The joint between the compression driver and waveguide doesn't always end up being very smooth. Is it worth smoothing this area more? Possibly adding some type of filler or sanding down any raised areas.
Many believe it is very important indeed because any turbulence will contribute to Higher Order Modes where sound waves are bouncing around before exiting the horn. Geddes describes the problem well. Many of us have gone to the length of making a horn that exactly matches the exit angle of the compression driver. It is also recommended to fill any tiny gaps at the interface with modeling clay although I've never figured out how to do that while retaining the insect screen.
I've measured frequency response and phase anomalies up around 9-11 kHz when a compression driver is not well centered on a horn flange (within 1/32 inch), and perhaps evidence of harmonic distortion generation, which is pretty difficult to hear at 20 kHz, second harmonic. This would depend on the length of the throat or snout of the compression driver from its acoustic center to the driver/horn interface.
There is some question as to whether there were any audible higher order modes generated but this is anecdotal only at the frequencies associated with the 1/4 wavelength distance from the compression driver acoustic center to the entrance to the horn--which are very high. I also believe that the driver sounded a little smoother after alignment on the horn--again implying audible higher order mode generation--but it was very difficult to hear. I suppose if you're under 20 years old without presbycusis, the effect would be more audible.
As far as what you can do about irregular transitions: modeling clay (the type that doesn't harden) to smooth the transition is an old remedy. If you have any material intruding into the bore, ream out the horn entrance carefully, then back fill with modeling clay after the compression driver is attached. You can stick a rag into the throat of the compression driver before sanding/reaming and modeling clay smoothing, and when complete, remove the rag with all debris that might have been generated and left in the compression driver throat.
I do have to say that I've not detected any audible effects of having a slightly different exit angle on the compression driver and the entrance angle on the horn, in fact, most horns will have transitions that have some change in the ramp angle. I wouldn't sweat the small stuff--just smooth the transition with modeling clay carefully.
Chris
There is some question as to whether there were any audible higher order modes generated but this is anecdotal only at the frequencies associated with the 1/4 wavelength distance from the compression driver acoustic center to the entrance to the horn--which are very high. I also believe that the driver sounded a little smoother after alignment on the horn--again implying audible higher order mode generation--but it was very difficult to hear. I suppose if you're under 20 years old without presbycusis, the effect would be more audible.
As far as what you can do about irregular transitions: modeling clay (the type that doesn't harden) to smooth the transition is an old remedy. If you have any material intruding into the bore, ream out the horn entrance carefully, then back fill with modeling clay after the compression driver is attached. You can stick a rag into the throat of the compression driver before sanding/reaming and modeling clay smoothing, and when complete, remove the rag with all debris that might have been generated and left in the compression driver throat.
I do have to say that I've not detected any audible effects of having a slightly different exit angle on the compression driver and the entrance angle on the horn, in fact, most horns will have transitions that have some change in the ramp angle. I wouldn't sweat the small stuff--just smooth the transition with modeling clay carefully.
Chris
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I had a feeling it may not make any noticeable difference, but it's easy enough to do. I don't care about the bug screen.
I have some of that plumbers puddy that comes in a brick that is super sticky. Might try that out.
Would removing the felt gasket from the CD be helpful? That would narrow that gap a bit. Since I would be filling the gap I can't see what purpose the gasket would sever at that point.
I have some of that plumbers puddy that comes in a brick that is super sticky. Might try that out.
Would removing the felt gasket from the CD be helpful? That would narrow that gap a bit. Since I would be filling the gap I can't see what purpose the gasket would sever at that point.
Chris, I always appreciate you sharing your thoughts and experience. Any idea on how much of a "small difference" in angle might begin to be a problem? I've been thinking about trying out the de500. I've seen others say it's angle is 17 degrees. I'm assuming that is both sides? So 8.5 degrees per wall? That would be very close to my 6 degree horn. But 17 per side would probably be a bit much?
The "bug screen" could very well be an integral part of the driver design made as a phase plug etc... beware... if it's just not a mesh.
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Analogy. Whats better, driving down the highway and having your car dropoff a 3" level transition or a 2 foot transition? ....Nuff said. Hahahaha drunk,sorry!
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Lol! Saturday night in Canada is similar to Saturday night in South Dakota. Hahaha.
Sure smaller is better, yet it is reasonable to ask at what level transition you are likely to bend the frame, or lose a wheel!
Sure smaller is better, yet it is reasonable to ask at what level transition you are likely to bend the frame, or lose a wheel!
I remember trying to smooth the transition in one of those plastic horns I bought at P.E. by filing and filling. Finally getting a smooth transition from CD to throat. I did before and after measurements and listening and I could find no find no difference in either. 
Have to say that surprised me. Still, out of what seems like "best practices" (or paranoia) I try to make the transition as smooth as possible.
On the other hand, I certainly have measured and heard differences at the mouth of the horn.
Have to say that surprised me. Still, out of what seems like "best practices" (or paranoia) I try to make the transition as smooth as possible.On the other hand, I certainly have measured and heard differences at the mouth of the horn.
I've measured this from a same-but-different cause:
Cheap PA cabinet. The horn mouth is continuous with the body of the cabinet. The horn throat/driver mount is a separate hunk of plastic. There was a deep seam between these two parts of the horn.
Filling in that seam filled in a deep notch around 10kHz, and gave slightly more HF extension.
Interesting update. Just got 3 DE250s and these already have some type of smooth black material filling in the gap. Older photos I’ve seen of the DE250s don’t show this. B&C must have made an update somewhere along the way. The walls of the tube running through the magnet now are smooth all the way up to the gasket. You cannot see any silver metal anywhere inside. Compared to my DNA360s the DE250 definitely feels a little nicer. Smoother edges. Gasket is nicer. The path inside the DE250 is clearly smoother than the 360. We’ll see if any of that actually makes any difference. If I was shopping in person and just held and inspected both side by side, the DE250 would be my pick.
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