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I am only beginning to learn something about field coil speakers. My question is, beyond the obvious difference in how the magnetic field is generated, what other things differentiate these two speaker types?
Do field coil speakers present a different kind of sound?
Are they always deeper or steeper in cone angle? Those few I have seen are very deep.
Any deeper insight into field coil speakers would be welcome.
Follow Ups:
Hmmm... saying that horns with abrupt mouth termination "sound better" than those with a smooth or soft termination seems odd to me. I have certainly not found that to be true.
Sorry to butt in, but found that hard to ignore.
Neodymium (NdFeB) N50 has a Coercive Force Hc of 12,500 Koe (Alnico 8 = 1,640 Koe). This is the measure of a materials ability to resist demagnetization. Hc is the Oersteds required to reduce the magnet's B to zero. The UH grade of NdFeB has a working temperature of 180 C, the transistion temperature above where the material starts to lose it magnetic properties.
It would be safe to run a 95db/watt SPL motor with this NdFeB magnet at 100 watts with just 0.2% B sag from reverse current EMF until it heated to 180C. The motor steel pole piece would always be fully saturated.
NdFeB is my choice for 95db/watt home speakers that seldom ever see 20 watts even for a few seconds.
I just purchased a 8" Tangband W8-1808 full range with NdFeB motor for $170, cut off the wizzer cone and discovered a world class midrange. The Xmax is 5mm, Qtc 0.44.
was the whizzer-ectomy difficult, what did you use? Scalpel?$170 each? or for two?
freight?
Warmest
Timbo in Oz
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
http://www.theanalogdept.com/tim_bailey.htm
Edits: 07/28/09 07/28/09
+1^ what xover slope and what size and type of enclosure did you put them in?
also can you comment on the sound when run fullrange before removing the wizzer?
Thank you gentlemen for your expertise. A few things were of particular interest in the replies. One was the concept that field coil speakers might offer some advantage at low levels. Admittedly, I listen at very modest levels, and so I might appreciate just such an advantage.
The remarks about OTL's were also interesting, as I am just beginning to learn about them. For clarity of concepts, I will post some follow up questions in the forum separately.
I do have a new question about field coil speakers.
Assuming I obtain all the correct specifications as per the schematics, could there be any advantage in driving the speakers with less voltage or less current than orginially specified?
Do field coil speaker owners tinker (downward) with these adjustments, and if so, how do they affect the sound?
Hi,
If you send a very low voltage to your drivers you less efficiency and you increase the QTS. When you increase QTS, you have more bass and mid-bass.
With less efficiency you need an amplifier with more power...
This is the egg and chicken problem...
A+
Cyr-Marc
Supravox importer and distributor for North America.
Permanent magnets are not ideal. When used for speaker motors, their magnetic fields may respond to the fields created by the signal currents in the voice coils. This response may have some granularity, depending on the magnetic domain properties of the motor. The result is a subtle distortion that increases in fraction of the signal as the signal decreases in level. This kind of distortion gives a mechanical character to the sound, especially in well-recorded natural music, where the decay into silence has musical significance. This kind of distortion limits the resolving ability of the speaker.
Two other areas in audio have similar problems. Push-pull output stages suffer from what is called "crossover distortion." The tube or transistor amplifier characteristics change as the signal approaches zero. A band-aid is to increase the bias, so most power amplifiers are called "Class A-B," with both sides of the push-pull topology conducting current at zero signal. A true Class B amplifier would allow each side to go into cutoff (no current) exactly at zero signal. A true Class A amplifier never cuts the output devices off, and is very inefficient. However, many audiophiles prefer this type because it avoids crossover distortion. Single-Ended Triode (SET) amplifiers avoid the problem completely because the output device does not get close to zero except at maximum signal.
Tube amplifier output transformers also have grain that depends on the core material. Different choices for core material give different tradeoffs in distortion verses ability to handle large bass levels, efficiency, and cost. The tradeoffs are so severe that some folks use so-called Output Transformer-Less (OTL) tube power amps, such as the Atma-Sphere amps. These amps are entirely free of the transformer grain problem and are capable of superb resolution.
Good reproduction requires the audio equipment to work well at the extremes of maximum and minimum signal as well as at average signal levels. Field coil speakers have an advantage over permanent magnet motor speakers at low signal levels. Whether they perform well at high levels depends on other design choices that are common with permanent magnet motor speakers.
Al, do I know you? Man you write the way I think. Anyway, keep it up!
I have long characterized the "field coil sound" as one of great dynamic range that does justice to the recording, accompanied by a relaxed presentation that places the listener at ease. After years of the teeth-chattering high end (hind end?) sound that Joe Roberts has described as "painfully accurate", I took to field coil drivers right away when I heard them. Like great old Alnico V drivers, but with even further refinements in the same direction. It seemed strange that 1930s theatre gear blew away 1980s high end salon stuff in the ways that mattered to me, but that's the way it sounded.
My theory, as yet untested, is that a well designed field coil motor permits less modulation of the gap flux by the voice coil currents than occurs with a permanent magnet motor. If this is true then the relaxed presentation of the field coil driver would simply represent greater accuracy in reproducing the input signal. Seems to me that to the degree the voice coil currents make the gap flux bounce around there might be all sorts of time-dependent interactions, like a pendulum swinging while attached to another swinging pendulum.
Very good description. Thanks.In addition to your response I add my two cents.
Except the sound property, "field coil" drivers have a big advantage than permanent magnet drivers. You can modify some TS parameters when you increase or decrease the voltage apply on your drivers. You can reach the better parameters for your enclosure or your OB pannels.
In any situation where you build enclosures, you build them in consideration of the TS parameters your permanent magnet drivers have. With "field coil" you can build your enclosures and after you can adjust with precision the interaction between drivers and enclosures when you change the voltage you apply on your drivers. With permanent magnet motor drivers you cannot make this.
A+
Cyr-Marc
Supravox importer and distributor for North America.
Edits: 07/20/09
Very well articulated. I'd like to make a few minor points to add to the discussion:
1) Even field coil magnets have iron in the magnetic path, and therefor some granularity.
2) Field coil power supplies will usually have an electrolytic capacitor, through which any induced signal current will flow. I suppose either a current source or a good (shunt!) regulated voltage source would help. Haven't given it any thought which is best?
3) OTL tube amps are usually Class B or AB, so they acquire crossover distortion in order to avoid magnetic hysteresis distortion. No free lunch I guess.
hey-Hey!!!,
The field-coil can have its core run well into saturation...well into the horizontal part of the B-H curve, so that a perturbation by the voice coil can not change the flux density much. Run it below saturation( where the dB/dH is steep ) and a small push by the vice coil will change flux density...
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
I suppose some field coils are run with the pole pieces saturated, but I don't suppose the entire magnetic loop is saturated even then.
There is no essential difference between any given field coil speaker and a permanent magnet speaker beyond the motor/magnet. Any field coil design can be made as a permanent magnet, or vice versa. A field coil motor in itself is no guarantee that a speaker will sound good. It is true, however, that many of the early, high efficiency drivers which used a field coil electromagnet out of necessity sound superb. While many attempts to do field coil speakers today miss the mark- probably because the knowledge, craft, art, or however you wish to call it for making such speakers has largely been lost through more than a half century of neglect.
I'm typing this while listening to modern field coil drivers on horns (Cogent) and the sound is indeed remarkable.
Jonathan
Anyone have any feelings about using the Cobraflex University horn as a midrange hi fi speaker? I may try these horns to replicate the brass ranks of a virtual theatre pipe organ.
The art has not been lost. It's just that those with the knowledge and experience necessary to engineer a fc are typically employed in making drivers with permanent magnets because they're more economically feasible for a larger audience. And speaking from experience, when you design drivers all day long, it's hard to come home and get excited about designing some more.
Sounds like a rough life there, John. Glad to see you have just enough energy after a long day at the driver design place to still want to post on the subject over here at AA.
Regards,
Jonathan
Yeah, it was rough - a half space anechoic chamber good to below 100hz, a few Klippel analyzers, robotic glue dispensers, a machine shop, a room full of random cones to pick through, and a few dedicated listening rooms at my disposal. It was rough I tell ya. ;> Sadly I don't design them for a living anymore, but on the bright side I have a bit more energy for it as a hobby now. In fact I spent a few hours tonight assembling a LeCleac'h horn out in my shop.
Hi John,
Why not take that LeCleach horn back to the anechoic chamber, measure it, cut off the big lip at the mouth, measure it again, and post the findings here? Love to see those measurements...
Jonathan
Hello,
Not measurements but BEM simulations performed by Bjorn Kolbrek which has been proven in several cases to be accurate compared to real measurements:
http://kolbrek.hoyttalerdesign.no/index.php/horns/bem
see #10, 11, 12, 13 and 14
Best regards from Paris, France
Jean-Michel Le Cléac'h
Because I'd rather have the big roll back so diffraction is reduced? Seems obvious... ;> Plus as I said, I don't work there anymore. Actually the anechoic chamber is not even there anymore... Very sad.
And if you're suggesting that a conical without a radius at the mouth would be an improvement, well, even the OS cabinet I'm working on has a rather generous radius at the mouth (along with everywhere else):
Actually, John, its that "seems obvious" approach to speaker design that probably accounts for the fact that there are no good sounding modern fullrange field coil speakers.
Jonathan
Jonathan,
I evidently should have been more specific. I meant that if I'm building a LeCleac'h horn, it seems obvious to me that one of the benefits of the rounded mouth is reduced diffraction and thus (also obviously) cutting off the mouth is not something I would be inclined to do. Further, it 'seems obvious' that reduced diffraction would be a good thing based on the work of countless researchers into the effects, audibility, and subjective preference for the results of low diffraction. So it seems obvious to me because I've had some exposure to this research in the course of my profession. Perhaps I was mistaken in assuming it would be obvious to everyone else on this forum. Similarly, many driver design issues are obvious to someone who has experience designing drivers. That's why they get paid to do it - they don't have to spend time figuring it out from scratch for every driver. I would agree that people who have no knowledge of a subject, don't take the time to educate themselves on the current state of the art of that subject before trying to improve on it, and then design and build something based on what they feel will work well won't necessarily get the best results.
John
Well, John, I would think a true audio professional, a full time speaker designer, would actually measure something like the lip on a LeCleach horn, to find out what exactly the effect of the lip was, and not just be "inclined" to follow one's prejudices. Then one might find out what the diffraction issues really amount to in the real world. But you'd have to do the work, which you don't seem "inclined" to do. Do you need that anechoic chamber to do it?As for the "current state of the art" on drivers, I do try and keep up, and my listening to such drivers informs my previous comments. There are no modern fullrange field coil drivers that are anything even remotely as good sounding as the vintage models with which I am familiar. If contemporary designers actually understood what was done in the past, they would obviously stand a much better chance of making a good sounding driver today. But, as you say, they are all so exhausted from a hard day's work designing permanent magnet speakers, that inspiration is hard to come by.
Thanks for clarifying that.
Jonathan
Edits: 07/21/09
Sigh... Well, at the advice of my audio therapist, here are some examples of horns without rounded mouths that I've designed, built, measured, and listened to over the years:
And a few more I don't have pics of because it was too long ago.
After that I only had enough energy left to cast my own aluminum throat plates, design and build my own CNC router, and then build some horns with that:
Yes, clearly I am not inclined to do any work.
John
Is that you in the picture on the sofa facing away from the camera? I must say that you have developed into a fine looking man, though I did'nt realize that you were old enough to grow a grey beard yet. Perhaps this is makeup, and you're picking up some extra money as an actor. Ah well, we do what we must to support our obsessions!
Paul
; )
Who was that bearded man? Something tells me you've already identified him, Paul. ;>
Ah, my college basement. How I do miss the dankness and my roommate's stolen beer sign. It was a nice horn system if I do say so myself, though. The horn sub was especially fun on organ pedals.
John
Caught in the act enjoying conical horns, in a mono rig at that! And after all the disparaging remarks I've made about conical horns! Judging by the comments of all the attendees of that meeting, this was an amazing sounding rig, and all the more so for the fact that it was mono. We listened to music more than we talked. Conical horns have been misunderstood, by me.
Here's to greater understanding here!
Paul
Hello John,
The fact that you have built horns in your garage and cast aluminum in your backyard is laudable, but you've made some very contentious claims about mouth diffraction and conical horns in your previous recent threads, and I've asked you or LeCleach to provide the data substantiating your position, something neither of you are "inclined" to do.
In a previous thread, your comments on a conical horn I manufacture included "That thing is a big diffraction mess waiting to happen," so lets be clear about the context here.
Live by the sword.... you know the rest.
Jonathan
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
...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
Don't be supercilious Weiss. If you knew John you'd also know he knows what he's about and has for quite a long time. John has great credibility and no axe to grind.
Edits: 07/22/09 07/22/09
Wow, where's the hostility coming from? Unless you're a driver designer now, my comments were not aimed at you and I took pains to write them that way.
In any case, it's good that you know so well what I have or haven't done in the development of this particular horn I'm building. And thanks for reminding me why I don't post here that much.
John
I sincerely hope you don't become discouraged from posting here. Your posts are always good, with just the right amount of theory, and some sawdust to back them up (I should also add metal shavings too). You have a propensity for tackling very outrageous projects, but I've never seen any outrageous claims by you. And you're a real engineer and scientist!
Things written here are always open for misinterpretation unfortunately.
Paul
As the original poster, and as a brand new member, I must strongly concur with Paul's remarks. Forums evolve the most peculiar etiquette at times. It is a product of the way dialog is exchanged. It would be a shame not to have benefit of experienced voices because of a minor misunderstanding now.In any event, I appreciate all the replies. Even when there is no exact consensus, everyone contributes something beneficial. This body of new information gets distilled by the astute reader, and forms the basis for a new comprehension. I now have a much better grasp of field coil speakers. Thanks to all.
Edits: 07/22/09
Mac
If you have'nt already heard them, you should definitely listen to the Cogent field coil horn rig (hopefully Cogent will again be at the '09 Rocky Mountain Audio Fest in Denver this Fall). It's difficult to put this rig in perspective. I heard it in 2007 and it's very much "a thing unto itself". The only thing to compare it to from the "recommended component list" audiophile stuff at the show was the Wilson Maxx's, and the Cogents imaged as well, or better even, and were more dynamic with much less power. I could'nt say what the field coils contributed to this unless I heard the rig with some permanent magnet drivers for comparison. Steve Schell and Rich Drysdale of Cogent have a preference for conical horns (which tend to be deeper than curved wall horns), perhaps they can address the reasons for their choice of horn type. I used to make fun of conical horns until I heard some very good sounding examples from John Sheerin and Tom Danley.
Paul
Thanks for the recommendation. I did a search of Cogent. If these are the ones you are referring to:http://www.teresaudio.com/haven/cogent1_lg.jpg
and if they sound as good as they look, then they would indeed be impressive. I would like to have the chance to hear them, although I am not sure when or where this might ever take place.
My interest in field coils is currently limited to this massive old 1945 Wurlitzer tone cabinet I am trying to get running as a guitar cabinet. In this function, it looks impressive with its solid wood construction, slightly Art Deco styling, and load of drivers with two 12" Jensen field coil, and two Magnavox 15" field coils.
Edits: 07/22/09
MacThe cabs you describe seem like they were for an electric organ originally. The better contemporary electric guitar speakers (which are also of interest to horn designers) are usually low Q/over damped types and, because of their relatively large magnets, they tend to have a bass roll-off an octave or more higher than the resonant frequency. With this in mind, it may be useful to adjust the Q of the driver, as some of the other posters have suggested is possible with a field coil, for your application
Paul
Edits: 07/23/09
Yes, it was for an old Wurlitzer organ. The cabinet does have amplifiers, but I wish to use my own, thus I want to power the coils separately and provide signal from my amp.A friend suggested modifying the amplifiers to provide just the field coil power, since they are already set at correct values. He is very competent, but far away and busy, so I am not sure when I will have advantage of his help.
I might have had a simpler time adapting a separate DC power source myself, but I just received the schematics, and if I am reading things right, the voltage is 342 volts for the small speakers and 394 for the large. This sounds like a lot, and I have not seen any DC power supplies on ebay that go past 300V, thus using the amps might be the most economical way to go, although I will lose the flexibility of tinkering with current. But I am now wondering if I can just plug in a variable AC power control for a simple control. Would this work?
Edits: 07/23/09
Mac
I would go with the original amps, as long as the iron is good, as this would be the simplest and cheapest way. Typical amp topologies from that time and application would be 6SN7 or 6SL7's driving 6L6 outputs, and this is similar to a guitar amp anyway. See the link for some ideas for converting old industrial amps for guitar amp duty. If the original field coils were run with D.C., then running them with A.C. is definitely NOT recommended, but someone else would have to advise you on this. You will probably wind up with more bass with the rig in question than you need, the bass E string on the guitar is 82 Hz. Good luck, and let us know how it goes.
Paul
Thanks Paul for the input and the link. It looks like an interesting read, and I will pour through it over the next little while. I should clarify that I was thinking that plugging the amplifiers into an AC power rheostat rather than the wall with its fixed 120V would allow me to control the amount of voltage seen at the coils through a different means, but perhaps this is still not a good idea. I will leave this thought aside for a while regardless.
You were spot on with the 6SN7's and 6L6's (there is a 5U4G tube as well), but I am hoping to use only essential amp circuitry to power the field coils. I have an excellent 100 Watt head with much more flexibility in gain and tone control, and I would like to use it for the voice coils. It is a Hughes and Kettner Triamp MKII.
I will ultimately have help from someone with plenty of experience for the actual rewiring, and I will report back once some measure of progress is made. I will also start a new thread instead just for simplicity in reading. Thanks again to all for the information provided.
hey-Hey!!!,
Now there is a neat topic. The device that measures better sounds worse( to generalize ). No secret to the devotee of the SE amp for instance...:) Not that it is universal, but the meat of it seems to be that the wrong things are being measured.
As far as the flare at the end of the horn goes, it seems to be a pleasant coincidence that without seems to sound better; neglecting that flare during construction makes things simpler.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Not this again... Devices that 'measure better' don't sound better - you're measuring the wrong thing. This is well understood with, for example THD as in your SE amp example (and with typical loudspeaker distortion). Typically enough things aren't measured to completely characterize a device, and then people complain that measurements don't tell the whole story. They should really be complaining that the one doing the measurements should improve their test methodology.
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