University Classic, Pictures & Drawing Plans

[ajsons]

Here's a very good find for you, DrewB,

The SK's woofer driver, the W1508S.

[ajsons]

With an Fs of 25.7hz, and Qes of .38, it dosn't meet satisfactorily Keele's figure of merit for bass horns.

Fhm = 2Qts/Fs (mass roll-off frequency, must be around 300hz and above).

Like the K33, it is not a good match for the Khorn, according to Keele. But they both work well with a 3 x 13 throat.

http://forums.klipsch.com/forums/627507/ShowPost.aspx

[D-MAN]

Here's my take on the horn "stuff"...

1) horns DO clearly operate below Fc - the F3 of a horn is "shorter" (or higher) than a direct radiator F3 certainly, for instance the -3db point of the Khorn's nominal 40Hz horn is 33Hz. This is the "halfway down" or 1/2 as loud as it was at Fc. Definitely "usable" output.

2) The Fs of the horn driver should be below the respective horn's Fc. There is some controversy over this one, though. Edgar contends that a higher Fs than the Fc of the horn is better as it can be adequately annulled for extended low frequency use. Historically, this has not been the case, however, there are now drivers available that simply did not exist "back then". I tend to stay with the known "what works" thing of Fs below Fc. As noted both the K33E and the Speakerlab horn drivers work admirably but the T/S parameters would certainly not indicate this to be the case and in both cases, the respective driver Fs is well below the Fc of the Khorn and its ilk. It might even be said that you would be safe using a driver with an Fs below the F3 of the horn.

3) Vb (back chamber volume) is used for 2 things in a front-loaded horn. First being to balance the reactance at the throat (front of the cone) with the compressable air behind the cone to enable equal forward and backward movement of the cone. Unequal cone extension would result in higher IM distortion such as with a direct radiator, etc. This came from PWK in his 1945 patent.

Second thing is that T/S parameters per Don Keele indicate that the driver can be further reactance "annulled" by achieving the appropriate amount of volume behind the driver to allow it to resonate at or SLIGHTLY below its normal Fs, in keeping with the PWK formula as described above. The K33E is not specifically annulled in the Khorn, for example; the theoretical annulled Vb would be 9343 cu inches vs. the nominal 4800+ cu.in in actual practice.

PWK tended to prefer to slightly undersize the Vb in his horns. This would raise the overall Fs of the driver slightly but increase upper-frequency corner response at the same time. The Keele T/S annulment formula is a technical refinement of the PWK approach and the mathematical results are more precise. It should be kept in mind that with lowering the Fs of the driver below a certain point, it also begins to increase IM distortion, of course, so there is the inevitable tradeoff.

As one can see from the design of the Jubilee, there is a great deal of leeway in using a combination of different expansion rates in a horn. The "rubber throat" of 97 Hz is then translated into a section of 38Hz and then the terminal section(s) use a 40Hz rate, giving the overall result of a target 38Hz Fc. Due to the small size of the throat opening(s) dictated by the use of 12" drivers, the throat reactance would be too high if the throat expansion was also 38Hz (rapid expansion rates and/or large throat openings are easier to push air through). Therefore a throat expansion of almost 3x was used instead. This rapid rate was compensated for by the next section with a much lower expansion rate of 35 hz followed by the terminal section at 40Hz. The use of multiple expansion rates in horns was originally out forth in a paper by Olsen.

The La Scala and Belle also use a multiple expansion rate and an undersized Vb. PWK horns (except the "Little Bastard") tend to display the following attributes: a) bifurcated at the throat, radius folds, c) undersized Vb, d) front-loaded, sealed back chamber.

The University Classic seems to quite a bit like a 50 Hz Fc La Scala except that it does not bifurcate the channels. It has a mouth size conducive to single-planar element placement (allowing it to be free-standing). It employes a single expansion rate, but it has 1 problem that I can see (IMO). The same 90 degree untreated fold as seen in the La Scala. This should use a 45 deg. reflector instead to reduce reflections back into the throat channel.

In the old days, it was generally accepted that the use of full-radius turns be employed at folds. I think that hard reflective surfaces are better employed at horn folds. Edgar has also published this (see "The Monolith Horn" SB article) and I agree with his assessment (go figure!). Cohen got the reflection part right on the other fold of the Classic, which would lead me to believe that he also could have "fixed" the throat fold for being a less turbulant choke-point. Why he didn't who knows...

I suspect that it was generally accepted at the time (this is still a great rule of thimb) that if you are going to "get funny" with anything, do it close to the throat where the waveform is relatively small.

I personally like the Dinsdale articles - even though he is a little "KEF-centric", the information is valuable and fairly concise. To find out more, you will have to dig into the source material. I find the Edgar complaints about the Dinsdale articles - while probably true - to be somewhat unimportant - who would REALLY build a giant horn using tiny little KEF drivers? Not me. I took it to be a primer on horns - not a "how-to" speaker building article.

DM

[ajsons]

Whew!! I have to read that at least 5 times before things started sinking in.
Based on these things you said, I am drawing some conclusions related to the Classic, and maybe I'll ask questions on things I didn't quite understand.

"1) horns DO clearly operate below Fc".

The Classic goes down below the 50hz Fc, per Steve Schell and Q-mans observations/listening tests. And mine. I watched Fleetwood Mac's video the other night, John McVee was hitting the low E's and F's, with no problem on the Classic. I plan on doing more low end tests this weekend.

"for instance the -3db point of the Khorn's nominal 40Hz horn is 33Hz. This is the "halfway down" or 1/2 as loud as it was at Fc. Definitely "usable" output."

Earlier in the thread, you mentioned, "The -3db point (half down) is approx. 43Hz for the 50Hz Fc."

Are those numbers formula based or rough approximations? I had always ASSumed that the final F3, when the horn is finished, will tell you if you did your homework correctly in designing the horn, and can end up higher than, lower than or at Fc.

"2) The Fs of the horn driver should be below the respective horn's Fc."

I was right then in thinking that the EVm15L (Fs = 43hz) is the better driver, specs-wise, for the Classic and not the C15W (Fs = 58hz), but the latter works well with it. (It puzzles me that Cohen used a high Fs driver when he himself ties Fs closer to Fc, well he has a (lower) theoretical Fc and a (desired) target Fc). I have to do more listening tests to figure out which one I really like. I wish I have another Classic so I can I do an A/B test. But that's coming.

"It might even be said that you would be safe using a driver with an Fs below the F3 of the horn."

Now, Qman also says use a low Fs driver, quoting John (Warren).

How does this then relate to the mass roll-off freq formula, which calls for a high Fs?

[ajsons]

Second thing is that T/S parameters per Don Keele indicate that the driver can be further reactance "annulled" by achieving the appropriate amount of volume behind the driver to allow it to resonate at or SLIGHTLY below its normal Fs, in keeping with the PWK formula as described above. The K33E is not specifically annulled in the Khorn, for example; the theoretical annulled Vb would be 9343 cu inches vs. the nominal 4800+ cu.in in actual practice.

PWK tended to prefer to slightly undersize the Vb in his horns. This would raise the overall Fs of the driver slightly but increase upper-frequency corner response at the same time. The Keele T/S annulment formula is a technical refinement of the PWK approach and the mathematical results are more precise. It should be kept in mind that with lowering the Fs of the driver below a certain point, it also begins to increase IM distortion, of course, so there is the inevitable tradeoff.

The La Scala and Belle also use a multiple expansion rate and an undersized Vb. PWK horns (except the "Little Bastard") tend to display the following attributes: a) bifurcated at the throat, radius folds, c) undersized Vb, d) front-loaded, sealed back chamber.

How does the back chamber volume relate to F3, or does it relate at all? (This was another assumption on my part...Reactance annulling determines where F3 is going to be, and when done correctly will put it close to or at Fc).

Based on the above, and even the Classic, a horn can have the wrong back chamber and still be a very good horn.

[ajsons]

The University Classic seems to quite a bit like a 50 Hz Fc La Scala except that it does not bifurcate the channels. It has a mouth size conducive to single-planar element placement (allowing it to be free-standing). It employes a single expansion rate, but it has 1 problem that I can see (IMO). The same 90 degree untreated fold as seen in the La Scala. This should use a 45 deg. reflector instead to reduce reflections back into the throat channel.

In the old days, it was generally accepted that the use of full-radius turns be employed at folds. I think that hard reflective surfaces are better employed at horn folds. Edgar has also published this (see "The Monolith Horn" SB article) and I agree with his assessment (go figure!). Cohen got the reflection part right on the other fold of the Classic, which would lead me to believe that he also could have "fixed" the throat fold for being a less turbulant choke-point. Why he didn't who knows...

I suspect that it was generally accepted at the time (this is still a great rule of thimb) that if you are going to "get funny" with anything, do it close to the throat where the waveform is relatively small.

That makes me feel I'm going in the right direction with the Classic. On my drawing, I steepened the angle of the throat reflector, added the 45-degree one, and this I'm not quite sure, changed the angle of the large reflector, based on the "angle of incidence = the reflected angle". I don't know if that should apply here.

I also plan on adding a center divider starting at the throat section, like the Khorn, maybe it'll improve the high corner end.

[D-MAN]

The -3db point of 43Hz is typical for a properly designed horn with an Fc of 50Hz (from Dinsdale's article). It has nothing to do with the Vb of the back chamber being annulled or not. The low frequency response (Fs) of the driver could be lowered somewhat by back chamber sizing, but the Fc and F3 point is more-or-less fixed by the physicality of the horn itself. So within the confines of the horns operating characteristics, the frequency response can be altered and IM distortion minimized by proper reactance annulling using the volume of the back chamber.

Are you talking about adding a horizontal channel brace to the throat ala Khorn? That would be fine, it would reduce wall vibration which will always result in better response. In addition the brace need not be thicker (say 1/2" ply) than you can easily get a screw to bite into safely. The stress is horisontal, and 1/4" ply could supress that, although it would be a bit tough to get a edge screw in, IMO. Even 3/4" ply could work if that's what you got handy, i.e., it will not take up too much channel volume in other words...

The 45 deg. reflector should be wider than shown in your drawing. What you've described is still a radius-type turn, while it would help, it could be better if it was wider - I know it looks like it would restrict the passage, but it won't in reality.

DM

[D-MAN]

Here is an example of a full reflector - not a radius-type!

DM

[D-MAN]

I would not change the reflector at the throat opening - it should already be the optimum 45 degrees.... I would only add a corner reflector!

The Classic is a truly fine design, its simplicity is outstanding.

DM

[D-MAN]

Armando, based on your drawing posted above, I think you've got the corner reflector down fine. The throat reflector change also looks pretty good to me. Here is my take on the reflection angles. I could be very wrong about this, though. This is based on the estimated angle of the throat reflector, which if 45 degrees entrance would turn a full 90, but this is not 45 degrees? If its less than 45 then what I've drawn is correct - if not ...

If the angle is too sharp, the reflections come "back" to the back chamber front baffle, not the outer side of the channel as I drew it. If that is the case, then there is a problem. The question would remain, then, whether a half radius turn at the throat is more acceptable.

DM

[D-MAN]

Here is the real scary news: Corners ALWAYS reflect the soundwave back to the source.

DM

[ajsons]

I remember seeing those details in Speaker Builder, but the name Huygen didn't register. You mentioned that in a couple of other threads and I couldn't recall what that was.

It makes sense though, since it agrees with "the angle of incidence equals the reflected angle", i.e in a 90 degree turn you use a 45 degree reflector. If the turn is not 90 degrees, then the reflector angle needs to be adjusted.

I did the first drawing (and the second) ASSuming that the sound energy is travelling in the middle of the horn duct (reference line). To accomplish that, the reflector's angle must be positioned so that the angles are equal on each side of the reference line. I screwed up in the first drawing, but this second drawing is what I really intended. Now, I don't know if this is right, but my reasoning is, if this is what works for a lot of things....the angle the ball lands on a wall equals the bounce angle, reflected mirror image angle equals viewer's viewing angle, angle of incident ray equals angle of reflected ray, ...then it should work in a horn.

The angle of the throat reflector in my first drawing is a compromise, as things really will not lign up with the edges of the throat cutout as seen in this second drawing.

I did not use the same approach ("incident angle=reflectance angle") when I did the small corner reflector in the first drawing, I used another method to position that, and you're right, it is still a radius reflector. After adjusting it, it turned out to be less than 45 degrees.

Like I said, I don't know if this is the right approach to this angles, but this is the one that makes sense to me.

I think the same thing is accomplished when you draw a straight horn duct and fold it. The fold angle becomes the reflector angle.

[D-MAN]

I sent you an email with what I think I would do if I were building one.

Let me know what you think about it.

DM

[ajsons]

(I attached the wrong drawing to my last post, took it out and reattached the correct one).

Dana,

I get my emails at work, I'll read that first thing tomorrow a.m..

Armando

[awsjr]

started my classic build...got all the outside pieces cut ...then went

to the inside stuff and the angles didn't make sense...so redrew it and

now have some dwgs with good dim's.....

I will cut these on the weekend and see how it goes together -Al

[awsjr]

another view.......

[ajsons]

Al,

You need to take a lot of pictures during construction, and post them on the forum. Good luck with your Classics.

If there's something I can help you with, like details not obvious from the plans, dimensions for the front grill cover, etc. let me know.

I'll start a new thread for the driver specs, and other construction details plus cad drawings of mods, additional parts, etc. You are welcome to input your drawings.

Armando

[awsjr]

hi Armando....thanks....haven't made much progress due to work getting in the way....I will input some stuff on the other thread too -Al

[triceratops]

Tricertops,

I was trying to understand the relationship between driver's Fs, horn's Fc to come up with a driver that's best suited for the Classic. The C15W's Fs is 58hz, the Classic's Fc is 50hz . Maybe a driver with a lower Fs will be a better choice for it, I don't know. Right now I have an EVM-15B in it. I have to do more tests.

I personally believe that the C15w and the other components of the Classic are very good drivers, although I have not tested (don't have) the HF-206 tweeter. I have the Cobreflexes, but they're waiting for final sandblasting.

You may want to consider this University system. It doesn't take up as much space as the Classic. It uses the same components.

Hi Ajsons,

Thanks for the follow up and sorry for the long delay in my response. I started a new job and was off at training with no email access.

I wonder how the EVM-15B compares to the old EV SP15WK (which looks a lot like an old EV SRO bass guitar speaker)! I've always liked EV woofers and have the SP15WK in my 1960 Klipschorns. I also have a single University tweeter in storage, but I can't remember if it is a 206 you talked about or something else. I think a shortened version of that tweeter was also used in some 1950's Khorns. The non-folded cabinet you pictured looks like it might be University's answer to the Cornwall.

The Classics threads are fascinating. Too bad the Classic cabinet I once purchased was stored poorly by the previous owner and I could only salvage the drivers and crossover and had to discard the cabinet...

Best in horns,

triceratops

[ajsons]

How do you like the sound of your Khorns with those EV drivers? My SK with the EVM-15B actually makes a good partner for the Classic. The difference is obvious, in favor of the Classic, but you can tell that the SK/EV combination is a very good system too.

Please check out the other thread, "Should I build University Classic or La Scala?" by Quisitive. There's a lot of good info there.

I'm glad to hear from another EV fan.