[TomW16]

If I remember correctly, Dave Ellis did some experimentation with different edge radiuses and he ended up with over a 1 inch radius to reduce diffraction effects.

Tom

[Jeff B.]

What a radius does is randomize the frequencies affected by the edge over a wider range and reduces the amplitude of the affect due to diffraction. If you leave one edge as a sharp corner then you will get some increase in ripple at frequencies whose wavelengths are associated with the distance to this edge. If the edge is 4" away, then there will be small hump at 1700 Hz, a small dip at 3400 Hz, and so on. This ripple will be reduced though compared to all sides being a hard break, so they will be on the order of 2-3 dB at 3400 Hz, decreasing with increasing frequency. Will it be measureable? Yes. Will it be audible, I really doubt it. The thing about diffraction is that off-axis it can fill in these peaks and dips fairly well, and what may look a little rough on one axis may look different on another axis, and overall sound very smooth. I really doubt that any affect will be heard by leaving a top edge sharp, without a radius. In other words you may not be getting much benefit for the cost, other than a slightly smoother measurement on a given axis.

Hey Jeff     Let me grab you on one of your infrequent visits.  Based on your real world experiences, and simulations with your software, can you say anything about how much benefit from roundover (let's say 3/4") you lose if you just radius the sides of the baffle around the  mid and tweet and not the top?  The former can be done much easier with veneer than the latter, so I'm curious what the cost-benefit is.  Thanks.

[Mudslide]

What a radius does is randomize the frequencies affected by the edge over a wider range and reduces the amplitude of the affect due to diffraction. If you leave one edge as a sharp corner then you will get some increase in ripple at frequencies whose wavelengths are associated with the distance to this edge. If the edge is 4" away, then there will be small hump at 1700 Hz, a small dip at 3400 Hz, and so on. This ripple will be reduced though compared to all sides being a hard break, so they will be on the order of 2-3 dB at 3400 Hz, decreasing with increasing frequency. Will it be measureable? Yes. Will it be audible, I really doubt it. The thing about diffraction is that off-axis it can fill in these peaks and dips fairly well, and what may look a little rough on one axis may look different on another axis, and overall sound very smooth. I really doubt that any affect will be heard by leaving a top edge sharp, without a radius. In other words you may not be getting much benefit for the cost, other than a slightly smoother measurement on a given axis.

Jeff, a question.  I don't know too much about the frequencies affected by the edge diffraction issue.  But won't the constant distance change (from the tweeter to closer or more distant cabinet edge areas) cause only an exceedingly minor amplitude increase/decrease, but extend across a broad range of frequencies?  Given your example...(in Dennis' case of top-only sharp edge) only at a single point would the cabinet edge be 4" from the tweeter, no matter the position of the tweeter in the cabinet (assuming a straight cabinet edge).

[Jeff B.]

You are correct. I simplified my answer for the sake of brevity, but in my longer article I will explain further. Typically, we look at it as rays emanating from a point, spaced by a few degrees radially. The 4" may apply only to the vertical ray, but the ones over the next several degrees in either direction will be fairly close to that distance, so the frequencies begin to become lumped in a specific range, so the affect is similar to my simple discription. The tweeter has rays that extend to the bottom edge too, but those frequencies are usually lower than the tweeter's operating range. Near the top the corners provide the longest rays, but the affect is less because the corners are points instead of lines.

If you have Excel, I have a program you can play around with and see all of this in action. It is located here:
http://audio.claub.net/software/jbabgy/jbagby.html

Jeff, a question.  I don't know too much about the frequencies affected by the edge diffraction issue.  But won't the constant distance change (from the tweeter to closer or more distant cabinet edge areas) cause only an exceedingly minor amplitude increase/decrease, but extend across a broad range of frequencies?  Given your example...(in Dennis' case of top-only sharp edge) only at a single point would the cabinet edge be 4" from the tweeter, no matter the position of the tweeter in the cabinet (assuming a straight cabinet edge).

[Mudslide]

You are correct. I simplified my answer for the sake of brevity, but in my longer article I will explain further. Typically, we look at it as rays emanating from a point, spaced by a few degrees radially. The 4" may apply only to the vertical ray, but the ones over the next several degrees in either direction will be fairly close to that distance, so the frequencies begin to become lumped in a specific range, so the affect is similar to my simple discription. The tweeter has rays that extend to the bottom edge too, but those frequencies are usually lower than the tweeter's operating range. Near the top the corners provide the longest rays, but the affect is less because the corners are points instead of lines.

If you have Excel, I have a program you can play around with and see all of this in action. It is located here:
http://audio.claub.net/software/jbabgy/jbagby.html

Great...thanks for the info and the program, Jeff!