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Old 31st January 2013, 08:19 AM   #1
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Default Dipole front wall reflection EQ

Not everybody can afford placing dipole speakers far enough from the front wall. Mine need to stay 60-70 cm close. An speaker with free-field linear frequency response will now show an obvious, rather broad and clearly audible peak at the wavelength that is 4 times the distance to wall - in my case, right in the midbass at ~140Hz . I am sure I'm not the only one having this issue

Options:
1. absorb the rear wave - not very practical or aesthetic
2. toe-in the dipole: it helps a bit, but not quite that much, because even at 45 degrees there is still significant output at the back due to the small distance to the wall
3. EQ the "hump": that's what I ended up with and so far did not see any issue.

Do you see any issues with option 3 ? Does anyone use it as well ?

I am actually thinking if in such cases it would make more sense to target a flat response of the SPEAKERS+WALL ensemble rather than speakers alone. One could use room EQ as in some newer receivers (Audissey EQ)
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Old 31st January 2013, 08:36 AM   #2
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4. Convert your dipole to a wide-baffled closed box? I'm a fan of solving problems acoustically.
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Old 31st January 2013, 09:13 AM   #3
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by bzfcocon View Post
Do you see any issues with option 3 ? Does anyone use it as well ?
I am actually thinking if in such cases it would make more sense to target a flat response of the SPEAKERS+WALL ensemble rather than speakers alone.
At 140 Hz and your small wall distance, you can't hear the speaker alone. It will always be the combination of speaker and early reflection. So it is perfectly ok to EQ for a flat response at the listening position.
I hope the speaker distance to the side wall is not in the 50-80 cm region.
While the midbass peak can be equalized, you are stuck with the early (4-5 ms) reflection throughout the complete frequency range. This will result in imaging problems in the 0,5-5 kHz region. What are you intending to do about that?

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Old 31st January 2013, 09:52 AM   #4
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Originally Posted by a_tewinkel View Post
4. Convert your dipole to a wide-baffled closed box? I'm a fan of solving problems acoustically.
That's not exactly a solution, rather a replacement of the problem and the introduction of others

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Originally Posted by Rudolf View Post
At 140 Hz and your small wall distance, you can't hear the speaker alone. It will always be the combination of speaker and early reflection. So it is perfectly ok to EQ for a flat response at the listening position.
I thought so too. Thanks !

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Originally Posted by Rudolf View Post
I hope the speaker distance to the side wall is not in the 50-80 cm region.
Yes, there's significantly more space in lateral, some 1.5 m (they're on the long side of the room)

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Originally Posted by Rudolf View Post
While the midbass peak can be equalized, you are stuck with the early (4-5 ms) reflection throughout the complete frequency range. This will result in imaging problems in the 0,5-5 kHz region. What are you intending to do about that?
Now that's an interesting topic.
As you say and according to the theory (Toole's book) there should be definitely some imaging shifts due to the front wall reflections. However, I subjectively find the imaging of the speakers very good to my taste:
- the center image is VERY well defined. This actually make sense, because the front wall reflections should cause an image shift TOWARDS the center.
- the sound stage is broad, stable and well defined in the whole range between the 2 speakers. This could be related to the particular polar response of my mid-high panels, which are a 2D array of 3.3" fullranges crossed over to a Neo 3 PDR. This makes them rather directional even above the dipole peak (>~500 Hz).
- the only impairment I could observe so far was the "depth" dimension: if I want depth, I need to roll-off everything > 5Hz at the back side. Which I do, being helped by the nature of the small fullrange drivers which roll off quickly on the back.

So I guess that the imaging could be "wrong" in terms of absolute source positions compared to, say, the same material on 2 normal speakers in anechoic conditions. But OTOH, imaging is something rather subjective and the absolute positions of sound sources are not really that important.
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Old 31st January 2013, 09:53 AM   #5
lolo is offline lolo  France
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+1. Last week end I had a real epiphany. I pushed the dipoles as far out in the room I could, 2.5m from the front wall and 1m away from the side walls ( I managed to keep 3m separation). Well, blown away is certainly an understatement . The rear wave was somewhat absorbed in the highs, but too much to my taste, it lacked "air" and "sparkle" on cymbals for ex (I know, how original is that!).
I would just not put a dipole in a small room, a small omni, nearfield, sounds way better to my ears.

Last edited by lolo; 31st January 2013 at 09:57 AM.
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Old 31st January 2013, 12:58 PM   #6
Rudolf is offline Rudolf  Germany
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Quote:
Originally Posted by bzfcocon View Post
However, I subjectively find the imaging of the speakers very good to my taste:
If, and how early reflections interfere with imaging will highly depend on the local situation - listening geometry, front wall condition etc. What I experience (without proper attenuation) is this:
front wall refl.gif
Phantom sources near left and right from the reflection point tend to be drawn to that point. The amount of draw can change with frequency.
Best strategy would be to keep everything far from the walls, as lolo suggests. Does he live in one of those beautiful spacious Loire chateaux?

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Old 31st January 2013, 01:37 PM   #7
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Well, your picture suggests two strategies:

- put speakers as far to the walls as possible (the obvious choice if you have Lolo's castle )

- second best: put speakers as _near_ to the wall as you can so you can still call it a dipole: in this case, the reflection point is very close to the speaker itself and the shift will be reduced. EQ might be needed again and there's some combing unless care is taken to absorb part of the back wave.
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Old 31st January 2013, 01:58 PM   #8
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I think people tend to forget that monopoles radiate as much to the rear as dipoles up past the mid-bass. If you are getting reflections at 140hz with a dipole, you also will from a monopole.
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Old 31st January 2013, 02:23 PM   #9
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Originally Posted by sfdoddsy View Post
I think people tend to forget that monopoles radiate as much to the rear as dipoles up past the mid-bass. If you are getting reflections at 140hz with a dipole, you also will from a monopole.
That's true, but instead of a peak, which is quite disturbing, with monopoles you'll get a dip, which is likely less audible and will make your speakers sound as if they had more deep bass.

How about a setup like this: L-wings from speakers to the wall on the inner side ? This would make sure no reflection occurs on the inner part of the front wall, at the expense of an asymmetric dipole response.

Anyone tried this ?
Attached Images
File Type: png Lwings.png (3.6 KB, 129 views)
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Old 31st January 2013, 03:08 PM   #10
lolo is offline lolo  France
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Nah, I don't have a castle dear Lords, sorry to dissapoint!

the room is 7.3x5.2m, but the open kitchen kicks in a 4.5m, otherwise I would have tried to push them even further!

Sure, omnis will give more reflexions, but you can listen to them really close, like 1m away or even closer, and then manage a larger ITD gap, that's the key!

I recently built an el-cheapo Demokrit speaker, they image better than the dipoles with much, much less room treatment. Sure, the SPL is not the same..nor do they reach deap bass but hey, they are dead fun to listen too! Very close only though..
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