 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
216.14.198.113
Just wondering (I know, I know, I should be working:-) but is it possible to build a back-loaded horn to take advantage of the back-wave of the driver used in a FLH?You could choose a coupling chamber for the BLH to roll off the output at the same point the FLH kicks in. And you could set this point at the same width (I'm assuming a rectangular mouth on the BLH) that the baffle step takes effect. Then all you would have to do would be to ensure that the entire length of the 2 horns (plus coupling chamber) is 1½ (or 2½) times the wavelength at the 'crossover" point.
Simple, right? There's got to be a catch.
Asking as I've got a pair of round hole basket Coral FE103's that would work in such a configuration - if it's possible.
Has anyone done this, or is there info somewhere out there? I don't actually know what to search for - at least, nothing is coming up when I do a search.
Cloth
Follow Ups:
Hello,i use this only indirect with the cheap BG20,
and a direct driver like FE206 and AN 8" with
a second horn, look:
ClothYou and fellow inmate John K. should start one (see post from next page: "Anyone Know dimensions for Olson rear horn rectangular enclosures?" of 1/29). As you can see from the patent link in DJK's post, this has been done before by the great H.F. Olson, though Voigt in England was also using this type of design in the late 30's. Dinsdale calls it a "double loaded horn" in his Wireless World articles, which you can download from volvotreter's webpage. The catch(s):
_Difficult to design "by the numbers" or computer sim.
_Some ripple in the response can be expected at the crossover freq. between the two horns. This can be minimized by adjusting this freq. to be an odd number of half wave-lenghts considering the total length of both horns.
_Complex big cabinet. Expect to do a lot of cut-and-try adjusting to dial it in.
I've been listening to double loaders (with mid horns and tweeters) for some 15 years, and they work great.
Paul
Hi Cloth ears,I'm in Melbourne too, in Seddon near Yarraville.
I wonder how many other horn addicts there are in Melbourne?Have you seen this article from the Melbourne audio club website?
I am not a member, they seem to be over the east side of town.Regards Philip
Hello Philip
I am in Melbourne (Northcote), and just about to embark on the ' horn ' journey. The help on this site has been great. I am doing reading on most of the legends of horn design, and hope to make an enclosure for a 15" Paudio co-axial, and possibly a smaller 8" Fostex horn enclosure. Local input and advice would be fantastic!
I hope to hear from you
best wishes
John
and this guy dragging me along
I too have the PAudio looking for a good home for it.
We should meet up in melb and exchange ideas.
Well, I'm a member, but I hadn't seen this, so thanks for pointing it out. And I'm not actually a horn addict (yet), more a multi-way, active, SS-powered addict. But when you get a good driver cheap, you may as well try to place it somewhere...
Thanks for (all) the replies so far.Agreed that a design by numbers may not be perfect, what is? But the length and size of the 2 horns, and the coupling chamber, are easily formulea'd. And I've found some 'soft' plywood in my local that I believe that I can 'bend' for the ends of the BLH and all of the FLH.
The rest of the design can be slightly adjusted by playing with the dims of the coupling chamber and the first part of the BLH (I think).
DJK, thanks for the link - I'll read it tonight.
ClothIt sounds like you have a good grasp of the challenges involved. Olson's design had a crossover (between the two horns) under 200 Hz. While the air chamber before the front horn should follow standard procedure, the air chamber before the bass horn is a different matter. The hair-pin turns in the top manifold (along with the other turns) create a muffler effect which can roll off the high freq. response of the bass horn roughly an octave below what you could expect from the driver mass roll-off/air chamber volume and a strait horn combination. I would'nt bother using curved horn walls in the bass horn, but the front horn definitely should have curved walls.
Good luck!
Paul
The FLH will definitely have curved walls. I was looking at doing this for the BLH simply for prettiness and to test my abilities (my guess was 5 layers of 3mm plywood). If they are allowed inside it will be because the missus says yes, and smooth curves are easier to veneer in my favourite (Tasmanian Oak) than losts of fiddly straight pieces.Regarding Olson's design - I found the patent lacking (from a laymen's perspective, anyway). Obviously the design changes that he's brought in are the 'double coupling' (the holes called 21 in the design drawings are the end of second coupling chamber), the section in the front-horn (23 and 25) which works kind of like blowing over the top of a beer bottle. In the actual description, only points 13 and 15 reference this - the rest is just re-iteration of 'prior art' (even his 'dividing said cabinet into a pair of compartments' - said so many times it becomes a litany - is a product of his second coupling chamber). And he doesn't even include this second constriction of the bass horn in his 'electrical equivalent diagram of the acoustic arrangement' - although he does make a big show of the (hmm-mmm) idea for the front horn.
IMHO
But the patent does show that double loaders were well established by this time (1937).
I'll keep on looking - I'm going to go with my design, but it always helps to know what other people have done (either successfully or unsuccessfully).
J
J.I'm glad to see you're planing ahead for a lo-Z WAF design. Making the BLH with curved walls certainly won't hurt anything, and I salute your wood working skills and dedication.
Olson's patent (as other patents I have read via the new google search engine) is rather light on the practical details of how to make a functional device, and long on covering all the bases of how the patent could possibly be applied. So he shows you how the magic trick was done, but not necessarily how a competitor could duplicate the device. So it's DIY R&D. Olson does illustrate an electrical analogous circuit diagram of the double horn, but he does not establish any definite acoustical perameters for this, as did Keele and Leach in the late 70's. I don't quite understand what you imply, that the top manifold section (with it's restricted termination to the lower chamber) is a resonant system and part of the design. Several factors lean against this. One is that the top manifold is an expansion, and the sound waves should proceed from an area of high pressure (the throat) to an area of lower pressure (the larger chamber where they will mix below the top manifold). This is in addition to the fact that the length of the chambers is too short to acomplish anything in the context of the bass horn. If the chambers are about 24" long (asuming the scale of an 8" driver) they would theoreticaly resonate at over 500 Hz. Olson showed frequecy response graphs of this design in his Acoustical Engineering book, and the bass horn does'nt go over 200 Hz. Olson does reference prior "...loudspeakers of the compound type..." and claims his design is an improvement, though no specific prior designs are mentioned. Anyway, it's good to see others attempting a double loader.
Let us know how it goes!
Paul
Hi Paul,RCA did build a compound horn system for small theatre use in the late 1930s which closely resembled Figures 1,2,3. I've never seen one of these rarities in person, and don't know if the front horn actually contained the resonating chamber(s). Seems like a hip idea though to keep the front horn from honking below its cutoff, since there was no electrical crossover to limit low frequencies being fed to the driver.
The partial obstructions of the rear horn shown as #29 in Figure 9 are described in Olson's text as being embodied by 1) the reduction in cross section from chamber 27 to the beginning of the horn sections 19a and 19b, and 2) the reduction of cross section at 21 where the horn path travels through panel 11.
RCA built the design shown in Figures 5,6,7 as the 64A Monitor (late 1930s) and 64B Monitor (mid 1940s). The two models differed in external styling but seem to be the same internally. I have a pair of the 64Bs, and (not having studied the patent carefully) have been puzzled by the restrictions to the rear horn path. As built, the first obstruction is actually much more severe than indicated in the patent. A crosswise panel at the rear of chamber 27 leaves only a vertical slit barely wide enough to admit fingers at the point where the path splits in two. Perhaps this was an example of Olson's cleverness as you indicated, describing the function well enough for the patent but not tipping the competition as to the actual embodiment. The second obstruction is similar to the patent drawing, perhaps just a bit more severe than shown.
Hi SteveThe rareness of the theater version of the double loader speaks to the power of the Altec monopoly which you described so well in your legendary presentation at MAF '03. I recall you mentioning the monitors before. It's interesting how they seem to depart from the patent in the size of the connecting holes. Olson never mentioned anything about customizing the cabinets to a particular driver, something which is almost second nature to us today. Maybe this is the reason for the variations in the hole sizes in your monitors. I did a sketch of the backloader version scaled up to accomodate a 15'er for John K.. I also did a second sketch with a larger bass horn mouth and a somewhat different top manifold section. In the second sketch I made the holes larger (almost as a reflex), with the idea that they should be adjustable, but I did'nt give it any more thought untill these latest threads. I was fixated on area 27, the back/front chamber, We now usually evaluate this chamber solely in how it effects the highs, but Olson concieved it as having both acoustic capacitance and inductance, which becomes more important in a double loader. Olson also investigated multiple chambers connected in series in his Dynamical Analogies book which has recently become available as a download on the net (thanks for the link Freddiy!). It's interesting trying to probe the mind of the master.
Thanks for your input.
Paul
Hi Paul,I just reread your post and caught the part about the "...legendary presentation...." LOL! I guess the 10 or 12 people in the audience were also legendary attendees. The MAF 2003 was a mighty fine audio show, and we have the irrepressible Mike Baker to thank for that.
Hi Paul,Olson was indeed a genius; just ask RCA-fan (himself a bass horn master) who knew him! In many of Olson's writings I also detect the uncredited insights of Frank Massa, who collaborated with Olson at RCA all through the 1930s and refined the science of voice coil and transducer design to an amazing extent.
The 64 Monitor design was no doubt tailored to Olson's dual voice coil driver. These amazing 8" (6" cone) relics still define what is possible in a single wide range driver. At the last Oswald's Mill gathering, Jonathan Weiss had scored a 64A Monitor cabinet with altered exterior cosmetics for something like $35 on ebay, and had procured the correct RCA dual voice coil driver to install. The assembled audio crazies listened to that system in stunned silence except for the occasional "Wow." Though a bit limited in bandwidth in both extremes, these systems possess incredible resolving power and see-through clarity.
Olson's dual voice coil driver design can be studied in detail in his U.S. Patent #2,007,748. The dual voice coil was incorporated in the driver (can't remember the model #) used in the 64A, which otherwise was the same as the MI-1425A 8" cone driver used in most of the early 1930s RCA horn systems. By the 1940s there were modernized versions of the driver, MI-4410 (permanent magnet) and MI-4411 (field coil) which were fitted to the 64B monitor; either available at equal cost. Then the trail went cold, and to my knowledge Olson's dual voice coil driver has not been heard from since.
The 64B contained at least one acoustical improvement over the A, a sheet metal vaned high frequency diffusor. It is decribed in Olson's patent #2,102,212.
Until carefully reading the patent we discussed earlier, I assumed that the purpose of the two necked-down sections was to limit the output of the bass horn. The output of the horn does seem a bit timid in comparison to the driver's front output. Perhaps Olson did seek to limit the bass output a bit, as these monitors were listened to in the nearfield in small control rooms.
SteveAdd my "wow" to the 64 Monitor, and I have'nt even heard it! Some of the patents from the late 30's I ran across had Massa's name on them. The double loader design of 1936 pictured in Acoustical Engineering has both Olson and Massa's name, but the 1937 patent application lists just Olson. Bruce Edgar told us at a CHC meet once, that Massa resembled Larry Fine of the Three Stooges, and was prone to play practical jokes on the rather stiff Olson. The RCA R&D lab must have been a great place to work in the 30's. BTW, I have some vague memories of some German field coil designs from the late 30's with dual, or maybe even tripple, voice coils (Telefunken?). Thorsten Loesch posted some pics on one of these forums several years back, with similar comments that a pinacle of driver design had been reached, and then forgotten. I hope you guys do get to incorporate the dual VC in your FC designs.
Paul
Hi Paul,I seem to recall Thorsten posting pics and discussing the System Eckmiller 015 coaxial speakers of the 1930s. There is quite a bit of information on these drivers at the site below. Click on pictures and scroll down to category 2.4. Drivers. There are plenty of other fabulous cinema products documented on this site as well.
Steve,How was the signal fed to the Olson dual voice coil? Was it used like the Watkins woofer, where the second voice coil would kick in at the point where response started to fall off, thus assisting the first voice coil and extending the low end? Or were the two voice coils used to create a sort of servo for tighter control of driver, etc.?
I'm asking because Mr. Sano's Exact drivers -- at least the later ones -- all used dual voice coils (actually it's a bifilar winding so at first glance it appears to to be a single coil, until you notice that there are four leads) and he used them in conjunction with a passive circuit that not only extended the bass but enhanced the treble as well. Feastrex dropped that approach and went back to a conventional single voice coil, because they felt the gains achieved by Mr. Sano's approach came at the cost of imparting the "sonic signature" (or "veiling effects") of the passive components, somewhat akin to what one tends to get with passive crossover components in a multiway loudspeaker (versus using an electronic crossover or just a single driver).
Feastrex is getting good results with their conventional approach, but I often wonder if maybe it wouldn't be worth going back and taking another look at dual voice coils in a fullrange driver. Not necessarily the way Sano used them, but perhaps the way Olson used them. I'm quite curious to find out exactly what Olson had them doing.
When Feastrex parted ways with Mr. Sano, it was definitely the right decision for them to drop the dual voice coils. They were going through the "reinventing the wheel" process that every transducer-building acolyte has to go through, and eliminating that variable probably made their life a lot simpler. But now that they have thoroughly debugged their initial product line, as soon as their balance sheet stabilizes I hope they'll go back and take a second look at that. As you know well Steve, there are too many gems that have been consigned to technology's trash heap without good reason!
Hi Christopher,"...too many gems that have been consigned to technology's trash heap..."
You sure got that right. When it comes to the technology trash heap, I'm a dumpster diver!
Olson's dual voice coil driver was fed a normal audio signal, but contained an onboard crossover of sorts, partly electrical and partly mechanical. A careful reading of his patent # 2,007,748 will explain it pretty well, but I can add a few things.
These drivers had 8" baskets and a 6" thin, seamed, concentrically corrugated paper cone that was sprayed on both sides with a dull black compound resembling auto primer. RCA's literature states that the compound was applied to help the paper resist weather changes; I believe it has a damping effect as well, like the Aquaplas used by JBL. The outer suspension was leather or cloth, and the spider was in the center of the 1.5" voice coil assembly, located with a screw (like some of the finer compression drivers :) ).
The voice coil assembly is the heart of the driver. Two half shells of thick copper foil comprise the bobbin, like brake shoes, and do not quite touch each other. The bobbin halves contain a half roll compliance midway across their width- this can be seen in the patent drawing. Two voice coils are wound on these half shells; an aluminum coil close to the cone junction and a copper coil in the rear, on the other side of the compliance. The half roll acts as a mechanical crossover, isolating the two coils above 2.5kHz. The coils are wired in series with a center tap lead brought out along with the other two. The patent drawing shows four lead outs, but there are only three on the actual drivers. A pair of paralleled oil caps (2 mfd. total I think) is fastened to the rear of the driver, and is connected in parallel with the copper coil to keep the high frequencies directed to the aluminum coil.
So what we have is Olson's stellar design where both voice coils drive the cone at low frequencies and the aluminum coil drives the apex of the cone at high frequencies. The corrugated cone likely decouples to progressively smaller areas at higher frequencies, eliminating excess mass and preserving dispersion in one fell swoop. The dispersion of the highs is augmented in the 64B Monitor by the vaned diffusor, described in the second patent I mentioned above.
The response curve shown in the patent is about right. About three years ago Rich and I evaluated three different drivers on a small TL; a 4" aluminum cone Jordan, a 6" Fostex, and the Olson permanent magnet dual coil MI-4410. First we listened, then we measured. Subjectively we thought the RCA had the sweetest, most extended highs and best, most lively sound overall, followed by the Fostex then the relatively inefficient, lifeless, but still not bad sounding Jordan. When we made LMS measurements the RCA driver tanked above 8kHz., the Fostex was strong to about 12k and the little Jordan was almost ruler flat to 16 or 18kHz. Despite the measurements, I would much prefer the clean, lively RCA driver for a desert island system. I am still not sure exactly what is at work here, but I think the RCA obtains its response honestly, with no whizzer nasties or other breakup BS to gain h.f. level.
A back burner project is to tool up and produce this voice coil design again someday, perhaps fitting it to a 20 kilogauss field coil motor. If someone like Fostex (or Feastrex? Exact?) were to do it they would have a unique, world beating driver; a real Lowther killer IMO. If anyone does get around to building the Olson driver before Rich and I do I'll be the first in line to buy a pair.
I'm not real good at explaining the bits but I'll have a go:In section 15 (on the last page) it says:
"... said horn increases uniformly in cross-section along its length and characterized further in that the opening in said partition is smaller than the cross-sectional area of the horn at the same point, the area of said opening being so related to the cross-sectional area of the horn at said point as to constitute an acoustic filter adapted to pass only vibrations below substantially a predetermined frequency."This "opening" is in the partition between the upper and lower bits of the speaker and after the 'labyrinth' in the top section. So, he's got the back wave going 1) into an acoutic chamber, 2) through the first part of the bass horn, 3) through another constriction ("acoustic filter") and finally, 4) through the bottom section of the bass horn.
In figure 3 of the patent, the "opening" are both 21's, "the horn" which is "uniformly in cross-section along its length" is 19a and 19b, and the acoustic chamber I mentioned is 27.
So it seems he's got 2 acoustic filters in series to reduce higher frequencies, one of which is about halfway along the bass horn. I'm not sure that I've ever seen this idea in use, and I'm a bit doubtful that it would work in the manner which he intends.
J
P.S. Anyway, I'm not going to put this into practice - I'm just looking at the basic framework of FLH+BLH.
J.He does characterize said chamber(s) as a low pass filter, so it's fair to say that they are traps for certain frequencies, like a car muffler. On my first read of the patent I missed the signifigance of the restricted hole as a tuning device, as I was mainly interested in the backchamber volume in this regard. It would be interesting to test the effectiveness of changing this. The hole in my Mellow Monster variants is lot larger, and not a restriction at all. The downside of restricting the flow would be to lower the output of the horn, perhaps more of use in a BLH with direct radiating front., and they are in fact relatively smaller in the version of this Olson pictured in Figs 5,6,7,8. I can attest that the design does work as an effective low pass filter as a whole though, my variants cross over at about 180 Hz. In any event, Olson certainly covered all of the bases.
Paul
hey Paul - be nice to have a few original mellow monsters to re-populate the world plus your mod.think I lost the scan :^(
why might BLH benefit from curves? - looks too hard to build!
Freddy
Hey FreddyMy scanner is finally fixed. I'll send you some more stuff, including a sketch for a BLH for a 15"er after Olson, which I came up with while recovering from the latest cold making the rounds. I hope you are well (I think you got the virus before I did!).
Paul
djk referenced a very good example. My old friend Jon Sutphen (Joth) used to make a somewhat similar rather monstrous 18" cab that were like a combo of an A7 front horn and a JBL "Scoop" rear horn. They worked OK, but he later stopped using the front portion. The time delay issues, and lack of a tight compression chamber really mucked things up more than any added "punch" you get from the short front horn.
If you are attempting to cross your woofer over above 400 HZ you might find this arrangement useful, otherwise I'd suggest a spiral horn along the lines of Tom Danly's Servo-drive or Lab Sub or Cerwin Vegas approaches.
I have made smaller versions of the above and they work great in multiples, but are not very impressive singularly.
.
;-)!
WarmestTimbo in Oz
The Skyptical Mensurer and Audio ScroungerAnd gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
