line source diffraction

[david yost]

Hello All:

I have searched the site, but didn't find any posts about diffraction effects with line sources.

I would think that, considering a line in a tall thin box would have essentially equal path length from source to cab edge, diffraction control would be more important than with conventional point source boxes with offset drivers. I haven't seen this issue addressed by either commercial manufacturers or diy'ers other than some that use tapered or eliptical baffles.

Any thoughts about this? Suggestions for minimizing diffraction effects through cabinet design or edge damping (foam or felt)?

Thanks in advance,
David

[Jim Griffin]

David,

You are not going to find much info on this subject as line arrays are just like smaller speaker systems for most diffraction effects. First, you need to flush mount the drivers--especially the tweeters for best performance. You can taper the baffle sides to disperse the effects just as you do for point source speakers.

For near field arrays remember that the soundfront is emitted parallel to the floor and ceiling, i.e., radially from the line source so you don't have much vertical spreading as you would get from point sources.

Finally, in reference to offsetting drivers asymmetrical from the edge of the enclosure: you can do this but I suspect that for most line arrays with side by side woofer and tweeter lines that little benefit will be attained.

Jim

[david yost]

Thanks, Jim.

In my application, I am using a 60" ribbon which has a mounting flange 5.5" wide. I think that I can safely cross this as low as 300Hz using a 24dB/octave filter. If the ribbon can in fact do this, I may just mount the midwoofs perpendicularly on both sides of the cab, essentialy straddling the tweeter line. I do have delay compensation available. Any thoughts?

David

[Jim Griffin]

David,

what you suggest should work but I have a caveat or two to throw at you.

First, if you would make sure that your ribbon (are you sure that this is a ribbon and not a planar tweeter?) can tolerate crossing that low. Many such as the B-G versions have issues with a cross that low but rather prefer to be crossovered a little higher.

Second, if you mount the mid-woofers beside the ribbon be sure that you watch the center to center spacing between these woofers on either side of the ribbon. Note that this spacing includes the width of the ribbon and its frame, any mounting separation for the woofers, plus one woofer diameter. You want to keep that spacing less than a wavelength (better would less than a half WL distance) at the crossover freqeuncy. The idea is to prevent these woofers from inner acting and causing off axis nulls in the horizontal dispersion. For example, let assume an 8" mid-woof with a 9" overall frame diameter, 0.25" distance between woofer frames and the ribbon frame on each side of the ribbon, and then the tweeter frame. This would yield a total c-t-c distance of 9"/2 + 0.25" + 5.5" + 0.25" + 9"/2 = 15". In this case this would be about one third of a WL at 300 Hz so it should work OK. If you raise the crossover freq. or if you use a larger mid-woof, then you need to recompute and verify the safety factor.

Jim

[david yost]

Hello again, Jim

Maybe "straddle" was the wrong way to describe what I have in mind for the midwoofers. What I mean to do is mount the mws on the sides of the aprox 6.25" wide cabinets with magnets essentially back to back. I can then use time delay on the ribbons. If my understanding of diffraction theory is correct, and the midwoofs are aprox. omni radiators at these frequencies, I should wind up with a fairly coherent wavefront along the vertical plane.

[planet10]

Originally posted by david yost
Maybe "straddle" was the wrong way to describe what I have in mind for the midwoofers. What I mean to do is mount the mws on the sides of the aprox 6.25" wide cabinets with magnets essentially back to back.

Sort of like they are here:




http://www.t-linespeakers.org/FALL/dipole.html

dave

[david yost]

That's pretty much the idea, except my box will be an infinite baffle tuned to Qt of <.5 and EQed.

Thanks for the link!

David

[LineSource]

Hi David,

I don't have complete faith that at 300Hz the sound coming off the rear sides of a "straddled pair" of speakers would be the best design. Great 300Hz sound would seem to demand a direct launch from the front cone. Some careful listening to a prototype seems important.

Linkwitz has proven that a W style baffle can work well for 80-100Hz with proper electronic compensation and cavity filters. For a dipole implementation to provide additional distortion cancellation, he puts the front of one driver opposite the rear of the other, not back to back. The back-to-front averages the non-linearity of the push-pull of the motor, and averages the front and rear chassis wavefronts. For a monopole or infinite baffle, just listening to the front cone of the speakers seems best.

For "all speakers face front" dipole linesources I got the best sound on narrow baffles ~18" with just rounding over the narrow side baffle edge, and for wide baffles >24" I got the best sound gradually widening the outer 6" to a 4.5" radius. This cylinder area at the outer edges is a great cavity for sand. I wrap Formica around plywood ribs to create this cavity.

For monopole linesources, I found a 4.5" edge radius to give the most even sound with the normal baffle compensation circuits. When I experimented with tapered sides that form a trapezoid, the compensation circuit set to the average width gave a larger set of dips and peaks in SPL.

[david yost]

Hello. LineSource

The only reason I am contemplating a "straddled" line at all is because I want to use it as a center speaker in a Trinaural set-up, and wanted symmetry of horizontal radiation. OTOH, if you guys think that a conventional MT line using 6.5" midwoofers snuggled up to a 5.5" wide planar/magnetic won't cause too many lobing problems off axis, I'll go this way, as cabinet construction would be much simpler.

Another possibility is a Newform or small Wisdom type layout with the ribbons in line centered over the woofer/s. Fact is, I have a bunch of reasonably high quality poly mid/woofers I would like to use for this project...that's why all the frettin'.

Thanks,
David