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Dipole speakers, subwoofers and that rear wall

I own modern quad dipole speakers (2912s). I’ve heard many stories about speaker position, but never something that rang as fully logical to me. I can imagine 3 choices:

 

1/ dipole pretty much against the wall, maybe slight toe-in. The reflecting sound will come quickly after the straight sound and might cancel out the direct wave

2/ dipole far from corner (I hear quad recommends 1.5m). Reflections will amplify the sound?

Both statements feel like they’re incomplete. Surely the frequency, or frequencieS being played matter a lot if the reflected sound is in phase (amplifies) or in antiphase (attenuates) the direct sound. I can imagine perfecting positioning for one frequency and its modes, but not for 20-20,000 hz full spectrum.

 

3/ Close the rear of the dipole or have sound-absorbing material behind the speaker

The third one seems somewhat more logical, since I can’t imagine a sinewave that’s being attenuated by a reflected wave being accurately-sine-y unless the reflection is exactly in counterphase with the frequency played.

But on the other hand, if I have an actual instrument that is somewhat reminiscent of an actual dipole (e.g. a snare drum pointing upward) will have similar reflections on the rear wall.

 

I guess it "feels" true that you don’t want to stuff a musician in a corner too much but I’m not sure if this will negatively impact his sound?

 

As for the second part, a proper subwoofer moves quite a bit of air, can that air damage a dipole eletrostatic speaker?

puntloos

Showing 2 responses by bdp24

bdp24's avatar
bdp24
9,510 posts
 

I can't let a discussion of dipole loudspeakers and subwoofers go without mentioning one combination of the two that is very unique: the dipole speaker of your choice mated with an OB/dipole sub.

Brian Ding of Rythmik Audio and Danny Richie of GR Research put their big brains together and created the world's only servo-feedback (all Rythmik subs are servos) OB/dipole subwoofer. Details on both company's website. THE sub for dipoles. 

bdp24's avatar
bdp24
9,510 posts
 

Sound travels at approximately one foot per millisecond. For two sounds to appear unrelated, they must be separated in time a minimum of 10 milliseconds (it’s a function of how our brain perceives and processes sound). For that reason it is recommended dipole planar loudspeakers be positioned a minimum of 5’ from the wall behind them. Why? The sound emanating from the rear of the planar travels the 5’ distance to the wall, is reflected off the wall and travels the 5’ back to the rear of the planar, 10 milliseconds after the same sound left the front of the planar. The rear wave therefore sounds like an event separate from the front wave; if the time difference is less than 10ms, the rear wave results in a smearing of the front wave.

But there’s more to the story: When the rear wave arrives back at the planar, if that wave and the front wave are out of phase with each other at any given frequency, the combination of the two may create a hole in the frequency response of the loudspeaker. Or they may combine to create a peak in response. We of course want neither of those to happen.

The way to prevent the rear wave from creating the above scenario---known as comb filtering---is to either absorb or diffuse the rear wave. Absorption needs no explanation, but it needs to be said that it may cause the sound heard in the room to be too "dead", lacking spaciousness, one reason for choosing a planar loudspeaker after all. A better tactic is to diffuse the rear wave; the random scattering of the rear wave prevents it from returning to the planar out of phase with the front wave, and keeps the sound produced in the room from becoming "dead" or lifeless.

 

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