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In Reply to: RE: Field coil vs permanent magnet -Fundamental differences besides the obvious? posted by jweiss on July 23, 2009 at 08:56:59
Well that's the thing - they're not very contentious claims. You just don't like them because you're selling a conical horn without a mouth termination. It's understandable. I'd suggest adding the mouth termination and adding an OS entrance as well (if there isn't one already - never been able to clearly see the throat of those horns). I know the OS entrance wouldn't work with the direct high frequency radiation from the Cogent driver, but if you use a super tweeter, that would not be an issue.
On to the data - I dug up some old FEA results to show a typical scenario. These are from a fully coupled model. In other words, the diaphragm, suspension, coil, phase plug, etc. of the driver are modeled along with the interaction of those elements with the air. The electrical / magnetic portion of the driver is included as a lumped element model. This simulation is from a 2kHz horn for a JBL 2402 ring radiator. The same driver / phase plug is used for both scenarios - only the horn is different. The conical horn is approximately the same size as the Le Cleac'h horn (it was drawn over the LC horn). This simulation has proven very accurate for directional effects. In other words while the frequency response may be off a bit from reality, the response at 0 degrees, 10 degrees, etc. will all be off in exactly the same way, and thus looking at normalized polar responses or directivity indexes will give an accurate prediction of reality. This has been tested many times using real parts in a calibrated, NIST traceable test system in an anechoic chamber. Also, if you wanted to extrapolate these results to a lower frequency range, you could expect the effected frequency range to be proportionately lower. For example if you were interested in a 200hz horn, the effect seen in these simulations at 10kHz would be seen at 1kHz. Here are screen shots of both models at 20kHz. The color scale shows pressure in a steady state anechoic environment. The scale is set to show primarily waves outside the horns, but you can also see the shape of the wavefronts in the horn. The grey color is where the pressure goes above the upper limit of the scale. Note that the phase plug for the 2402 is not the JBL plug.
First, let's look at the normalized response curves every 10 degrees from 0 to 180 degrees for a 2kHz t=1.4 Le Cleac'h horn. If you eq'd the on-axis response of the horn to dead flat (the line at 0dB), then all the response curves at the various angles would be as shown in this chart. Although there are differences in the on-axis response of various horns, this method levels the playing field in that regard (since you could, and arguably should, equalize the horn in the design of the crossover). What we are looking for are similar response curve shapes as we move off axis. Ideally, at least if we wanted constant directivity, we would see flat lines but at decreasing levels as we moved off axis. What we don't want to see is changing response shapes at various angles, as this will screw up the spectrum of the reflected sound.
So what we see in the LeCleac'h curves are that the responses are all similar except for 3 at the very bottom that have a few bumps in them. These are 140, 150 and 160 degrees from on axis.
Now let's look at the same curves for a straight conical horn with no mouth round over. These look decidely less orderly. You can see there's a big dip around 9kHz in the 50, 60 and 70 degree curves (brown, green, blue), but then the 80 degree curve (magenta) and the lower curves don't have this dip. Also, this dip is above 10kHz in the 40 degree curve (light green) and maybe a little higher in frequenc in the 30 degree curve (yellow), although it's tough to say if that's the same effect or not because it's less pronounced. Also, notice the dip in all the curves at around 3khz. Now notice how this dip progress from ~3kHz at angles close to on axis to ~4kHz as the angle moves further off axis. Lastly, lets look at the 10kHz to 20kHz region. Notice the chaotic behaviour from one curve to the next. There's a second notch that starts to crop up in the middle of that frequency range but is absent in some curves and moving around in frequency in curves where it is present.
Basically all this is not good. It means that no matter how you eq the horn, the response will never be uniform or even close to uniform at all angles - the spectrum of the reflected sound will always be different than the direct sound. While the Le Cleac'h horn is not constant directivity, the spectrum of its reflected sound will at least be a simple smooth rolloff of the highs (which is very typical for speakers as almost all speakers increase their directivity at high frequencies). Whether that smooth rolloff is desirable is debatable, but if you don't want it then the solution is an OS waveguide with a round over at the mouth.
Now let's look at the same data in another way - directivity index. Directivity index shows how directional a speaker is. A directivity index (DI) of 0 is an omnidirectional speaker - it radiates the same in all directions, or at least averages out to doing so. A 3dB DI would be a speaker that radiates only into half space, IIRC. What the DI can show us is how directive a speaker is and how that changes with frequency. It's an easier measure to look at than the raw response curves to get a quick idea of how the speaker's going to image, how and where to cross it over and to what speaker you might cross it to. Once again, we'd like to see a smoothly changing DI. For constant directivity, the DI would be flat versus frequency.
Below we see a chart showing the DI for both the Le Cleac'h and the conical. Note the low end of the frequency range (~2kHz). For the Le Cleac'h, DI smoothly changes from about 1dB at 1.4kHz to 4.5dB at 5kHz. Now notice tha the conical horn actually has a negative DI between 1.2kHz and 2.2kHz and then rapidly rises, falls again, and then rises at a faster rate than the LeCleac'h horn before plateauing around 9kHz. Since this is the range you'd likely cross over to this horn (3-4kHz), this is where you'd like a smoothly changing DI. While all horns will have their DI decrease at the low end of their range as they become acoustically small relative to the wavelengths being produced, if they do this smoothly it will be easier to match up their directivity to the device working in the frequency range below them as there will be more overlap of the DI's. This is really what we want - a smooth transition to the next driver. This keeps the spectral balance even in the reflected energy. The oft cited example of a big offender in this category is crossing a very small dome tweeter at a relatively low frequency to a large woofer, as this causes a large jump in the DI. The woofer will have significant direcitivity at the top end of its frequency range while the small tweeter will be omnidirectional at its low end. This is what we're trying to avoid.
Now look at the early reflection DI curves in the above graph. These curves show the DI but only looking at 0 to +/-30 degrees horizontal and 0 to +/-10 degrees vertically. Although I have not heard of a completed study on this, this measure should show what the imaging will be like. Once again, the Le Cleac'h is smoother and flatter - DI does not rise as much at high frequencies.
In both these curves, the advantage of the conical is that once it becomes acoustically large, the DI is somewhat smoother. This is seen above 10kHz - the LeCleac'h has the dip in DI at 16kHz while the conical does not. The Le Cleac'h also has a dip in DI at 20kHz, but because this is the very top end of the simulation, I would not put as much confidence in this. The element size in my model might have gotten a bit too large.
Now let's look at data from a transient analysis. A force was applied to the voice coil and then removed in the time span of one sample at 44.1kHz. The acoustic pressure was recorded for ~10ms with a 44.1kHz sampling rate following this impulse at points at every 10 degrees around a 1m arc as above. This data was then processed to produce energy time curves which are displayed as a normalized (the response is flattened) waterfall plot. Note that these plots are displayed on a linear frequency scale which isn't great, but it's a limitation of Excel. Also note that I have an overlayed line plot of the response at various times that I'm using to pick off frequencies and levels, but it's kind of confusing to look at on its own so I'm just showing the waterfall. Anyway, let's look at the on-axis waterfall for both horns.
Below is the waterfall plot for the Le Cleac'h horn. Notice it generally decays smoothly at a somewhat constant rate in the middle of its range. It makes it down to -60dB in ~7ms in the 4-6kHz range. It has resonances at ~7kHz and 10kHz. In the 8kHz to 10kHz range, there is a dip and then the level rises back up (to the right of the 10kHz resonance ridge). It also does this to a lesser degree below 3kHz. Ideally we'd like it decay smoothly, quickly, without resonances, and not bounce back up in level at any frequency, but we'll take this performance as our baseline - decaying well in the middle of the horn's bandwidth and not quite as well at the upper and lower extremes.
Now let's look at the same waterfall plot for the conical with no mouth termination. What we see first are the same resonances at 7kHz and 10kHz. This probably means they're caused by the driver. But we see a much smaller amount of decay in the 3kHz to 6kHz range in the same time as compared to the LeCleac'h horn. While the Le Cleac'h horn decays to -60dB, the conical only gets down to -25 to -30dB in the same period of time. Beyond this interpretation becomes somewhat subjective in my opinion, as there are pros and cons in other frequency ranges for both dvices, but I would note that the conical appears to decay better between the two resonances around 8kHz.
Also, while it could be argued (in the absence of data) that some of these effects are just the conical flair versus the Le Cleac'h flair, I would add that I have also simulated a comparable OS waveguide with a mouth radius and it does not have the problems this conical does - polar response is smooth and decay performance is much closer to the LeCleac'h. (For those unfamiliar, an OS is an oblate spheroidal flair - basically a conical with a round transition at the throat which reduces throat diffraction.)
These are just some of the issues caused by not having a mouth radius. Other issues might include production of more higher order modes. Also, if there isn't a smooth transition out of the driver at the throat, this will cause even more HOM's. Read Geddes to find out more about this. There have been tons of discussions on diyaudio about this for the last year or so. He has identified this as the cause of horn sound - waves bouncing around inside the horn and reemerging later as delayed energy.
Having said all this, please understand I'm not saying that conical horns with no mouth termination sound horrible. From my previous post I've obviously listened to quite a few and they can sound pretty good. However, mini monitors can sound pretty good. MP3's can sound pretty good. But if we're looking at horn systems for home usage, especially large horns, and especially in this discussion where we're talking about custom field coil drivers, it's obvious we're neurotically pursuing the last little bit of sonic performance. And these issues are much more than just the last little bit of performance - I'd say they're at least moderately significant.
John
Follow Ups:
...John...
...as the smartest guy in the Chicago Horn Club. In certain quarters (figurative Bavarian castles surrounded by fortified walls, which are themselves surrounded by moronic peasants with torches and pitchforks, and with an overlord Raving Over Morons Yesterday) this may seem like a back- handed compliment, but the said peasants have better things to do. So as long as you don't try bossing us around, but we could use some direction at times ; )
The phase plug in the sym looks like a bisected "Hershey's Kiss", unlike the bullet shaped plug in the original, and you did state that this was so. This was presumably done to smooth the transition of the wave front as it exits the horn mouth, which may have shown some anomalies due to the bisected bullet? So would a Hershey's Kiss shape make a better phase plug than the usual bullet shape, or was this just a convenience in the sym to remove this as a point of contention?
I had no difficulty locating the vertical frequency divisions in the polar plots of the Le Cleach horn and the conical horn examples, they are at: 100; 1000; 10,000 and 100,000 Hz. These plots would seem to address the main performance point of contention here for the difference between the conical and Le Cleach horns.
Of all the horn families, the Le Cleach horn seems most like the tractrix horn, though the curve is slightly different, and there is that round-over at the mouth with the Le Cleach horn, where the tractrix curve by comparison ends at 90 degrees to its axis. That is if I have read JML'C's website correctly.
Paul
Hello John,
Thanks for your informative post.
Simulations are heuristic devices. In other words, simulations are a way to try and predict what might happen in a given situation, and to learn how the real world works in relation to our expectations. Simulations in speaker design are just the same- they are no substitute for the real thing, which is building actual speakers and testing them. This is what we’ve been asking from you, and which you evidently have not done yet. A simulation is no substitute for the real work involved in designing good sounding horns, something I doubt you would disagree with.
When you do simulations, which we do a lot of, by the way, its best to choose ones which will tell you useful information, something you really want to know. The same goes for comparisons.
In your first simulation, shown below, for example, you have a LeCleach horn modeled against a conical horn. You choose a 20kHz frequency to do this. 20,000hz is at the very end of the audible frequency spectrum, which is a rather ridiculous place to locate any such comparison. We don’t make a 20,000 hz horn, and I doubt this simulation has any value, except to show that you were really looking hard for places to make strong visual contrasts between the LeCleach and a conical horn.
For that matter, its best to also be clear in comparisons, and to compare apples with apples, and oranges with oranges. Meaning, compare a LeCleach 500hz or 800hz or whatever cutoff horn with a similar conical, such as the ones we make (if you are going to attack our products, at least use them for comparison, and don’t erect “straw men” ), and specify actual horn length, mouth area, etc. This you did not do, or if you did, its not very clear (to me, at least.) Furthermore, do this with a given driver, one which makes sense for the context of the comparison. In fact, we don’t do any modeling of horns without the driver being part of the equation, as it ALWAYS IS part of the equation. At least in the real world, which is where we make our products. Not in SimVille.
I don’t want to belabor this with a discussion of how your sims were done (they don’t look like ours, I can tell you that) but I can suggest that your conclusions, for example with off axis directivity are very, very strange. The conical profile has superb off axis response, and the LeCleach does not, at higher frequencies, where the wavefront does not even “see” the rapidly expanding walls of the horn and simply beams like a headlight. Any session with a LeCleach horn and actual listening will prove this- no computers are necessary.
So we are back where we started- we asked you to provide actual, hard measurements of a LeCleach horn with and without the mouth rollback, which you credited with reducing HOMS, etc, and improving the sound of the horn. Such measurements should be done at 10 degrees, 20 degrees, 30 degrees off axis. Outside, preferably, unless you can get back into that anechoic chamber of yours. We contend such actual measurements of the LeCleach horn with mouth rollback and without will show negligible real world benefits, either measured or audible. But you have not done any such measurements, or if you have, you have not posted them here. In fact, the LeCleach is really just a hyperbolic horn with a very squeezed throat, which colors the sound greatly, even if it simulates well, and that is the actual cause of the “honky” colorations which you, Geddes, and LeCleach have mistakenly taken to be a result of a non rolled back horn mouth termination.
As for your comments on Geddes, I think his credibility here has already suffered greatly, as when Tom Brennan linked to the thread that you mention over at DIY Audio in which people were complaining about the truly absurd, cheap, and poor quality of the horn kits he was selling. But this is a respected scientist, who has published major papers, you may contend. Such papers as to how there is no difference between the sound of different compression drivers (!!!!!?????) If that is so, there must be a lot of deluded people on this Forum, as a good deal of the traffic here is about the difference in sound between the various compression drivers, but which Geddes cannot hear or differentiate between. When he posted references to that work on this Forum originally, I read the abstract, and his methodology involved hiring college kids to hear 10 seconds of a Talking Heads song from the album Stop Making Sense, I believe at high volume. From that setup, Geddes derived his conclusions.
I find that about as satisfying and useful as your simulations and discussion of horn design.
Jonathan
Wow Jonathan,
I don't know what to say to that other than you have no clue what you're talking about. Simulations were not done at 20kHz. They were done at many frequencies which is how the plots versus frequency were produced. The pressure plots at 20kHz were merely to show the geometries simulated. Those geometries are comparable - a given size LeCleac'h flair to a same size conical. The driver is the same in both simulations and is appropriate for the horn - it produces a plane wave at the entrance to the horn. As I said, this simulation method has been tested against reality in a calibrated, repeatable test setting which is better than what you recommend and found to be completely in agreement with measurements of real horns. Thus as an engineering tool, these can be expected to be almost identical to real measurements. If we were picking nits about 1dB here or there, yes, measurements of the real device would be needed. Since we're talking about gross trends, these simulations are more than adequate. And these simulations are very representative of larger horns as long as they have the same profile (ie, not truncated early like is sometimes done with bass horns). The frequencies would just scale with the size of the horn as I mentioned.
I'd suggest you leave the debating of technical topics to those who have some understanding of them and go work on improving your horn.
Hello John,I respect the fact that in the past you have had some professional experience in the speaker world, but until you post measurements of a LeCleach horn in a realistic fashion, meaning a horn meant to do minimum 700 hz, both with and without the famed LeCleach rollback (which you believe to be a breakthrough in reducing horn "colorations") we won't have anything worthwhile to discuss on this forum.
The 2402 JBL you used in your sims is an " ultra-high frequency transducer engineered for tight pattern control and extremely high on-axis sensitivity." This is what you are using to sim the difference between a conical and a LeCleach horn? A supertweeter?"
Simulations are simulations. Real measurements do matter. Would you fly in a jet that was never actually tested, just simulated?
I wouldn't.
Jonathan
Edits: 07/28/09
Hi John:
This was a great post. Thanks for sharing a lot of high quality information!
Jon Ver Halen
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