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Old 7th January 2013, 08:48 AM   #461
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True - if the rear wave is absorbed (or diffused), a dipole radiates much less energy into the room than a monopole. And more importantly - the energy is far more linear across the frequency range.
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Old 7th January 2013, 09:00 AM   #462
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True - if the rear wave is absorbed (or diffused), a dipole radiates much less energy into the room than a monopole. And more importantly - the energy is far more linear across the frequency range.
The interaction with the room is certainly different but I'm not sure if it's generally better. As soon as wavelengths reach room dimensions a dipole stops working as a dipole. The behavior is probably better described as two "out of phase" monopoles with virtually no distance between them. Is such a configuration generally better at delivering low frequency large room behavior that might be preserved in certain recordings?

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Old 7th January 2013, 10:11 AM   #463
lolo is offline lolo  France
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here you go, again.. I thought we were talking of freq 1khz up this time? With your definition I allegedly get "true" dipole behaviour down to 45hz, not too bad I would say?
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Old 7th January 2013, 10:23 AM   #464
Elias is offline Elias  Finland
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As soon as wavelengths reach room dimensions a dipole stops working as a dipole. The behavior is probably better described as two "out of phase" monopoles with virtually no distance between them.
Two opposite phase monopoles with infinitesimally small distance between them is called ideal dipole, by the way.

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Old 7th January 2013, 10:25 AM   #465
Rudolf is offline Rudolf  Germany
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also remember the rear wave cannot really be used "as such", it has to be diffracted or absorbed.
This is too much generalization for me.
The rear wave is as good as any other wave direction. What we really don't want is a few specific reflections: the ones reaching the ears too early. And in those reflections we only need to eliminate those frequencies which give wrong clues. The reflection points and frequencies in question are easily detectable. It is probably less than 5 % of the dipole energy reflected from the front wall which can be called harmfull at all.
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Originally Posted by StigErik View Post
True - if the rear wave is absorbed (or diffused), a dipole radiates much less energy into the room than a monopole.
Even with the rear wave included a true dipole will radiate much less energy than the monopole.
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And more importantly - the energy is far more linear across the frequency range.
Which allows us - and should encourage us - to eliminate only those small portions of energy in selected frequency bands which really do give bad spatial clues.

To come back to the LX521: SL has to consider that many builders of a LX521 will not have the ability or means to find and eliminate bad front wall reflections. For those people it might make sense to omit the rear tweeter completely.

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Old 7th January 2013, 10:37 AM   #466
lolo is offline lolo  France
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Rudolf, you may well be right. (as usual.. ) but could you be more specific? What freq are you talking about?
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Old 7th January 2013, 11:24 AM   #467
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This is too much generalization for me.
The rear wave is as good as any other wave direction. What we really don't want is a few specific reflections: the ones reaching the ears too early
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. For lateral reflections, the IACC is lowered compared to frontal reflections (there is a larger difference between the 'comb' at both ears) and the comb filtering is more easily 'filtered out'. Lateral reflections are less likely to cause colouration and more likely to add spaciousness.
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Old 7th January 2013, 11:33 AM   #468
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As soon as wavelengths reach room dimensions a dipole stops working as a dipole. The behavior is probably better described as two "out of phase" monopoles with virtually no distance between them.
This is certainly true but I would not suggest that for a typical dipole woofer (or U-frame cardioid) that the distance is virtually zero. I've done a number of simulation and measurements that clearly show that with the 18" separation between front and rear of my Note dipole woofers the response of each source is. One in the modal region, dipole or otherwise, woofers behave as multiple sources with varying phase (and perhaps amplitude). Additionally, it is often said that dipole are better because they don't excite modes perpendicular to the dipole axis (provided the dipole axis is aligned with a room axis). However, with a stereo pair of woofers it should be realized that the dipole can excite the same front to back axial modes as a monopole pair and in the cross room direction, with symmetric placement, the monopoles will excite odd order modes to the same degree but with opposite phase. Additionally careful placement can also prevent excitement of at least the 2nd order mode in the cross room direction.

The most simplistic way to look at woofer systems is to recognize that in the modal region the response at a listening position depends on 1) the near filed response of the source(s), the position of the listener and source(s), and the room transfer function between each source and the listener. Free field response means nothing.
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Old 7th January 2013, 11:41 AM   #469
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Originally Posted by Rudolf View Post
To come back to the LX521: SL has to consider that many builders of a LX521 will not have the ability or means to find and eliminate bad front wall reflections. For those people it might make sense to omit the rear tweeter completely.

Rudolf
When I put the Note II together I initially omitted the rear tweeter because I could not really hear a difference with or without it. I only decided to keep it at the last minute because if it should be a problem in some rooms it's easier to turn it off than to add it later. My tweeters come in about 6k Hz.
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Old 7th January 2013, 12:56 PM   #470
Rudolf is offline Rudolf  Germany
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Originally Posted by john k... View Post
When I put the Note II together I initially omitted the rear tweeter because I could not really hear a difference with or without it. I only decided to keep it at the last minute because if it should be a problem in some rooms it's easier to turn it off than to add it later. My tweeters come in about 6k Hz.
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.

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