whatever sounds best in the room. Ive had around 20 or so open baffle designs both planer magnetic and cone type. Seems they ALL were best at between 4 and 5 feet from wall. They all sounded less convincing at less than 4 feet( mid range color and spatial issues) and at more than 5 feet didnt seem to gain anything.Never went more than about 6 feet so cannot say BUT there may be advantage in bass at large distances.
I see two issues.1- The bass cancellation due to wall distance
2- mid range coloration and less depth if too close to wall
The bass cancellation thing tends to happen around 120 hz give or take and with some playing around with position you find the best compromise. Sometimes I hear people say that dipoles dont sound dynamic. I suspect some of this is the suckout around 120hz that gives that impression
I see two issues.1- The bass cancellation due to wall distance
2- mid range coloration and less depth if too close to wall
The bass cancellation thing tends to happen around 120 hz give or take and with some playing around with position you find the best compromise. Sometimes I hear people say that dipoles dont sound dynamic. I suspect some of this is the suckout around 120hz that gives that impression
Did you mean "minimum" distance? There isn't a maximum distance (or you would not encounter one in a residential interior space).
The rule of thumb is that you want the reflected sound to have a delay of at least 6 milliseconds (0.006 seconds) compared to the direct sound. This minimum delay time comes from psychoacoustic/perception studies into how the brain interprets (e.g. "hears") sound. I won't get into it here, but it is a very interesting subject.
Given that the speed of sound is roughly 345 meters per second, you need the reflected sound to travel at least a distance of 2.1 meters farther than the direct sound. Assuming your listening position is in FRONT of the speaker and the wall/boundary is BEHIND the speaker, the rearward radiation first travels towards the rear wall, reflects off of it, and then travels in the forward direction back towards the listener. Since it traverses the distance between the speaker and the wall twice, the distance only needs to be half of the required pathlength difference.
Thus it is advised that the distance between the dipole loudspeaker and the rear wall be 1 meter MINIMUM.
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Hi Charlie, thanks for your answer, yes I did mean maximum. I'm aware of the requirements for a minimum distance. I was just wondering if the 3D imaging and spaciousness could be impaired by having too long a delay. Is there a sweet distance as it were? 
BTW, I have open baffle wideband and separate U frame woofers
BTW, I have open baffle wideband and separate U frame woofers
Hi Charlie, thanks for your answer, yes I did mean maximum. I'm aware of the requirements for a minimum distance. I was just wondering if the 3D imaging and spaciousness could be impaired by having too long a delay. Is there a sweet distance as it were?
BTW, I have open baffle wideband and separate U frame woofers
OK, I see. Thanks for clarifying.
There probably is not a hard and fast "maximum" distance like there is for the minimum distance. But let's think about it here a bit. Some of the acoustic benefit of an open baffle system provides arises from the reflected sounds. The farther you place the OB from the boundaries (e.g. walls) the longer the delay AND the weaker the sound pressure will be when it finally reaches the listener. Making the distance "too far" will weaken the reflections to the point that they are much lower SPL that the direct sound and will become masked. Also, the brain is able to make out a repeat of the same sound with a delay of about 40-50 milliseconds or more IIRC as an ECHO whereas when the delay is less than that it becomes lumped together with the direct sound and together they provide a sense of "space". 50 milliseconds is about 17.2 meters, or a distance to the boundary of half that, e.g. 8.6 meters (more than 28 feet). To have a room in your house where you can put the loudspeaker that far away from boundaries, you would need a monstrously large room! So, practically speaking, I think you would not be able to reach this "maximum" distance.
As lowmass noted, locating the speaker around 1-1.5m from the back wall is likely near the optimum and will maximize the reflected sounds that your brain will enjoy!
Since I brought up the MINIMUM distance earlier in the thread, this web page discusses placement of dipole loudspeakers in a "small" room with that in mind. I will note it here for anyone interested:
Loudspeaker Placement in Small Rooms | Richard's Stuff
Example of interesting stuff from the linked page:
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Thanks, that puts my mind at rest somewhat, it had been bugging me, so long as it's within that time window and doesn't become perceived as and echo there's no problem. My room is quite small about 12 foot wide 15 foot long and I have the placement of the speakers and myself about right. That link you posted is interesting as it mentions the distance for lateral reflections being greater than off the front wall, is this yet another advantage of dipoles due to their reduced radiation in that direction?
Edit...ah I've just got to the bit where he says it's probably ok for dipoles!
Edit...ah I've just got to the bit where he says it's probably ok for dipoles!
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For the same physical distance to the side and rear wall, the reflection will arrive sooner from the side wall due to the actual total distance traveled to the listening position being shorter. It's just geometry. So you need to make the sidewall distance longer to compensate.
It's possible that the lateral reflections would arrive too soon (before 6msec) and be "louder" than rear wall reflections because the side walls are "closer" that the rear wall to the listening position. This might make them more problematic unless you can reduce the loudness and increase the delay somehow, and one way to do this is to use a wide room. Linkwitz, who has spearheaded much of this kind of research, has more of a long/narrow room than wide, as seen from his listening position.
Side wall reflections are just another reflection of the acoustic energy emitted by the loudspeaker. Those pretty point-source dipole models that show a true "null" at +/-90 degrees don't agree with real-world driver measurements that I have conducted, in which the SPL is only down about 10-12dB WRT the on-axis SPL. This is true even at low frequencies. It's a bit puzzling, but it seems like only a partial null is formed to the sides, so there is still some energy coming out that way.
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Yes, I've found that, as I walk between and behind my speakers the sound doesn't change as much as I thought it would. Also the idea off bass cancellation doesn't work so much in the real world, thankfully!
Even if there was a perfect "null" in the sound emitted at +/-90deg from a dipole, in a real room you would still hear sound. This is all the reflected sound, which still reaches you. Moving to the "null" does maximize the ratio of reflected to direct sound. I kind of like it there! I have even done some listening with the loudspeakers rotated OUT slightly more than 60 degrees, with my listening location where the two baffle planes intersect. Try it! I have a pretty wide listening room, and maybe that is why this "works".
Right, I'll give that a go. I've found my favourite position so far is speakers 6ft apart and about 5ft distant facing straight out into the room so I'm listening about 35-40 degrees off axis. As I mentioned they are wide bands so high frequencies are lacking a little to the rear, do you think there's any advantage in having a rear firing tweeter to balance out the radiation?
Supposedly, or according to what some other posters have said, once you reach about 5kHz the rear radiation is not all that important. It might mostly get adsorbed by the wall and other soft objects (room dependent). If you are using a single driver, or larger driver with whizzer, you may not have much rear radiation above 1kHz or 2kHz. It depends on the design of the basket, size of magnet, etc. I have started to measure my drivers in the rear horizontal plane so I can guesstimate what the power response will do when/if rear radiation is attenuated. It seems not to be consistent with size, e.g. a small 3" fullranger may crap out at 2kHz to the rear while a 6" driver might continue up to 5kHz or more. The former might have the rear mostly obstructed by the magnet, while the latter not. I just measure to the rear, then I know for sure.
A dipole acts as a dipole when in free space, strictly speaking the speaker doesn't need the wall at all. That said, your situation is much more complex than that.
Nice comments. Like Charlie said, most open baffle speakers have very stranges response laterally and on the rearside.
When I measured my speakers indoors, even REW/UMIK system was fooled by room reflections! REW picks the loudest signal even if it is delayed and shows that as response! Be careful when measuring 60 - 120 degrees laterally, check impulse response and move "0" time point if needed!
The magic of dipoles is in having that rear radiation, and it (all directions actually) should have same spectral balance to sound really good. Wall abrorption is also frequency dependent, this makes thing even more difficult. Rotating speakers is generally good, it reduces energy of early reflections with dipoles.
When I measured my speakers indoors, even REW/UMIK system was fooled by room reflections! REW picks the loudest signal even if it is delayed and shows that as response! Be careful when measuring 60 - 120 degrees laterally, check impulse response and move "0" time point if needed!
The magic of dipoles is in having that rear radiation, and it (all directions actually) should have same spectral balance to sound really good. Wall abrorption is also frequency dependent, this makes thing even more difficult. Rotating speakers is generally good, it reduces energy of early reflections with dipoles.
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Omnis are omnis, they have too much radiation sideways. But I think that they are second best!
The problem with monopoles and most "dipoles" is uneven dispersion. Bass will be omni/dipole up to say 500Hz, but upper mids and treble will loose in total power response/spl. This will definitely lead to problems to get the reverberant sound field's spectral balance to correlate with direct sound.
Highly directive speakers dominate with direct sound and I don't like that. Many others do.
The problem with monopoles and most "dipoles" is uneven dispersion. Bass will be omni/dipole up to say 500Hz, but upper mids and treble will loose in total power response/spl. This will definitely lead to problems to get the reverberant sound field's spectral balance to correlate with direct sound.
Highly directive speakers dominate with direct sound and I don't like that. Many others do.
I think most people want room involvement. It really becomes a question of where and when the room is involved. Narrower directive speakers can succeed here if the maximum reverberant field is achieved. It gives ambience/space, but it is delayed. It doesn't dominate but it is significant all the same.
Since a speaker doesn't 'fit' a room, it is also the best way to force a fit by filling the room the most evenly.
Dipole gives the user a quite effective way to control room reflections. Speaker distance and angulation/rotation in relation to front and side walls does change the delay and amplitude of early reflections. When listener positioning is added, the ratio of direct vs (late) reflected sound and also spectral balance can be adjusted to some degree.
This is not possible with in-wall speakers at all. Omnis and directivity-controlled monopoles don't react to these changes so dramatically. This phenemenom is generally considered to be a problem or disadvantage of panels/dipoles, but I think differently!
One important thing is that the listener must be at least 60cm, preferably more than 1 meter from back-wall! I would like to say that this is the most obvious fault at least here in Europe and Asia, based on photographs!
This is not possible with in-wall speakers at all. Omnis and directivity-controlled monopoles don't react to these changes so dramatically. This phenemenom is generally considered to be a problem or disadvantage of panels/dipoles, but I think differently!
One important thing is that the listener must be at least 60cm, preferably more than 1 meter from back-wall! I would like to say that this is the most obvious fault at least here in Europe and Asia, based on photographs!
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I have/do consider omnis. However, all the ones I've seen require an enclosure, I consider the lack of an enclosure to be another advantage of dipoles
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