Bratislav said:
You're only correct in that it effectively extends the baffle without even touching it. The energy supposedly nets to zero and it doesn't, period. Where these 2 opposing energies meet they create a greater effect than with boxes regardless of how rigid your baffle is, because the pressure change is greater. I don't really care what you call it, although edge diffraction seems appropriate, but this pressure change does create audible sound and it is a problem due to the delay in time relative to the main signal.
Lynn,
I'm sorry that this diffraction topic is being argued in your thread, but I believe it is on topic because you appear to be shooting for an ultimate open alignment. Maybe the pourous edge baffle idea will help reduce edge diffraction, but I have my reservations unless the construction damps it instead because I doubt the wavefronts will see the pours. The alternative is edge geometry that spreads the pressure change over a broad enough time frame that it become immaterial. This is no easy task, since the effect is double that off boxed speakers, and whatever you do for the front to minimize it affect the rear wave dispersion too.
This is what lead me to dipole waveguides. It turns out that my 2" radius roundovers where grossly insufficient. Maybe filling in the space between my front and rear roundovers will reduce the problem, but I don't think it will completely resolve the issue.
I'm sorry that this diffraction topic is being argued in your thread, but I believe it is on topic because you appear to be shooting for an ultimate open alignment. Maybe the pourous edge baffle idea will help reduce edge diffraction, but I have my reservations unless the construction damps it instead because I doubt the wavefronts will see the pours. The alternative is edge geometry that spreads the pressure change over a broad enough time frame that it become immaterial. This is no easy task, since the effect is double that off boxed speakers, and whatever you do for the front to minimize it affect the rear wave dispersion too.
This is what lead me to dipole waveguides. It turns out that my 2" radius roundovers where grossly insufficient. Maybe filling in the space between my front and rear roundovers will reduce the problem, but I don't think it will completely resolve the issue.
Bratislav said:
Wrong again. They don't cancel. The front wave bends around the baffle to full space expansion, as does the rear wave. Only where they are travelling in the same direction and have travelled the same distance do they net to zero and effectively cancel in that direction. There isn't destructive cancellation at the edges, a common misconception. That may very well be the flaw in the simulations. There's no net pressure travelling 90° to axis, with or without the baffle. Where they reach the edge, there absolutely is a pressure change in the on axis direction.
ucla88 is probably correct that the new source resulting from the pressure change at the edge is dipole in nature and not omni directional, but a dipole source mounted at the edge of the baffle definitely doesn't result in zero sound.
Maybe this discussion will lead to a good enough understanding in this area that the simulations will be modified to more accurately predict the behavior of open alignments. Until then, I'll just treat them as tools, and I'll be darned if I'm going let a hammer tell me how to design and build a house.
Well, I'll try only once more.
Soundwave is a physical change of pressure in a medium (air) that starts as a perfect hemisphere on both sides of a baffle. As it moves along the obstacle (that is baffle) those pressure differences are kept apart by the baffle itself (which is non-compressive by definition). So there is pressure difference on both sides, right ? If there was no baffle, they would cancel right there (*).
Now, you adding a piece of paper in the air as a baffle extension does the same thing - prevents pressures in the air equalizing/canceling. But paper is far less stiff than a baffle - hence it moves.
I'm not saying that diffraction does or doesn't matter for open baffles. My exposure to diffraction is through optics, and there we deal with mostly planar waves (sometimes approaching 2 pi, never ever even close to 4 pi like in acoustics) so I reserve my conclusions about this until I understand it a bit more.
I am simply saying that your paper experiment is flawed and conclusions drawn from it cannot be taken as valid.
(*) assuming wavelength is much greater than baffle/driver itself
Soundwave is a physical change of pressure in a medium (air) that starts as a perfect hemisphere on both sides of a baffle. As it moves along the obstacle (that is baffle) those pressure differences are kept apart by the baffle itself (which is non-compressive by definition). So there is pressure difference on both sides, right ? If there was no baffle, they would cancel right there (*).
Now, you adding a piece of paper in the air as a baffle extension does the same thing - prevents pressures in the air equalizing/canceling. But paper is far less stiff than a baffle - hence it moves.
I'm not saying that diffraction does or doesn't matter for open baffles. My exposure to diffraction is through optics, and there we deal with mostly planar waves (sometimes approaching 2 pi, never ever even close to 4 pi like in acoustics) so I reserve my conclusions about this until I understand it a bit more.
I am simply saying that your paper experiment is flawed and conclusions drawn from it cannot be taken as valid.
(*) assuming wavelength is much greater than baffle/driver itself
Bratislav said:
In one sentence you say they cancel at the edge, and next you say they don't. If there was destructive cancellation when out of phase sound waves cross paths out of phase, it would completely change sound as we know it. For example, there would be no reflections. In the simple demonstration I suggested to hold the paper near the edge (where the pressure change is greatest), not against it. If there was destructive cancellation at the edge, there'd be nothing to move the paper.
If you want to apply what you know about optics, then it's probably better to look at edge diffraction of sound more like a reflection of a light at the edge of a boundary than to apply principles of light diffraction.
I do not understand it well
I think the paper will never be absolutely correctly situated in the symetry plane and this is enough to make some resonances throug it.
Some say the problem of edge diffraction should correctly be named edge reflections. If there is some significant reflection of wave at any mid frequency, the very light paper should never be absolutely calm.
I think the paper will never be absolutely correctly situated in the symetry plane and this is enough to make some resonances throug it.
Some say the problem of edge diffraction should correctly be named edge reflections. If there is some significant reflection of wave at any mid frequency, the very light paper should never be absolutely calm.
Hi JohninCR,
Just trying to visualise various explanations here.
If we bring any material into the perimeter airflow at the edge of a driver or baffle (as opposed to anything in the path of either front/rear radiating pressure wave) it will be acted upon by that null plane pressure differential, and if the material is paper it will re-radiate like a parasitic loudspeaker cone.
If the object has mass it will not physically radiate, but its edge will become a size related pressure differential radiation source in its own right at higher frequencies.
If a circular object is brought close it will be almost inaudible; if square there will be a pair of spaced sources; if sharp the pressure redistribution disturbance will be greatest and thus most audible; all disturbances relating to radius of shape plus spacing of edges and distance from LS centre.
As you state, this is demonstrable, and is most audible at higher frequencies because the resulting re-radiation is delayed wrt driver transduction, and thus it stands out as an incoherent (has its own sound) and frequency varying amplitude response.
I cannot visualise any difference between the LS or baffle edge actually self causing this re-radiation compared to it being caused by another identical shape being brought close.
Conclusion - round off a baffle edge to the greatest diameter possible, and 'D' or even '8' shape the entire baffle to maximise front-rear cone distance whilst ensuring that any re-radiation is minimised wrt primary transduction and forward radiation.
Cheers .......... Graham.
Just trying to visualise various explanations here.
If we bring any material into the perimeter airflow at the edge of a driver or baffle (as opposed to anything in the path of either front/rear radiating pressure wave) it will be acted upon by that null plane pressure differential, and if the material is paper it will re-radiate like a parasitic loudspeaker cone.
If the object has mass it will not physically radiate, but its edge will become a size related pressure differential radiation source in its own right at higher frequencies.
If a circular object is brought close it will be almost inaudible; if square there will be a pair of spaced sources; if sharp the pressure redistribution disturbance will be greatest and thus most audible; all disturbances relating to radius of shape plus spacing of edges and distance from LS centre.
As you state, this is demonstrable, and is most audible at higher frequencies because the resulting re-radiation is delayed wrt driver transduction, and thus it stands out as an incoherent (has its own sound) and frequency varying amplitude response.
I cannot visualise any difference between the LS or baffle edge actually self causing this re-radiation compared to it being caused by another identical shape being brought close.
Conclusion - round off a baffle edge to the greatest diameter possible, and 'D' or even '8' shape the entire baffle to maximise front-rear cone distance whilst ensuring that any re-radiation is minimised wrt primary transduction and forward radiation.
Cheers .......... Graham.
Hi JohninCR,
"If there was destructive cancellation at the edge, there'd be nothing to move the paper"
I think this logic might be flawed, if we put a paper perpendicular to a speaker driver cone, this might happen, because the pressure on both side is the same, and of the same phase.
Edit: come to think of it, your argument do have some merit, but the logic above is not right.
Hartono
"If there was destructive cancellation at the edge, there'd be nothing to move the paper"
I think this logic might be flawed, if we put a paper perpendicular to a speaker driver cone, this might happen, because the pressure on both side is the same, and of the same phase.
Edit: come to think of it, your argument do have some merit, but the logic above is not right.
Hartono
Hi
What can be seen in John's simulations at post 1333 is that BECAUSE the edge does contribute less with thinner baffles the on axis FR gets worse - though with the thicker baffle there is a overlay of bumps caused by the edge-diffraction-FR.
Look at LS's baffle and you will notice that he does THICKEN his baffle edge.
Earl, Is this meant that you say the zero thickness edge establishes a secondary dipole ? What direction does it have ?
If I got you wrong, could you outline the difference you make between " dipole diffractions .... monopoles diffractions" please?
Isn't it that with the finite thickness the two monopoles at the front and rear edge form a secondary dipole as also was said by John K ?
John, your sheet of paper is NOT a pressure mic. All along the symmetry plane you have flow with no SPL. Same thing as with standing waves in boxes / room modes where you have points in space with max pressure (max SPL) and max flow (min SPL).
Greetings
Michael
What can be seen in John's simulations at post 1333 is that BECAUSE the edge does contribute less with thinner baffles the on axis FR gets worse - though with the thicker baffle there is a overlay of bumps caused by the edge-diffraction-FR.
Look at LS's baffle and you will notice that he does THICKEN his baffle edge.
Earl, Is this meant that you say the zero thickness edge establishes a secondary dipole ? What direction does it have ?
If I got you wrong, could you outline the difference you make between " dipole diffractions .... monopoles diffractions" please?
Isn't it that with the finite thickness the two monopoles at the front and rear edge form a secondary dipole as also was said by John K ?
John, your sheet of paper is NOT a pressure mic. All along the symmetry plane you have flow with no SPL. Same thing as with standing waves in boxes / room modes where you have points in space with max pressure (max SPL) and max flow (min SPL).
Greetings
Michael
gedlee said:
The original question was not "which has more diffraction" it was " What happens when the edge thicness goes to zero." For the moment, lets assume that the finte edge thichness problem does not collapse to the zero thickness case (a position with which I strongly dissagree.) That still leave use with looking at the finite thickness case for varying T for which my previous results show that the diffraction effects become smaller as T get samller.
johninCR said:
Think about it on molecular level. You have a spherical wave of denser than normal air propagating from the front. The same from the back, but in this case it is a wave of less dense packed molecules (opposite phase). As they meet after solid baffle ends, air mixes, pressure equalizes, the cancellation occurs.
What happens when you insert piece of paper in between two fronts ? The air cannot mix anymore. You have created pressure difference by very act of attempting to measure .
Hence reference to Pauli.
No diffraction to be discussed anywhere in this example.
johninCR said:I challenge all the believers in computer simulations verses the real world to a simple test. Take any relatively thin baffled dipole. A raw driver will suffice. Start playing some music through it at a reasonable SPL and hold a small piece of paper very near the edge parallel to the plane of the baffle right where the null should be greatest, and where the computer says should net to zero. You'll feel the forces at play acting on the paper. Then come back and try to say that this energy nets to zero, or what is there is so low in magnitude that it shouldn't be addressed.
For those who think this has something to do with asymmetry, just play content you know is low enough in frequency to have symmetry.
A real world experiment is worth several thousand simulations when the programming is flawed.
If you do the simularion correctly it will give to the result you observe. At 90 degress off axis the acoustic pressure is zero, but the partial velocity (velocity of the air) is not. It varies + and - in the direction of the dipole axis. The force on the paper is rerlated to the momentum transfered to the paper by the air being blocked by it.
I worked for 30 years developing and applying simulation codes to complex engineering problems in fluid dynamics, electron transport in semi- and super conductors, microwave tranmission, etc. Computer simulation works.
johninCR said:
John, to simulate an infinitely thin baffle, are you using extremely thin paper, like maybe rolling paper? I would like to repeat this experiment myself. Sounds interesting.
johninCR said:
So your Wave Flow Visualization + paper method is more accurate than computer simulations & measurements? Could it also be applied in aeronautics or is it strictly an acoustics application? How does one acquire these visual skills? Is a training video in the works down in CR for your fellow DIYers? Rather fascinating to say the least.
cheers,
AJ
With regard to the diffraction-problem I want to suggest that the secondary (edge-)sources behave like "dipoles" in any case, be it the edge of a conventional box or an OB. Only with an OB we have to consider 2 edge-"dipoles", generated by the front- AND backwaves.
As is suggested in this article by Andy Unruh.
Of course he is speaking of 2 "impulses" with opposite polarity instead of edge-"dipoles".
So I think the term "dipole" may be reasonable only as far as the directions "on-axis" (towards the listener) and "backwards" (away from the listener) are concerned. What exactly happens "to the sides" I have no founded idea at the moment; at least not with "box"-edge-diffraction ...
Regards
Bernd
As is suggested in this article by Andy Unruh.
Of course he is speaking of 2 "impulses" with opposite polarity instead of edge-"dipoles".
So I think the term "dipole" may be reasonable only as far as the directions "on-axis" (towards the listener) and "backwards" (away from the listener) are concerned. What exactly happens "to the sides" I have no founded idea at the moment; at least not with "box"-edge-diffraction ...
Regards
Bernd
From my perspective this irritation about OB edge diffraction seems to be semantic to a large extend. John Ks diagrams clearly show the full dipole effect for the thin baffle – no more/less cancellation between front and back wave than everybody else would argue. As I understand it he simply doesn´t call it diffraction but “dipole summation” or “primary dipole” effects and reserves “diffraction” for the “secondary dipole” effects in baffles with measurable thickness. That way he can easily deny any “diffraction” effects for an infinitely thin baffle.
So thank you (not so much) John K for inventing your personal dipole nomenclature, but thank you (honestly) for your thought inspiring simulations of different baffle thickness.
@ JohninCR
Obviously there is no discrepancy between your observations and the theoretical explanations or simulations. For my part, I always try first how my experimental results could conform to a physical theory /simulation before calling the theory /simulation at fault.
Rudolf
So thank you (not so much) John K for inventing your personal dipole nomenclature, but thank you (honestly) for your thought inspiring simulations of different baffle thickness.
@ JohninCR
Obviously there is no discrepancy between your observations and the theoretical explanations or simulations. For my part, I always try first how my experimental results could conform to a physical theory /simulation before calling the theory /simulation at fault.
Rudolf
john k... said:
JohnK,
I'm not saying that simulation doesn't work. I am saying that if the simulation indicates that there is less edge diffraction with a dipole than with a monopole under similar conditions, then the simulator has a hole in it.
We agree upon what moves the paper, so why can't we agree that this increased flow due to the greater pressure change than with a monopole corresponds directly with increased edge diffraction? Is it simply that we are talking about two different things with the term "edge diffraction"?
Rudolf,
You're probably correct and it's a terminology issue. If so, the issue remains unresolved, and that is the increased secondary source of sound occuring at the edges of the baffle compared to the same baffle with a monopole. I submit that this secondary source can be quite audible.
Bratislav,
There really isn't destructive cancellation of the 2 wavefronts at the edge. You have to get past that concept on the road to understanding the behavior of open alignments.
AJ,
I'll not spend any more time responding to your non-constructive posts.
Graham,
Yes, making the pressure change more gradual using large radius roundovers seems to be the only cure without decreasing the pressure change by increasing the baffle size, which also lowers the frequencies where problem occurs. Doing so leaves the dipole purists in a pickle unless the front of the baffle is no longer flat, but that introduces new issue. The practical answer may be an oval shaped baffle (when viewed from the top) using a second driver in the rear to create the dipole, and box issues with that are easily addressed, since there is net 0 pressure put into the box.
Everyone else,
After the paper demo, add a panel behind the baffle to block the rear wave from getting to the edge to imitate monopole behavior. Repeat the test to see the quite obvious reduction in energy at the edge.
With monopoles the physical construction of the speaker is taken to the nth degree, but with OB's it's all but ignored with the "experts" focusing instead only on XO's and electronic correction. Until the physical issues are addressed with a similar thoroughness as with boxes, open alignments will never be all that they can be.
Rudolf said:
What I was trying to do, because it is important to understand when designing a dipole speaker system, at least IMO, is to separate out the fundamental dipole behavior from what would be considered conventional diffraction effects. I said there is no net diffraction effects for an infinitely thin baffle because when considering point sources on a baffle of zero thickness the result is identical to two independent point sources suspended in free space separated by a distance D equal to 1/2 the baffle width. I would agree that for an infinitely thin baffle, or any baffle, the turning of, for example the wave radiated from the front source around the baffle edge is a "diffraction process". But I'm trying to show what the effect of the process is in the net response. I believe the original question was asked in the regard.
At the same time, when we consider real world drivers a lot of this is academic because there are so many other complicating issues like the directionality effects (which I tried so show) but also the lack of symmetry between front and rear response, not to mention baffle shape.
Most of these arguments address primarily what happens above the dipole pole peak and, as I believe I said elsewhere in this thread (could have been a different thread), for a high quality dipole we shouldn't use driver in that area. This is where it gets to be a combination of art and science, again IMO, because the optimum baffle shape is going to be related to how far above the dipole peak we try to extend the response and that will be strongly dependent on driver directionality and asymmetries between the front and rear response.
Anyway, this topic has really been done to death here. Earl and I are in disagreement, perhaps semantically. In any event, I've enjoyed the discussion and the analytical aspects of it because that is what I like to do. But at this point it consumed too much space (almost 100 posts?) so I suggest we leave it as is and move on. I will probably do something on my web site about this for kicks which might give a little more insight. It’s probably easier to put my thoughs down in a clear (to me
) concise manner and let people interpret as they will.gedlee said:
In all deference, you have not made an effective arguement why symmetry fails. Regardless of the nature of diffraction, once the diffracted wave leaves the edge, and a identical but out of phase wave leaves the other edge, well, this is characteristic of a dipole radiation pattern. To argue that there is a singularity with twice the diffraction at the path length equal to zero would be a peculair and unexpected result given that the behavior taking the limit is a John stated. Any secondary diffractions should still be subject to symmetry considerations.
Anyway,
as has been mentioned, any real world dipole design has a diffraction signature which would be quasidipolar in nature and highly dependent on driver and baffle geometry. Exactly how this diffraction signature sounds subjectively, well ?
Time to mount the drivers in my dipole baffle...