Hi Earl,
If an OS wave guide ultimately is to mate smoothly with a flat baffle then can't this be considered a transition from an OS wave guide with specified 1/2 angle expanding through a second wave guide (or flare) where the 1/2 angle then increases with distance from the throat from the "joint" 1/2 angle value to a 1/2 angle of 90 degrees? Wouldn't the flare introduce the HOMs the OS part is trying to avoid?
Yes, of course, but it's a matter of degrees. The greater the flare radius and the further away it is from the throat the less of a problem this is. As I have often said, there is no way to have no HOM, only ways to minimize them. All of my work has shown that the lower they are the better the sound quality. Treating the mouth is a critical factor, just as the throat is. You can't just do part of the job and expect completely satisfactory results.
But analytically the better way to treat the mouth is as a radius rather than another OS contour. This is shown in my book.
Is this saying that 100degree (total included angle) cone is not as good as an 80 degree cone?
If an elliptical cone were being formed, would the wider angle, set to control horizontal wave, be set to less than or equal to 90degrees?
What is the range of vertical angles that could be used? 90degrees (for a circular cone) down to 60degrees? 40degrees? 30degrees?
Yes, but lets not split hairs. 40 to 50 degrees is not a huge change. 30 to 60 would be and the 30 degree would have much less HOM than the 60.
Any angle CAN be used, but I would never go beyond 90 degrees for a small room application. And if I were to try and go to > 90, I would use a different technique (see my patent applications for this).
Thanks Earl,
Let me ask another question. For a given 1/2 angle the OS form actually reaches the specified 1/2 angle at an in finite distance from the throat. Due to the nature of the OS form, it comes close to the exit 1/2 angle a few throat radii from the throat. So, for a given flare radius how do you decide where to start the flare, by over all depth of the WG, by over all diameter, or what? Also, would a constant radius flare be better than one with an elliptic profile? It seems there is as much to designing the flare as there is to the OS part, maybe more.
Let me ask another question. For a given 1/2 angle the OS form actually reaches the specified 1/2 angle at an in finite distance from the throat. Due to the nature of the OS form, it comes close to the exit 1/2 angle a few throat radii from the throat. So, for a given flare radius how do you decide where to start the flare, by over all depth of the WG, by over all diameter, or what? Also, would a constant radius flare be better than one with an elliptic profile? It seems there is as much to designing the flare as there is to the OS part, maybe more.
Hello Jean-Michel,
Yes, I have read these before. The scope of what I am trying to acheive is a bit more aggressive as well. Optimizing beam and acoutic impedance from 1K~20K. Since the driver characteristic is known, the conditions are slightly more difficult to handle.
Note that Earl addresses HOM from a contour point of view, and in the Newell and Holland book, they address acoustic impedance as a source of "horn sound". We also want some form of well behaved beamwidth. So the effort seems to be trying to find some optimum design point.
Last edited:
I won't answer your questions in detail as some of the answers are trade secrets, but basically I just try and make the waveguide as big as possible.
As to the elliptical versus constant, I don't know what would happen and this level of detail would be very hard to model. I've thought of doing that, but the molds would get quite complicated and I have enough trouble as it is.
I would agree with the impedance being a symptom of "horn sound", but not with its being the source.
As far as I understand, the anti-parallel wavefronts generated within the device are a product of reactive forces. Could we define these as HOM? If so, couldn't we quantify them by analyzing the acoustic impedance at various locations within the device (ie throat, mouth, phase plug entrance/exit etc)?
If we minimized reactance, wouldn't we also be minimizing HOMs?
I don't understand " the anti-parallel wavefronts generated within the device are a product of reactive forces." so I can't comment on that question other than to say that "impedance" as a concept is not really applicable to HOM in the standard way. You have to understand/define them in a way that makes sense.
No, there is no unique relationship between HOM and "reactance". All modes have complex impedances, but take note that the "bulk" impedance, what is usually measured and siscussed, is zero for all HOM. The "bulk" impedance is that impedance seen by the throat as a surface, you might think of it as an "average" impedance across the aperature. Only the lowest mode has an average velocity across the throat aperature that is non-zero and hence only it has a non-zero bulk impedance.
One can talk about the point to point impedance, but trust me, in this case it wouldn't lead anywhere. What you want to talk about is the modal impedance. The impedance seen by the "modes" of the decomposed wavefront in any cross-section.
If I decipher you question in the right way you are asking where actually the *origin* of the OS contour has to be seen or set ?
You should know best as you have put the geometry of that contour into a formula with ease (after lots of guess working and a loooot of dancing around that topic from Earls side – to say it politely)
Where to put this origin "in praxis" is a prolonged anger for Earl as he would like to see compression drivers to be specifically made to fit the purpose.
Simply put – there is no compression driver that holds for the plane wave front supply, a OS is deliberately intended to transform into a spherical.
At dimensions of the phase plug and all channels below transmission line wavelength – sure – but actually all those usual compression drivers are a little bit deep with respect to upper frequency limit we like to reach.
This is far less of a problem with a contour that starts out with low flare rate – as the exponential or LeCleach for example - as outlined in detail to jzagaja some postings back.
Seen from that perspective most OS horns we have seen measurements from perform pretty well
Given above restrictions of compression drivers its interesting that Earl has not arrived at planars, in particularly at AMT
On the other hand I possibly too would not have, if not been coming along from another base.
Also I possibly had the "luck of birth", as with old school Heil / ESS I might also not have tried...
But possibly - I did not understand what you actually was asking...
########
Regarding the discussion about larger than 90 deg included angle of conicals versus OS, it might be worth to note that "smoothest diffraction" claimed for the oblate spheroide horn contour gets soon towards a non existent throat round over - when included angel gets increased
So one easily gets a feeling that "smoothest diffraction proven by math" in praxis, is also pretty restricted to certain opening angles – besides it is ill put IMO anyway...
Michael
Last edited:
One can calculate the effects of HOM by measuring the far field radiation pattern for a "known" configuration and subtracting off the field that is due to the lowest order mode. What remains are HOMs. These can then be further decomposed into the "orders" if desired. The "known" configuration can take many form, but perhaps the flat baffle is the easiest to follow, but its not the easiest form for the calculations - that would be a sphere. The mouth configuration (radius, etc.) becomes a major issue as there isn't a good model of it so its hard to know what is the lowest order mode and what are HOM. If the mouth has no flare then the ambiguity and difficulty are gone, but the sharp discontinuity of the mouth will now create its own set of HOM.
Its relatively easy to calculate the HOM from the compression driver in a plane wave tube and we did that at B&C.
Bottom line here is that its not trivial and web posts are not the place to discuss complex math.
No that was not what I was asking. The OS geometry starts at the throat with some prescribed throat angle and then extend to infinity where it will asymptote to the prescribed exit 1/2 angle. But the WG doesn't extend to infinity. It is truncated at some distance from the throat. If the WG is to ultimately go to a 90 degree 1/2 angle, to blend with the plane of the baffle, then at some point a 45 degree OS WG is mated to some type of flare that opens up from 45 degrees to 90. In other words, at dome distance from t he throat the WG is no longer following the OS contour but something else. So the WG is actually a hybrid, OS from the throat to some distance out, and then something else: constant curvature, elliptic, log sprial..... It leaves a lot open for experiment.
Someone once suggested a reversed OS contour, which makes sense, but when plotted out is almost indistinguishable from a constant radius. I'm willing to bet that they will all work about the same for a given waveguide size.
The one thing that stands out to me is that when a flare is added to the OS WG, if a constant radius is used then there is more than likely a discontinuity in surface curvature. That is usually not a good thing.
I beg to differ. It is reasonable and possible to fit a radius such that there is no discontinuity in surface curvature.
I recommended this concept. However, it is not a direct reverse. Maybe I should patent it.
But for the sake of audio advancement, I will not do so. During calculation of the lip curve, the radius to use is in the magnitude of the mouth radius, and the angle should be somewhat that the intersecting point would be tangent to the original OS curve. The direction of curvature would be like a WG perpendicular to the WG under design.
Last edited:
The "termination of the mouth" is actually a issue put wrong.
In a more holistic view there is not precisely a "mouth" – you definitely should read the first postings in Jean-Michel's thread too.
In short you may come to the same conclusions as I – looking at diffraction as *the need* to bend wave front vector all the way around to finally fill space available (whole listening room).
So you always *have* (at least) 180 deg of diffraction to manage (90 deg in case of an infinite baffle setup).
Sure – as further out the "end" respectively the discontinuity in horn contour is set – the less odd SPL overlay that's causing sound field defects...
Michael
I think it would be interesting to compare the 2nd~5th harmonic contents of OSWG and LeCleach horn to see how they differ. CSD comparison is also good information. Not that I am trying to compare which is better, but how to look at various data to see what optimization posibilities are possible.