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Old 10th June 2012, 07:47 PM   #7701
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My primary mentor on horns is Bjorn Kolbrek, who has the best simulations I know of. He guided the parameter selection for the AH425 and provided the hard-to-find information about the exit angle of the Altec/GPA 288 (which is 7~8 degrees, depending where you measure).

From what I can tell, there's a tradeoff between "smooth loading diaphragm loading vs frequency" and "constant-directivity" horns. The conical and modified-conical horns (with a smoothed transition between compression driver and horn flare) are optimized for constant energy over the intended radiation angle, commonly 90 degrees, but other radiation angles are common too. Conical and near-conical horns require a transition region between the (near) plane wave coming out of the compression driver and the spherical wave that expands through the conical portion of the horn. This transition region is accomplished with either a sharp or gradual bend in the throat of the horn, close to where the compression driver and horn-throat meet, and if sharp enough, this transition region creates diffraction, which in turn creates reflections in the time domain and ripples in the frequency response of the horn.

So conical and near-conical horns may require in-band equalization, and occasionally in-band notch filters. However, if diffraction is severe enough, equalization may not work, since the response ripples will be different at different angles of emission, with no single EQ curve applicable to the entire horn. Conicals and near-conicals also require a gradual HF boost to offset the drooping power response of the compression driver - with some compression drivers (typically those with titanium diaphragms), this boost is applied in the breakup region. Mylar and plastic diaphragms have more self-damping than metal diaphragms, so they are better candidates for applications requiring HF boost.

As a far as I can tell, the LeCleac'h and Holland & Newell horns do not prioritize pattern control (they are not constant-directivity), but optimize diaphragm loading over the working bandwidth. This results in less need for in-band equalization, and if the horn is well-designed and exit angles and horn-throat angles are well-matched, minimizing diffraction in the throat region. The impulse response I've seen from the AH425 is the shortest I have seen from any horn, which tracks with the overall goals of the Newell and Holland project.

The tradeoff is a narrowing polar response at higher frequencies. The AH425 mimics a direct-radiator about 4" across in terms of its polar response; the dropoff with angle is very smooth (and sounds it), but a supertweeter is definitely needed, for dispersion reasons alone. Since I am using a compression driver with a large aluminum diaphragm (about 3") that has the first breakup above 7 Khz, that's another reason to use a supertweeter.

Here's a simulation that Bjorn Kolbrek did that compares the AH425 versus a conical. It shows the resistive versus reflected energy as seen from the compression driver diaphragm. Like a radio antenna, an ideal horn sends all its power out the device and into the environment, reflecting nothing back to the source. Any power reflected back to the source is wasted, and worse, creates a reflection off the rigid diaphragm, creating ripples in the time and frequency domains.

The thin black trace is the resistive part of the AH425, and the thin red trace is the reflected energy (as a percentage). The transition is around 500 Hz, as you would expect. The curves are telling us that the horn ceases to work below the cutoff frequency. Above 700 Hz, more than 90% of the energy goes into the horn, and less than 10% is reflected. That was one of the key design goals of the AH425 project, and that goal is reflected in the real-world measurements of the AH425.

The heavy black trace is the resistive part of a conical horn, and heavy red trace is the reflected energy. Note the uneven diaphragm loading versus frequency. As a result of diffraction in the throat, the horn doesn't present a resistive load to the diaphragm until you get to frequencies above 3~4 kHz.
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Last edited by Lynn Olson; 10th June 2012 at 08:15 PM.
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Old 11th June 2012, 08:41 AM   #7702
SunRa is offline SunRa  Romania
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Hi Lynn,

This simulation is great, very revealing. A question though, was the conical profile optimized for transitioning the wave front from plane to spherical? And if yes, which was the method? I recall Earl Geddes argued that oblate spheroid was the only geometrical solution for this transition. This is what was used?

Also, LeCleac'h has an intrinsic mouth round-over due to the actual profile, was the conical treated the same?

Thanks,

Florin
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Old 11th June 2012, 02:28 PM   #7703
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I don't recall if the conical was fully optimized - like an OS - or the simpler constant-radius transition, like the Quadratic Throat Waveguide. The degree of roughness directly correlates to the sharpness of the planewave-to-spherical transition.

It was clear in the simulation that an intentional diffraction in the throat, although a useful (and common) way to create the desired polar pattern, has the penalty of creating a large reflection. The conical horn simulated above was much better than the JBL Bi-Radial and Altec Manta-Ray horns, where the diffraction throat is much sharper.

In a LeCleac'h with the right T ratio, the roughness can be brought down to nearly zero (the AH425 has a T ratio of 0.707). The penalty, like the similar Newell & Holland horn, is a gradual increase in directivity at higher frequencies.

From what I could see in the simulations, there seemed to be a general rule: directivity control (using diffraction throats) goes in the opposite direction to time and frequency optimization. Attempts to optimize one would degrade the other two, which follows the conclusions of Newell and Holland (Loudspeakers for Music Recording and Reproduction, pp. 108 through 113).

The fine detail and edges of the polar pattern (I need to dig up more of Bjorn's sims) are different as well. The conical has a sharp edge with ripples (fingering), while the LeCleac'h has a soft edge but narrower overall directivity. The polar patterns looked like a high-gain Yagi antenna (for conical) versus a low-gain cardioid-like pattern for the LeCleac'h.

Throat diffraction has negative effects in the polar, time, and frequency domains, and shows up in simulations and real-world measurements.

Last edited by Lynn Olson; 11th June 2012 at 02:38 PM.
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Old 11th June 2012, 03:44 PM   #7704
tomtom is offline tomtom  Slovakia
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Hi Lynn,

I also have followed BTA thread and also made system based on these ideas /albeit not strictly/.

It is big WTW with AE's lambdas TD12M and 1.4 inch compression driver with 3 inch diagram. /18s titanium nitrid/ Horn is asymmetrical - so at 500hz XO CTC is small enough to be "point source".

I've also added Supertweeter /which off course cannot be integrated without lobing/ and I had great deal of difficulty with its implementation and every time it was a question whether adding STW is positive VS running CD all way up.

Contrary to your finding I've found crossing to STW quite sensitive and problematic. I've tried many tweeters including 3/4 dome, compression driver with small horn and ribbon tweeter with and without small horn. /I've ended up with the last at ca. 10k/.

Would you be so kind to share your observations about adding STW, of course to the extent you feel comfortable with it, because of your upcoming speaker project?

Many thanks
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Old 12th June 2012, 12:09 AM   #7705
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Well, if the supertweeter is off to one side, then imaging will be somewhat unstable, since the listener will hear a crossover with different phasing for each ear. Even a slight movement of a few inches to the left or right will shift the relative phasing between horn and supertweeter.

In principle, a supertweeter located directly above the horn will have the same phasing issues, since any supertweeter (that isn't coaxial) will be several wavelengths apart at the crossover frequency. If the supertweeter is directly above the horn, though, at least there won't be left/right differences between the ears. What happens instead is a vertical null that comes and goes with shifts in seating height. If the null is at 7 kHz or above, it creates a false impression of additional image height, but not a coloration that is tonal in character. If the null is 5 kHz or lower, tonal colorations are heard, and if the null is 3 kHz or lower, then more severe colorations will arise.

A supertweeter appears to benefit from a 3rd or 4th-order highpass crossover, despite conventional wisdom. The lower the supertweeter reaches into the region where pitch is audible the more noticeable it seems to be - and a supertweeter should never be be directly audible as a driver. All switching it on and off should do is add a touch of sparkle and air at the top - there should never be any tonal shift to the loudspeaker as whole - if there is, the supertweeter is too loud, too peaky, or has too much energy below 7 kHz. It has to remembered that very little spectral energy is above 7~10 kHz, and a moderate-slope crossover has to work against the steep spectral tilt of the original program material.

A simple way to test this is listen to the supertweeter alone - if you can hear what sounds like music coming through, the crossover isn't steep enough, or is too low, or both. All you should hear is an occasional transient - basically, clicks, and not much else. It should like it is doing almost nothing when you listen to it by itself.

Regarding the comment that a 500hz XO CTC is small enough to be "point source", I'm not sure I agree. This has to be determined by subjective experience - preferably with broadband program material like massed chorus, or large-scale symphonic music. Some driver layouts will disappear into a single virtual point source, and others won't, and it doesn't always agree with theory.

I've had bad experiences with large MTM's - it was a struggle to get the 5.5" drivers of the Ariel to sound like a single coherent driver. I've yet to hear a large WMTMW or large WTW sound coherent, regardless of crossover, and I've heard lots of them at hifi shows and at friend's homes. Small phasing differences - on the order of 10~20 degrees between drivers - that are acceptable in a compact MTM sound quite unpleasant in a larger system, and the higher the top woofer is off the floor, the worse it seems to be.

A subtle problem for large WMTMW and WTW arrays is the floor image is always present - carpets absorb less than 1 dB below 5 kHz - and the floor image will couple to the lower woofer more efficiently than the upper woofer, which unbalances the WMTMW or WTW pairing. In effect, the floor image adds deep bass to the lower woofer, while the upper woofer sounds thinner, since it doesn't couple as well to the floor image.

It could also be purely subjective; bass coming from above ear height sounds weird and unnatural to me - it's something I try to avoid in my own designs. However, personal tolerance for this seems to differ. Plenty of other audiophiles enjoy large WMTMW and large WTW systems, so it could just be the way I hear things.

Last edited by Lynn Olson; 12th June 2012 at 12:38 AM.
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Old 12th June 2012, 12:22 AM   #7706
limono is offline limono  United States
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"I've had bad experiences with large MTM's - it was a struggle to get the 5.5" drivers of the Ariel to sound like a single coherent driver. I've yet to hear a large WMTMW or large WTW sound coherent, regardless of crossover, and I've heard lots of them at hifi shows and at friend's homes. Small phasing differences - on the order of 10~20 degrees between drivers - that are acceptable in a compact MTM sound quite unpleasant in a larger system, and the higher the top woofer is off the ground, the worse it seems to be. It could be that bass coming from above ear height just sounds weird and unnatural to me."

What about 70-100hz crossover point? (2x altec 416 and Tannoy 10" gold in the middle)
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Old 12th June 2012, 12:45 AM   #7707
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The last post timed out, so I'd like add this point: listen to each woofer by itself (run them full range), quickly switch between them, and see if they have the same tonal character. I'll bet they don't, even if carefully pair-matched.

That's the effect of the floor image. Moving a woofer just a few inches vertically can noticeably affect the subjective balance of a woofer; 6" + 6" + horn size is a much larger vertical distance.

If the woofers sound different - as I would expect - then the underlying advantage/principle of a D'Appolito array is lost, since the drivers are no longer phase-matched (phase follows frequency response in a minimum-phase system).

We get into the much deeper waters of independently equalizing the two woofers so they phase-track each other through the crossover region. I'm not sure a passive method exists, since the "warmer" sounding woofer is effectively more efficient in the 30~100 Hz region, and there's no good way of passively reducing its efficiency in that range. At a wild guess, it might take a separate power amplifier for each woofer, along with independent digital equalization, in order to offset the differential effect of the floor image so the woofers can be brought into mutual phase-tracking.

What if the tonal difference is ignored, and the woofers share a common lowpass filter? That means there is a phase difference between the two in the crossover region, which tilts the polar pattern. The crossover is no longer a pure D'Appolito array, but something else, and the model start to break down, unless you include the floor image as part of the overall design (which I would recommend).

Last edited by Lynn Olson; 12th June 2012 at 01:05 AM.
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Old 12th June 2012, 01:01 AM   #7708
limono is offline limono  United States
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Speaker showed is early Kevin Brooks creation. I think around 12-15 Cu. ft , originaly with Tad 1602 wofers and 260hz tractrix horn in the middle with Tad TD4001. Now, I have Tad 4001 for a few years already and know that 4001 + relatively small tractrix horn =headache .It just doesn't work. The cab built in Tannoy tradition with Tannoy grill cloth begs for 15" Tannoy but those are fetching obscene amount of money. I figured out that small Tannoy 10" in 2cuft closed horn compartment with biamped HE woofers would get pretty close if not better than single 15" gold. If not , a single 416 or 515B would do nicely in that big cab (although it would probably be a waste to cross 515B at 70-100Hz. I have lots of ideas about that cabs , also with horn arrays fitted . Sorry for OT post just the remark about big MTM not sounding natural caught my attention.
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Old 12th June 2012, 01:14 AM   #7709
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Quote:
Originally Posted by limono View Post
Speaker showed is early Kevin Brooks creation. I think around 12-15 Cu. ft , originaly with Tad 1602 wofers and 260hz tractrix horn in the middle with Tad TD4001. Now, I have Tad 4001 for a few years already and know that 4001 + relatively small tractrix horn =headache .It just doesn't work. The cab built in Tannoy tradition with Tannoy grill cloth begs for 15" Tannoy but those are fetching obscene amount of money. I figured out that small Tannoy 10" in 2cuft closed horn compartment with biamped HE woofers would get pretty close if not better than single 15" gold. If not , a single 416 or 515B would do nicely in that big cab (although it would probably be a waste to cross 515B at 70-100Hz. I have lots of ideas about that cabs , also with horn arrays fitted . Sorry for OT post just the remark about big MTM not sounding natural caught my attention.
I tried for many years to get Tannoy drivers on an OEM basis and the answer was always no. It's not easy getting them for replacement purposes; you have to prove to the factory you already have a dead Tannoy before they will cut loose a replacement driver. They are dead set against an OEM market developing as far as I could tell. Which is unfortunate, because the Tannoy really is a unique coaxial driver with a very good integral horn assembly.

I listened to a lot of small-format horns before I gave up on them. I suspect they don't work very well below 2~2.5 kHz - they might measure OK, but they sound strained and harsh below that frequency, even more than ordinary 1" soft-dome tweeters. Large-format drivers can be coaxed to much lower frequencies, but still demand low-diffraction horns - or the unobtanium multicells. Sectoral, MantaRay, and Bi-Radial horns with a diffraction "pinch" in the throat are all hopeless - once diffraction happens, no amount of EQ magic can undo it, and the horn-throat is the worst possible location for diffraction.

Last edited by Lynn Olson; 12th June 2012 at 01:17 AM.
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Old 12th June 2012, 01:26 AM   #7710
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Quote:
Originally Posted by Lynn Olson View Post
I listened to a lot of small-format horns before I gave up on them. I suspect they don't work very well below 2~2.5 kHz - they might measure OK, but they sound strained and harsh below that frequency, even more than ordinary 1" soft-dome tweeters.
I noticed you had positive things to say about the audiokinesis stuff earlier in this thread. Isn't that a small-format horn crossed around 1.2khz or so? Is that still strained sounding to you?
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