Tom E
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Old 1st September 2009, 03:12 AM   #9
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Yeah, balustrade shape is what I'm thinking of, but difference of scale here. I don't mean a small profile, but something like a 3" wide by 1-3/4" deep french curve. That would make a 6" wide balustrade! This won't be machined into solid material, but molded near net shape, to maintain constant wall thickness.

This is considered only for tweeter/midrange cab. I would be interested to see evidence that a chamfer works as well as a rounded profile, especially a large scale profile approaching one wavelength, for mids/highs.

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Tom E
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Old 1st September 2009, 04:09 AM   #10
dlr is offline dlr  United States
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Quote:
Originally Posted by madisonears View Post
DLR:
I see your point about sectioning a roundover at any point other than the four 90 degree quadrants of the driver's center being an ellipse. However, any elliptical "roundover" will be even more elliptical than that simple radius at any point.

However, it's not the elliptical shape that I think might be significant, but the large size of radius tangent to the baffle front, getting smaller only as it "turns away" from the listener.

Changing diffraction--more specifically, reducing it--is precisely what I'm trying to do. Do you think it would be driver dependent? Say, a 1" dome tweeter with a 4" flange and very good dispersion, mounted on a baffle with a flat surface no wider than the driver itself? This would be close to the egg-shaped ideal. And I'm don't mean a flat surface sliced onto an egg, as so many are improperly implemented, but a radius tangent to the baffle (or tweeter flange), similar to B&W Nautilus design, or the Cain & Cain design.

Peace,
Tom E
The egg shape is probably ideal, but it also does limit a tweeter the most due to the transition from 2-pi to 4-pi being much higher in frequency. Just one the many tradeoffs that we always have to consider.

Diffraction is definitely driver related specifically having to do with it's directionality. At higher frequencies where a driver may still have some off-axis (90 degree) output, there may be some non-linearity in how the edge is "illuminated" as some refer to it. At lower frequencies, it's more related to just the effective diameter (acoustically effective diameter). A tweeter shows the highest impact due to being more dispersive at frequencies related to typical baffle dimensions. But even small midranges can see significant impact, only it will be at lower frequencies than for a tweeter for the most part, since tweeter output drops off higher than a mid does. There's a rather wide range of possibilities, it's all a continuum of driver diameter to baffle dimension ratios.

There are some other variables as well. Which section of an ellipse will be cut into the edge and what is its impact? Let's just consider the nearest point profile, that edge normal to the ray from the driver center. I'm not sure which one you referenced.

Will the ellipse be a full quarter section of an ellipse, will the tighter radius portion intersect the baffle front surface or will it intersect a side with the shallower section intersecting the baffle front? This will change the transition and how it affects different frequency ranges. I suspect that the tighter radius section should intersect the baffle front.

Almost any curved edge will help in one way or another on some axes, but part of what any edge radius does is affect the diffraction in the off-axis as well as the on-axis. To say that a particular ellipse improves the diffraction (reducing it, as you say) may work on one axis and not so well on another. This is pretty much the case for essentially all edge treatments. You can study a number of baffle edge shapes and dimensions coupled with a baffle shape and driver directionality in things like the Baffle Diffraction Simulator, but complex shapes and ellipses are not covered in software I've seen, though I'd like to see it. In the end I see it as a lot of work for possibly dubious results since a plain, round edge of sufficient radius should provide enough improvement.

Or you can use square edges and use surface treatment and get results nearly as good if not better, but it is quite a different aesthetic. The effectiveness is always case dependent.

Dave
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