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Old 19th November 2008, 12:02 AM   #101
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Quote:
Originally posted by Rybaudio

I can't tell even horizontally if the systems really behave like dipoles up into the upper end of the midrange, through the crossover point, and up into the HF, and if not, how they deviate.
For the Orion, with its >8.5" effective path separation (13" baffle width divided by 2, plus 2" depth), the mid looks like it stops acting like a dipole around 500Hz, judging from these two sources:

1) http://www.musicanddesign.com/Dipole...n_baffles.html (Fig 3)
2) Linkwitz's spl_max spreadsheet - I input 215mm path separation and arbitrarily chose frequencies until the dipole SPL was barely 6dB over the monopole SPL, corresponding to the dipole peak shown in fig 3 (d/w = 0.5 at the dipole peak).

Note that the low-pass for the mid is 1400Hz. Also, the average path separation is greater than ~8.5" as the vertical separation is much higher than the horizontal, meaning that true dipole behaviour should be restricted to... I don't know. Less than 500Hz.
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Old 19th November 2008, 12:52 AM   #102
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What you have written is essentially the motivation for me asking that question. I know that above about 500 Hz the system is not safely in the range where it will certainly behave like an ideal dipole. Below 500 Hz the theory can tell us to a large extent how it will behave. The question is above that what does it look like?

Of course it's not going to be the ideal dipole, but what exactly does the polar pattern look like? It'll have a blob in the front and a blob in the rear, but how do they deviate from the ideal dipole? Some of the simulations on the musicanddesign site have some predictions, but they're still only a pointsource on a baffle. This doesn't include the directivity of a cone source, the fact that the front and rear radiations are different (basket), possible breakup of the cone, an oddly shaped baffle, the crossover to the tweeter, etc. Also, it's not just a matter of polar pattern vs. frequency- it's how these are pieced together amplitude-wise to produce the radiated soundfield, a function of position and frequency. There are many variables here (not just the physical construction, but decisions made while tuning)... things that I can't determine from what I've seen. That's why I want to see FRs at various off-axis angles... doing these horizontally and vertically is IMO a minimum to really see the relevant behavior (at least the linear behavior) of the speaker.
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Old 19th November 2008, 01:40 AM   #103
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Quote:
Originally posted by Rybaudio



By the way, does anyone here have some useful measurements of the Orion and Nao speakers? I'm talking:

- decent frequency resolution (1/24 octave though possibly smoothed)
- 15 deg (or less) increments horizontally and vertically all the way around (since dipole)
- on a decent graph (40-50 dB scale, NOT 120 dB)

There's Peter Aczel's measurement (reprinted on SL's site) that seems fairly high resolution scan :

http://www.linkwitzlab.com/TAC-review.htm

It also includes 45 deg off axis measurement as well. Seeing how little deviates from on-axis one I'd imagine those at 15 and 30 deg would not differ much (if at all).
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Old 19th November 2008, 03:34 AM   #104
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I've seen those. The resolution and scale are ok, but they represent a very small sample of what the speaker does. Also, they aren't plotted on the same graph so it is kind of hard to compare them. It looks like after he took the measurements he normalized them to both be around 0 dB nominal... is that right or are they with the same drive level? Notice that there is more output in the 2-3k region in the second graph... does that mean it was actually louder at 2-3k at 45 degrees off axis or does it just look that way because the graph was shifted to be nominally 0 dB?

At best they tell you what the speaker is doing 0-45 degrees off-axis, horizontally, in the front hemisphere.

There is also some evidence that things are going to go to hell after 45 degrees... the midrange (due to dipole) and high end (directivity of tweeter) appear to be dropping down, but the low end of the tweeter (1.5-4k) does not (makes sense as a monopole). If the trend continues, at 60-90 degrees you'll have a rainbow shaped response with a huge bump 1-4k and nothing outside of that... not good, considering 60-90 degrees consititutes a significant part of the radiated sound field. One of the main reasons I want to see the measurements is to see if the second tweeter kills off some of the output of the front tweeter in this 60-90 degree range. When I played with dipoles a few years ago I found this to be a real problem and I wasn't ever able to get things quite right.
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Old 19th November 2008, 03:42 AM   #105
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Quote:
Originally posted by Rybaudio


Of course it's not going to be the ideal dipole, but what exactly does the polar pattern look like? It'll have a blob in the front and a blob in the rear, but how do they deviate from the ideal dipole?
SL himself on Orion's dipole polar response. Yes it's not an ideal dipole all the way to 20kHz.

http://linkwitzlab.com/orion-faq.htm#Q20
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Old 19th November 2008, 04:28 AM   #106
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Again, words (quite general, imprecise ones), not measurements. There is a huge amount of flexibility in how those words translate into reality. I could have written that description just by looking at the speaker.

I've also looked at the Pheonix measurements (they do show the issue I mentioned above)... much more comprehensive, but still he only shows the vertical within 15 degrees of on-axis, and nothing for the rear hemisphere.
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Old 19th November 2008, 07:21 AM   #107
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Not orions but something similar. This is 10in baffled in 32cm wide OB with angled circa 7cm wide wings. Little bit exaggerated scale - 3dB per line. Horizontal front polars 0-75 deg (7.5 deg steps)
Click the image to open in full size.Click the image to open in full size.

Front "power" response across 0-75 deg compared to 23deg. Unfortunately a bit misleading because of my mistake. I scaled and count with levels across the area and not with intensity. Trends are probably right, relative levels not.
Click the image to open in full size.Click the image to open in full size.

Dtto back (90-180 deg).
Click the image to open in full size.Click the image to open in full size.

Expected cone-basket resonance is well below 500Hz, dipole pattern is diminishing above 600Hz, overruled by increasing directivity.
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Old 19th November 2008, 07:54 AM   #108
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Thanks for adding that. The measurements show a lot of what I was experiencing when I played with dipoles... neither constant nor smooth directivity... here it's moderate at 400, wide 500-600, and then really narrow at 1k. Is there a way you can normalize the curves to say, an average of the first three (listening window) so we can see what the speaker would look like off-axis if the listening window was EQed flat?

For example, here is the raw half-space IB voltage response of a B&C 12PE32 in 10 degree increments from 0 to 80 degrees off-axis:
Attached Images
File Type: jpg 12pe32 raw.jpg (96.5 KB, 504 views)
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Old 19th November 2008, 07:58 AM   #109
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and here is relative to the listening window
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File Type: jpg 12pe32 relative to lw.jpg (99.8 KB, 491 views)
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Old 19th November 2008, 08:02 AM   #110
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Pardon the frequency scale... as far as I can tell, if you use a log scale in Excel, the limits have to be powers of 10. Does anyone know how to get around this?

Anyway, for comparison with the dipole speaker from 100-1k Hz, here is the 12PE32 relative to the listening window, using the same amplitude scale (30 dB). This is a half-space measurement, but in a sealed box the response in full space is not much different. It certainly is a lot smoother directivity-wise (though it does increase in directivity at the top end of it's bandwidth) than any dipole setup I've ever measured.
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File Type: jpg 12pe32 relative to lw2.jpg (63.7 KB, 496 views)
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