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Old 20th June 2013, 10:15 PM   #191
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This is a really productive discussion, in my opinion. Thanks for your input, guys.

The thread really has two topics, one a subset of the other. It is about loudspeaker configurations that can provide uniform directivity and the various patterns they create, which ones are most useful and effective. And it is also about the horn/waveguides chosen to build them, their aspect ratios, flare profiles and other details, and how effective they are when used in a loudspeaker system.
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Last edited by Wayne Parham; 20th June 2013 at 10:18 PM.
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Old 20th June 2013, 10:22 PM   #192
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This is true about the mirror image, but I don't think it means that the sound appears to come from the center of the virtual array (the floor level driver). Like most line arrays, I think the perceived source location is more or less the ear level driver (or the driver even with a line perpendicular to the array going through the viewers ear level).
Do you think on a mirror floor in otherwise full space they would image on the axis from the listener through r behind the base of the arc on the ground plane?
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Old 21st June 2013, 01:00 AM   #193
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Do you think on a mirror floor in otherwise full space they would image on the axis from the listener through r behind the base of the arc on the ground plane?
I don't know if that is universally true but it would certainly be a good approximation. If the CBT drivers are in a true semi circle and the network doesn't add a lot of excess delay to upper elements then it is probably true. I think the apparent location is primarily tied to the first arival of the HF wave, and the driver it came from.

Somewhat similar is the older JBL diffraction slot CD horns. If you move laterally around them the diffraction slot seems to be the origin. If you move vertically then the entrance at the throat becomes the source. I think Don Keele called it a sort of astigmatism.

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Old 21st June 2013, 01:08 AM   #194
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You know, the floor (or any other boundary) mirrors a point source too. It's just that with a point source, the mirrored source becomes a second and that's different than multiple sources. Interference is course. With multiple sources, interference becomes dense enough the sound field starts averaging. It's kind of like the various forms of multisub configuations, which are actually all arrays of one form or another. Just something to think about.
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Old 21st June 2013, 04:01 AM   #195
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Well Wayne, as these types of discussions continue, the underlying reality is almost always revealed as a conflict of objectivity vs subjectivity in regards to what an individual 'enjoys' from a listening experience.

Designing through objectivity is smart engineering 'almost' all of the time. Taking many of the room induced variables out of the equation through constant directivity is a sound approach of course. Using drivers with low harmonic distortion and a smooth frequency response are equally responsible choices.....a design that employs all of the above is sure to please 99% of the people.....it's the 1% that play the wildcard for one reason or another. The factors that define the small percentage of audiophile listeners are quite broad.....some based in real world, quantitate measures and others derived from bias, memory, or other psychologically based rationale.

I think we all need to accept that as we get older and out hearing degrades, we find ourselves constantly searching for more of an audible 'memory' that becomes increasingly difficult to replicate as we age. Once we hit a proverbial wall, we tend to substitute 'new' experiences that never really fully pay off........sad but true IMO....LOL.

As a younger man, my listening focused on the top and bottom of the frequency range, always pleased by HF 'air', 'smoothness', and the overall definition that a well produced top octave could provide. Now that half that octave is gone forever, my brain tends to focus on the midrange.....as there's simple less...um....distractions? and my brain is now looking for intelligibility and less fatigue.

I suspect as an 'older' man in the years of retirement, listening will again shift in priority.....and I'm glad to not yet speculate on what that shift might be. For now, I'll enjoy my middle aged preferences while I still can......LOL.
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Old 21st June 2013, 02:29 PM   #196
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Yes, I think that's true to a large degree. Once you get to the point where you're talking about subjective differences, things get murky. And if you need double blind tests to determine which of two things is better - If it's that close, then both are pretty darn good. But I think we still have some objective, measureable facts we can look at, and I think it's worthwhile to talk about what differences are most audible. That's what the whole "baby and the bathwater" comment from the first post was about.

If we know that what's really important is smooth on-axis response and spectral balance of the reverberent field, then that can help dictate our design choices. If we know what directivity patterns work well in small rooms, that can really help guide our designs too. To build the best loudspeakers, we need to know what optimizations are most important to sound quality, e.g. on-axis response, shape of polars, change of directivity with frequency, etc. So I think it's useful to discuss what aspects are most important to optimize for high-quality uniform-directivity loudspeakers used for home hifi and home theater applications.
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Old 21st June 2013, 07:19 PM   #197
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Yes, I think that's true to a large degree. Once you get to the point where you're talking about subjective differences, things get murky. And if you need double blind tests to determine which of two things is better - If it's that close, then both are pretty darn good. But I think we still have some objective, measureable facts we can look at, and I think it's worthwhile to talk about what differences are most audible. That's what the whole "baby and the bathwater" comment from the first post was about.

If we know that what's really important is smooth on-axis response and spectral balance of the reverberent field, then that can help dictate our design choices. If we know what directivity patterns work well in small rooms, that can really help guide our designs too. To build the best loudspeakers, we need to know what optimizations are most important to sound quality, e.g. on-axis response, shape of polars, change of directivity with frequency, etc. So I think it's useful to discuss what aspects are most important to optimize for high-quality uniform-directivity loudspeakers used for home hifi and home theater applications.
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Old 21st June 2013, 07:37 PM   #198
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Most of these speakers with somewhat constant or slightly narrowing directivity for home use tend to be based around 90-ish once the control gets going, right? What do you folks think about going a little tighter?
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Old 21st June 2013, 09:31 PM   #199
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I actually always go a little tighter. My HF and MF profiles tend to be just shy of 90, in the 80 to 90 range.

This is only partially related, but I think it's a somewhat useful and interesting aside. The area expansion of a 90 trihedral corner (three-sided pyramid) is the same as a quadrahedral (four-sided) pyramid with 70 wall angle.

So for DI-matching (this time, talking index, not polars), a 90 axisymmetric horn is not matched with the 90 expansion of a room corner. For DI-matching, one would want a narrower pattern.

In truth though, I think this is almost irrelevant. I agree with Dave Smith, that what we really care about is a near-match in the polars, and not so much the directivity index.

But what I do think is important is that the pattern not be too wide. Ideally, I like 80 to 90 horizontal beamwidth and 40 to 60 vertical. Since beamwidth is defined as the -6dB angle, we're really not talking about a hude attenuation at the edge of the "beam". Even if the stated beamwidth is 70 (+/-35), out at 45 it's only about 2dB lower than a device that generates a 90 beam. So I'm pretty happy with 70 - 90 horizontal beamwidth, and that puts my design target around 80 or 85. By happy coincidence, conical horns and waveguides are often smoother with a slightly narrower profile too.

Beamwidth being just under 90 is especially useful for dealing with the ipsilateral reflection, when combined with toe-in. So I like the MF/HF beamwidth to be slightly narrower than 90 for constant directivity cornerhorns or matched-directivity two-way speakers using the (heavy toe-in) crossed-axes placement method. I think we all talk about 90 flares, but that's mostly as a quarter-sphere approximation. In truth, I think it's useful to be a smidge under that.
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Old 24th June 2013, 11:59 PM   #200
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I was actually thinking pretty much along the lines of your "interesting aside" there - been wondering if a 60x60 or 60x40 -6dB might be worth considering in a lot of home situations. Brings up a question of how best to taper from omni, I suppose, but it seems like that's still a somewhat open question for 90 too.

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