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La Scala bass bin


jheis
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Thanks Todd:

Wow, that looks like a major project - far beyond my skills.

What I am really curious about is how the woofer is positioned. It appears that it is in the "doghouse" with the magnet facing towards the peak and firing to the rear of the cabinet through a rectangular shaped opening. Is that correct? Is there something to be gained by firing through a rectangular opening rather than a circular opening?

I'm trying to get a handle on the folded horn concept - I'm sure I'm going to have to explain it to the first "audiophile" who hears my system with the new La Scalas - and frankly, I don't have a clue.

James

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The rectangular opening is just easier to build and I think the diffraction characteristics make the sound transfer to the rest of the horn better.

The concept of a folded horn is exactly the same as the concept of a normal horn in that the flare rate and dimensions of the mouth and throat are all the same. The complexity that gets involved is that it's harder to get higher frequencies to bend because the wavelengths are too short. Likewise, lower frequencies bend easier because the wavelengths are longer. Of course it's more complicated than this, but it's the general idea. All the rave going on about the jubilee bass bin right now is that the folded horn is such that the bass bin response can go up to just under 1kHz, versus around 300Hz for the khorn and lascalas...making it plausible to have a 2 way system (one step closer to a point source).

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Yep, you got it. I do not know why the rectangular slot. More compresion? Maybe some of the technical guru's will chime in here to answer that. It probably has been discussed before too, if you try some searches you may find something.

Oops, there you go, Dr Who posted while I was writing

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The rectangular slot is doing 3 things:

1) it is the proportionally the same shape as the horn channels cross-section (rectangular) which prevents discontinuities (reflections and turbulence) at the horn throat. The rectangular shape is determined by the horn channel shape, i.e., a round horn channel COULD use a round opening, although it could also be rectangular or square - it effects efficiency and upper freq. bandpass, but is usually a performance and manufacturing determination.

2) it acts as a physical acoustic filter; i.e., the physical distance (width of the slot) there is some indication that it is somewhat directly related to wavelength size, that is it will restrict the passage of smaller frequencies and has little effect on lower frequencies propogating through the slot (ala the 1960's Karlson cabinet with uses an exponential slot). However, the slot also increases reactance (physical resistance to air movement) on the diaphragm, and that certainly reduces the production of higher frequencies.

3) the slot size also effects efficiency, where efficiency lowers when the slot is smaller than the horn throat cross-sectional area, of course. Note that the slot itself is NOT the same as the throat, per se, rather it is referred to as being the "throat cavity opening". The throat (proper) is the area of the horn (i.e., the small end) where the cross-sectional area is the smallest.

PWK quote(s) on the slot...

PWK_throat.jpg

It is well known that a narrower slot will NOT raise a so-called "dip" in the response - someone here is pulling a fast one! The rule of thumb is "the wider the slot, the HIGHER the crossover point can be". This indicates that more high-frequencies are passed in a wider slot than a more restrictive narrow one. I have proven this out on my own as well.

So I know that the K33E has a PEAK around 400Hz, and that the slot was narrowed to reduce it, NOT increase it!

Do not take EVERYTHING that PWK reportedly said as gospel, because it ain't. I'm sure even he would tell you so. Also, some of the above is certainly not a direct quote from PWK, but I assume that PWK either "approved by inaction" or somehow supplied it originally, so it too needs to be taken with a grain of salt.

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D-Man, you are very wise in the ways of science. Please tell us again how sheep's bladders may be employed to prevent earthquakes! LOL (that last bit was a Monty PYthon rip-off)

But you do have it durn right in some of your recent posts. PWK's ramblings should not be taken as gospel. Many of his thought have been readily disproven. But he did quite a bit of it right.

Michael

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Michael, I'm not PWK-bashing, hope nobody takes it that way. In trying to research horn "slots" I ran across this stuff. I regard PWK's scientific work to be unassailable and his horn designs to be instrumental to the industry to say the least. He was a force to be reconned with.

It's the misquotes and/or falacies in the audio media that get me. You just can't rely on them as being factual or even being scientifically sound. I would have thought that all concerned would have known better, aren't they professional audio people, too?

That may have more to do with the media than the subject, of course.

Notice in the above post that Edgar asked when PWK changed the throat size in the Khorn, and he replied concerning the throat cross-sectional area, NOT the cavity opening size.

Perhaps confusing to those who assume he was talking about the "slot" - he was not. Note that the Khorn throat area (6x13") is still the same today (78 sq. in.), although the SLOT was reduced smaller (3x13") as stated in the second reference.

I also tend to believe that in advertising and related publishing (including Dope-from-Hope) PWK felt free to state his opinions on things rather than remain scientific, of course there is nothing at all wrong with that. He tended to move around more freely between fact and fiction more there, I think.

We tend to perpetuate some of the fallacies here, too. I just wanted to point that out.

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D-Man and I are sometimes at odds. Actually I think this could be like a cop - buddy movie where they start off in conflict and then wind up as friends.

Some years ago it was my thought that PWK was trying to form a front chamber in front of the woofer. You see articles about how this should extent the response at the high end. Essentially, without the chamber you have a second order low pass filter. With the added chamber you have a third order low pass filter. However, its high end cut off is a little bit higher.

Aside: This is pretty much a fine point of filter theory. Even if you're stuck with values of a second order filter, you can get better results in a narrow band by adding another component to make a third order. The added one is the front chamber. The front chamber (third component) really doesn't exist without the restriction.

I did a SPICE simulation based on Don Keele's circuit simulating horn drivers (his article is posted by me previously). I found the benefit was very mild.

Bruce Edgar had an article (1) proposing that the restriction created a horn with a smaller mouth. He did an about face on that in a later article (2), saying his methodology was incorrect.

However, then the folks at Klipsch sent him some data in the high end. It showed a peak at 400 Hz but a dip at 250 or so, with the restriction, as compared to the open throat. This was published (3). Unfortunately the two graphs were switched and this lead to a correcting article (4). So there are 4 publications. You have read them all to see the progresson of inadvertent errors and corrections.

About a month ago we had a question here about the AA and sucessor crossovers. These have a second order low pass filter. I.e. there is the addition a caps of various values.

I estimated the electrical resistance of the bass driver and put the crossovers into LEAP crossover simulation.

As you may know, if you have a second order crossover, you can get a peaked output at the crossover point, or nearby. (Again, a second component can lead to a peak, you don't get it with just one). The patent on the Valerie discusses the use of peaking effects. The peak in output is accomplished at the cost of a lower electrical input impedance. That is to say, you really are sucking more power out of the amp. There is no magic.

The LEAP simulations showed a peak in filter output at 250 to 300 Hz. Then a steep roll off above 400.

My conclusion is that the restriction does indeed peak up output at 400 Hz at the cost of a dip farther below. This lower dip is made up for by the AA and related crossovers.

I did not write it all up because it would take most of day.

Gil

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Well, I dunno.

Please note my long standing avitar. It it the paper target shot out by Martin Riggs. Have a good day.

But yeah, you can assume that personality, should it please you.

Right now I'm working on being Dr. Morbius (sp?) as played by Walter Pideon (sp?). The mission philologist from Forbidden Planet.

Smile,

Gil

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I tried to find the link to JimE's post for his LS drawing so I could post the link. I can't find it at all. Sometimes the search function works fine and at others....

Gil,

Did you ever get those Belles set up?

Marvel

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Yes I did get the Belles set up. I built a pair of dollies to put under them. Belle on Wheels.

They're fine in the corners. However, I find I need a center channel. That may take a while.

Gil

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Gil, I disagree with your assessment that a compression chamber is present in ANY PWK horn design.

PWK horn designs do not feature a compression chamber in the horn mouth. Here's a discussuion of a throat compression chamber:

The compression chamber

Finally there is a possibility to create a compression chamber (Vf) between the driver and the throat (Ah) of the horn.

image0012.gif

The volume of the compression chamber works like a low-pass filter (6dB/oct.), the higher the frequency the higher the attenuation. The crossover point (-3dB) can be calculated with formula:

Vf is the volume. Note that a larger volume lowers the crossover frequency. The nice thing of these acoustical filters is that the output can be corrected without the use of extra electronics.

Another -3dB "filter" frequency is due the driver moving mass corner of the driver according to the formula mentioned before (Fh).

When you decide to build your own hornsystem, I strongly advise an exponential design with the largest possible exit-surface. The used speaker unit(s) requires a strong magnet (Qts <0.3). The early decline in the low area can be increased actively which works well in the area where the horn is active (>Fg).

{edit} I WOULD agree that the original 1945 Khorn patent, which features a 12" field coil driver, DID INDEED have a compression chamber discribed in it. The current version of the 15" Khorn does not, nor do the LS or Belle.

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Here is my rendition of the PWK designs with an acoustic filter (the slot is smaller than the throat).

image_mod1.gif

This is different because the horn channels proper are NOT RESTRICTING, they are expansive in comparison the the previously posted diagram, although they may be the same exact cross-sectional area.

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I'm still of the same mind. I'd say you have drawn a chamber.

In my view, the illustrations we see in publications are somewhat schematic in form and do not show some of the issues. That includes the first one that you've posted.

If you start with a K-Horn, LS, Belle with a 6 x 13 slot, you really don't have an effective chamber. There is a potential for one just by the structure of the cone and the surrounding gasket. The problem is the 6 x 13 slot itself.

The effective diaphragm diameter of a 15 inch speaker is about 13 inches. We subtract out an inch on either side for the basket and surround. So the area is 132.7 square inches. The 6 x 13 slot is 78 square inches. So the openning is 59% of the diaphragm area. I have doubt there is going to be much compression of the air in the "chamber" when a greater portion of it is open to the throat.

On the other hand, if you reduce this to 3 x 13 you're down to 29% of the diaphragm area. Now I can see some compression going on.

I will admit it looks kludgy in view of the jump to two 3 x 13 ducts. But I believe the chamber is there (with the restriction), or not (if there is no restriction), regardless of the throat on the other side.

I've also read about the purported 6 dB filter effect. I'd refer people to Don Keele's article on driver selection based on T-S parameters, which I posted. He is saying that the voice coil, diaphragm mass, and front compression chamber each add 6 dB eventually. His model at high frequencies show a third order filter.

None the less, the theory of extending the high end is that the compression of air in the front chamber and the mass of the diaphragm cause a resonance, which can cause a peak. And it is possible to show this in the electrical model.

Best,

Gil

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Yes, the compression "chamber" would be directly in front of the cone in such a case of the narrow slot. And also one that changes its size with frequency, extension and and polarity as determined by the cone movement; probably not real calculable as a useful tool (without a fixed phase plug, too). But like I said before, it therefore reduces excursion (and high(er) frequencies) through reactive impedance, not by increased capacitance (or does it?).

Let's try it another way, what's the effect on a NON-K33E in the same horn, same slot? I got a kick out of Speakerlab copying the Khorn to the letter, INCLUDING the 3" wide slot, even though they were producing their own 8 Ohm horn driver. The ones I heard produced astonishing bass, even with the technically "too-small-for-them" slot.

Bruce Edgar reports that the slot should be wider in a Khorn for using an 8 Ohm driver. I also find this to be true by trying other drivers in both slot sizes. So in the technical sense, the wider slot reduces reactance at the throat. {edit} 1:1 with the horn throat cross-section and therefore, no extra reactance or acoustic filtering other than mass falloff.

PWK said the vc resistance went up (the degree remains a question) in the woofer when horn loaded, which is directly related to the overall slot size, presumably.

PWK_WOOFER.jpg

Where this is leading is that the real effect of the slot on the driver depends on: the vc impedance, the throat size, the frequency high-band pass, the overall efficiency desired, I think.

Perhaps you (and PWK) are right concerning the K33E reponse in a Khorn. What horn drivers do under a resistive load is surely strange.

I did try different slot sizes with the K33E and the Kappa 15LF, and can only confirm that the slot effects the high-pass of the horn for sure, I never noticed an actual dip on the K33E, but the lower bass does seem to "weaken" a bit but not objectionably IMO. Maybe that's what PWK was referring to as a "droop". I just took it to be change of "flavor" rather than a drastic "dip" or something.

Thank you for your discussion, good points all.

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The impedance of the driver has NOTHING to do with the slot size.

The 16 ohm number comes from taking the root mean of the minimum impedance and the maximum impedance at the first peak above the Fc of the bass horn (as explained by PWK in one of the Klipsch papers reprints of his various technical articles, not DFH).

The slot does several things, it acts like a cavity with the air trapped between the cone and the slot, this causes the roll off to be 18dB/oct (6dB from the mass corner, increasing to 12dB from the woofer Le and Re, and then 18dB from the shunt capacitance of the air in front of the cone. This follows the Keele horn model refered to by Gil.

The slot and the splitter also act like a very short very fast taper rate horn at high frequencies (400hz region) and as such will produce increased output over a very narrow range. If you consider Olson fig. 5.11 (posted by

Gil) and look at the impedance of the last horn, figure the Fc of the expansion of the slot to splitter, it looks like the first peak above the Fc might be 420hz with the Fc peak about 240hz, and a third peak around 580hz. This follows Olson, response of current Klipshorn bass unit from fig.9 of the JAES Jubilee article.

Woofers other than a K33 will produce different results due to different mass corners, different Le corners, different cone geometry resulting in different front cavity volume, and different reactance annuling due to different Fs, Qts, and Vas.

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Just some further thoughts. As D-Man says, it is nice to have everyone's input. We're not really at each other's throats. Ahem.

I can't quite agree that there is a mini horn effect. For the most part, small sections of a transmission line don't have much effect as transmission lines. But see infra.

I do agree, overall, that the Rvc of the driver itself should have little effect. We're just looking a the mechanical system. On the other hand, these beneficial resonances are damped out by motor strenght. So that damping will have some effect.

For example, I believe that we get a boost at the low end of the the K-Horn response just by the resonance of the the diaphragm and back chamber at 35 Hz. This is somewhat at odds with theories of reactance annulling. It also implies that very low Q drivers are overdamped and defeat this beneficial resonance at the bottom end. Maybe the top end too.

I was sitting on the 156 LaSalle bus and contemplating D-Man's diagram. I wonder if there is not an additional joker in the pack. The restricted motor board and the area of the splitter does create a little mass of air in that port. Hard to say how to model this or what the effect might be.

Something to think about.

Gil

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