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Old 7th January 2013, 12:36 PM   #471
Rudolf is online now Rudolf  Germany
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Originally Posted by a_tewinkel View Post
The research of Bech indicates otherwise. Strong reflections < 5 ms are harmful indeed. But reflections from the front wall, floor and ceiling can cause audible colourations because of comb filtering.
A friend of mine had tried to make the reverberation time of his room quite low and as frequency independent as possible. On the way to that goal he had to get rid of many reflections in his room. When he had achieved his goal, the first floor reflection stood out more colouring than ever before.
Why? Because he had lost all the other (later) comb filters from the side and rear walls, ceiling etc., which would have (to a certain degree) compensated the influence of the floor reflection.
There should be only two direct front wall reflections from a dipole (one at probably ~1/3 of the front width and one from the room corner). Absorb those above 1 kHz and let the rest of the wall in peace.
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Old 7th January 2013, 01:26 PM   #472
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Yes, that is possible. Reflections can cause colouration but they may also reduce the audibility of a colouration. It's complex and different for every speaker, room and layout.
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Old 7th January 2013, 01:50 PM   #473
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Originally Posted by Rudolf View Post
I believe your findings are exactly in line with my personal experience and what Toole found (fig. 5.2):
Click the image to open in full size.
Image shift and source broadening is nothing you repair at 6 kHz plus.
My dipole tweeters come in at 2 kHz though.

Rudolf
The tweeters on the NaO II come in at 2.2k. Turning the rear tweeter on and off makes a significant difference. They actually sound rather dull without the rear tweeter, except on some rock recordings. With the Note II the difference is insginificant at best, in my room. FWIF, my room's RT60 is about 350 msec from 250 to 1k Hz. 300 msec at 2k, 288 at 4k and 250 at 8k.
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Old 7th January 2013, 09:42 PM   #474
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I appreciate Toole's knowledge and experience, but David Griesinger of Lexicon suggests that envelopment actually goes all the way down in frequency. From about 1kHZ on down you've got inter-aural crosstalk as well as the room modes damaging the stereo effect and image location (apparent separation). The question might be what to do about that. One of the Griesinger papers shows a circuit that cancels L+R more and more as you go down in frequency below 800HZ (or 300HZ - it's switchable). I'm using that (frequency selective L-XR), the inter-aural crosstalk cancellation method (modified Carver for my main system and Polk method on my stereo soundbar), and aiming my woofers toward the side walls rather than directly toward the sweetspot.
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Old 7th January 2013, 10:04 PM   #475
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Back in about 1985 I noticed that the top of the line Snell speaker (not OB) had a rear firing tweeter, that according to their literature only operated above about 10kHZ (if I remember correctly). I suspect that they tried several cutoff frequencies before settling on that one. This was a speaker that was calibrated to be flat +/- 1dB. With what I know today, 1kHZ - 6kHZ is all about amplitude comparisons for image location, and above 6kHZ horizontal image location becomes rather vague. Above 6kHZ is apprently responsible for up/down imaging cues, due to the shape of the ear and ear canal, 8kHZ allegely being a key frequency for that. Perhaps Snell wanted to put air in the high treble (cymbals and such) without messing with uppermid imaging (1kHZ - 6kHZ) or the up/down imaging around 8kHZ (?).

One of the people on this forum made the comment that the 1 inch domes coming in at 1.4kHZ in the Orion speaker were tedious to listen to over time. He seemed to blame it on the 1 inch dome. I figure it was because of the off axis response having a jump when the 8 inch driver got directional at 1.4kHZ. This issue would perhaps be doubled by the presence of the same thing happening on the back side of the Orion speaker. Going from an 8 inch to a 1 inch is never going to be optimal.

In my OB system I used a vertical line array of four 5 inch drivers instead of the 8 inch, to minimize this issue (and others). In the Orion, the off axis response is still a big issue even if you're directly in front of the speakers, since the room will react to it, and color the final perception. I think the Seas millenium 1 inch dome is very good sounding down to 1.4 kHZ (w. 4th order active crossover). I think the problem was that it picked up where an 8 inch left off.
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Old 8th January 2013, 08:43 AM   #476
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that's right, SL choose the 8" to prioritize the low mids register and avoid using two 5" or 6".

Note 1: Since these tests were done Seas introduced the W22EX001. Like the W18EX001 it has very low distortion, but with 1.75 times the cone area it can move more air. This driver is my first choice for any new open baffle speaker design

The LX521 solves both issues, directivity of the 8" and power of the 1", for the ones who were not totally happy with it.
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Old 8th January 2013, 10:43 AM   #477
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There is more to it... here's SEAS' own distortion measurement of the W22EX001, taken from the original datasheet. As you see, distortion rise significantly above 1 kHz.
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File Type: png w22ex-dist.png (36.7 KB, 395 views)
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Old 8th January 2013, 10:44 AM   #478
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The 8" is not the primary issue. It is the tweeter blooming that causes extra off-axis SPL and might become audible depending on the room and placement.

If the 8" would have been the main issue then he would have avoided it in the LX. But since the issue were the tweeters, it is solved by the 4" on a narrow baffle, which avoids blooming at the cost of an additional driver.
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Old 8th January 2013, 10:49 AM   #479
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And the Orion baffle is much too wide for good dipole polar pattern. Thats why the LX baffle is almost a no-baffle.
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Old 8th January 2013, 08:15 PM   #480
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Originally Posted by 6.283 View Post
The 8" is not the primary issue. It is the tweeter blooming that causes extra off-axis SPL and might become audible depending on the room and placement.

If the 8" would have been the main issue then he would have avoided it in the LX. But since the issue were the tweeters, it is solved by the 4" on a narrow baffle, which avoids blooming at the cost of an additional driver.
I'm not sure what you mean by "blooming"? What does that mean technically? If you mean that it causes a sudden increase in dispersion at 1.4kHZ, then we're saying the same thing (?).
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