Greetings all,
in the thread 2 way Synergy inspired by SPL Runt, post # 46, mark100 argues that the distance from the MEH entry hole(s) should be measured from the middle of the hole(s) to the wave-guide to CD interface along the wave guide-wall. It seems consistent with other people's practice.
Is this because this interface is the first reflecting discontinuity causing the notch and this discontinuity has higher (perhaps even masking effect) than the reflection of the diaphragm?
If so, should this also apply to the case wherein the exit angle of the CD is matched to the exit angle of an axially symmetric wave-guide, where the discontinuity is minimized? Or, as mabat suggests extending the wave-guide all the way to the phasing plug, where there is no such a discontinuity?
Kindest regards,
M
in the thread 2 way Synergy inspired by SPL Runt, post # 46, mark100 argues that the distance from the MEH entry hole(s) should be measured from the middle of the hole(s) to the wave-guide to CD interface along the wave guide-wall. It seems consistent with other people's practice.
Is this because this interface is the first reflecting discontinuity causing the notch and this discontinuity has higher (perhaps even masking effect) than the reflection of the diaphragm?
If so, should this also apply to the case wherein the exit angle of the CD is matched to the exit angle of an axially symmetric wave-guide, where the discontinuity is minimized? Or, as mabat suggests extending the wave-guide all the way to the phasing plug, where there is no such a discontinuity?
Kindest regards,
M
There shouldn't be a discontinuity where the driver/horn attach. Of course often there will be, but not necessarily in this significant a way.
What about the acoustical significance of the cross sectional area back to the diaphragm. We can see a horn that goes back that far, but it does get small at a certain wavelength on the way back.
What about the acoustical significance of the cross sectional area back to the diaphragm. We can see a horn that goes back that far, but it does get small at a certain wavelength on the way back.
On my built unit, distance along the wall from the center of the hole to the inside of the compression driver corresponds pretty well to the cancellation notch seen in the FR measurement. Hornresp predicts it a bit higher (or I am doing some other mistake, I am no expert by any means, I just copied the Runt design from the available drawings and pictures).
The reflection comes from the front surface of the CD's phase plug which is some distance inside the CD's exit in many cases
Hi Allen B,
thank you for your reply.
O.K., so if it is not the wave-guide to CD interface that causes the notch, then what is it? The phase plug as pelanj appears to have used, and nc535 assets? Well, if so, why is the position of the hole(s) corresponding to the length of a 1/4 wave calculated and measured form the wave-guide to CD interface and not from the phase plug??
Can you please explain this with more details?
Do you mean that if one does not modify the compression driver, the portion of the horn inserted into the CD will affect the wavefront produced by the phase plug? Perhaps, but this is an implementation detail, not a conceptual issue.
Hi pelanj,
thank you for your reply.
When you wrote inside, did you mean from the phase plug?
Hi nc535,
thank you for your reply.
Hence, my question: Why is the position of the hole(s) corresponding to the length of a 1/4 wave calculated and measured form the wave-guide to CD interface and not from the phase plug?
Kindest regards,
M
thank you for your reply.
O.K., so if it is not the wave-guide to CD interface that causes the notch, then what is it? The phase plug as pelanj appears to have used, and nc535 assets? Well, if so, why is the position of the hole(s) corresponding to the length of a 1/4 wave calculated and measured form the wave-guide to CD interface and not from the phase plug??
Can you please explain this with more details?
Do you mean that if one does not modify the compression driver, the portion of the horn inserted into the CD will affect the wavefront produced by the phase plug? Perhaps, but this is an implementation detail, not a conceptual issue.
Hi pelanj,
thank you for your reply.
pelanj said:
When you wrote inside, did you mean from the phase plug?
Hi nc535,
thank you for your reply.
nc535 said:
Hence, my question: Why is the position of the hole(s) corresponding to the length of a 1/4 wave calculated and measured form the wave-guide to CD interface and not from the phase plug?
Kindest regards,
M
Yes, the DE500 is pretty shallow, I estimate the end of the phase plug to be some 5 mm behind the bug screen.
If you don't believe its from the phase plug, measure location of cancellation null using drivers with two different distances from exit to phase plug and see if it moves. You don't even have to use different CDs, you could use plugged tubes of various lengths.
To form a notch, you need obviously 180 degree phase shift. What is the effective acoustical round trip path length? (rhetorical) Don't forget its delta path length out to the microphone for the direct and reflected paths that you are measuring. This quarter wave length rule is only a rule of thumb. Best practice is to measure for one path length and CD and use that measurement as a calibration for figuring out where you really want/need the port to be located for that same CD on the same horn.
To form a notch, you need obviously 180 degree phase shift. What is the effective acoustical round trip path length? (rhetorical) Don't forget its delta path length out to the microphone for the direct and reflected paths that you are measuring. This quarter wave length rule is only a rule of thumb. Best practice is to measure for one path length and CD and use that measurement as a calibration for figuring out where you really want/need the port to be located for that same CD on the same horn.
Now I see! The further away from the horn, the closer will be the path difference to the axial distance of the port from the CD - at 1 meter and 10 centimeters horn wall distance it is already really close.
Hi pelanj,
So, I take your answer is that you measured form the phase plug.
Hi nc535,
thank you for your answer.
It is not a matter of what I do or do not believe, but what is the correct way of doing it as the tile asks. If my search turns people calculating and measuring from the wave-guide to CD interface, and I took care to match the exit angle of the CD to the wave-guide thus mitigating the mismatch, it is only natural to ask the question before I start drilling holes.
I measured the horn using ground plane.
Could you please explain this in more details? I already have measurements of the horn and the driver in 10 deg increments and, as best as I could model, the lowest cross-over frequency can be about 870 Hz.
I have also measured the distance between the exit of the driver to both the inside end of the conical "snout" and the phase plug; the phase plug is actually behind the inside end of the "snout".
Kindest regards,
M
So, I take your answer is that you measured form the phase plug.
Hi nc535,
thank you for your answer.
nc535 said:
It is not a matter of what I do or do not believe, but what is the correct way of doing it as the tile asks. If my search turns people calculating and measuring from the wave-guide to CD interface, and I took care to match the exit angle of the CD to the wave-guide thus mitigating the mismatch, it is only natural to ask the question before I start drilling holes.
nc535 said:
I measured the horn using ground plane.
nc535 said:
Could you please explain this in more details? I already have measurements of the horn and the driver in 10 deg increments and, as best as I could model, the lowest cross-over frequency can be about 870 Hz.
I have also measured the distance between the exit of the driver to both the inside end of the conical "snout" and the phase plug; the phase plug is actually behind the inside end of the "snout".
Kindest regards,
M
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Hi Jack, well it might not make it that far on mine.....will probably hit the dang CD exit/ mounting plate / horn wall mismatch first 😛 Lol
My best sounding yet...go figure ?🙂
Good reminder it's only a rule of thumb...
I'm thinking delta path length out to mic, between direct and reflected paths doesn't really effect the mid notch. Seems like the destructive summing is occurring in the horn before it even gets out, without further separate paths out of the horn.
(and is why mid to cd xover should always be below the mid notch.)
Do you see more to it?
hi mark100,
I am not ignoring you; you posted while I was typing a repsonse to pelanj and nc535.
It would seem that the sensitivity to the hole(s) placement, and consequently the notch is a function of the length of the driver's "snout". Since I have measured all the dimensions, I can calculate whether the length of the "snout" of my driver will make a difference.
Kindest regards,
M
I am not ignoring you; you posted while I was typing a repsonse to pelanj and nc535.
mark100 said:
It would seem that the sensitivity to the hole(s) placement, and consequently the notch is a function of the length of the driver's "snout". Since I have measured all the dimensions, I can calculate whether the length of the "snout" of my driver will make a difference.
Kindest regards,
M
From EE transmission line theory,
reflection coefficient = (Z1-Z2)/(Z1+Z2)
anywhere there is an impedance change, there is a reflection sized per the magnitude of the change. Substitute slope change for impedance change in a horn
Undoubtedly there are multiple reflections, a large one from a slitted phase plug and hopefully a small one from the horn/CD interface because the latter is also in the forward path