Dipole speaker wall treatment (for Quad, Magnepan, Martin-Logan, etc)

Friends:

Next year I will relocate and create a dedicated listening room, and want to get the design right.

I have read two books on acoustics, scoured online forums and articles, and checked several websites of manufacturers (RPG, Real Traps, A/V Room Service, etc) finding mountains of information on acoustical treatment for conventional box speakers. But I have not found one bit of information regarding how to treat the wall BEHIND dipole ESL's.

The manual for my Quad 2905's merely suggests setting the units at least two feet away from that wall, with 5~20 degrees of toe-in.

Quad-hifi.UK informed me only that their engineers treat their test room front wall with the same egg-shaped foam used in their box speakers (!) - but did not suggest that for everyday listening - and steered me to yet another acoustical engineering book. (I suppose I could purchase a bunch of Quad box speakers and rip out the foam, but that solution seems a bit, ah, over-refined.)

Understanding that much of dipole spaciousness results from the the out-of-phase back-wave reflecting from the wall, I would not want to deaden that reflection too much. Nor would I expect wall treatment to improve imaging, but I don't want to further blur it. (Current setup is three feet from a brittle plaster wall built in 1913, and imaging is vague.)

I'm hoping some of you dipole owners can share results from experimentation with various wall treatments that absorb, reflect or diffuse that back-wave.

Thanks for any input.

SCC

1 Answer

  • In general, dipoles require less acoustical treatment than other speaker designs, in fact, the treatment that works for boxes tends to suffocate them. Most dipole owners prefer diffusion at the first reflection points behind the speakers -- that is, the points at which you see the speaker reflected in a mirror when you're sitting in the listening position. A QRD diffuser should work.

    Dipoles don't generally require treatment on the sidewalls, because they radiate less sound to the sides. You can experiment. No treatment on the floor or ceiling, though a carpet or rug is OK.

    Other things you'd want to consider are bass trapping -- dipoles are less likely to excite bass modes but they still do excite some, depending on orientation. Also whatever HF absorption you need to get a good balance. Rear wall absorption if you sit very close to it, maybe diffusion if you don't, plus whatever diffusion you need to get rid of slap echo.

    Since it's a custom room, remember that the first rule of amazing dipole listening is BIG! The image keeps getting more impressive until they're at least 15' off the front wall. But the minimum you want to aim for is 5'. The further out they are, the more depth you'll get.

    Front wall itself should be (aside from the diffusers) a flat plane. Maybe a bit rough, e.g., brick, but no real features, dipoles are like sonar. If you can, make the wall pretty wide as well, the sound will start to wrap in where it hits the side wall. It might be advantageous in that case to experiment with diffusion or absorption at the corner reflection, which is a second reflection (speaker to front wall to side wall to ears). With dipoles, this is generally more significant than the sidewall first reflections since it isn't in the dipole null.

    As with any room, go for a rectangle, non-parallel walls cause more trouble than they're worth. A custom room can have bass trapping built in very economically, I'd look into that. I'd prefer wood for the floor, build a raised one if you have concrete. Don't go for a cathedral ceiling or anything like, just a flat one, and don't go for a high one, line source dipoles work best if they touch both floor and ceiling. Just make sure it has enough height for Acoustats, which I think are the tallest speakers commonly seen (about 8').

    Finally, don't make the common mistake of putting your equipment racks between the speakers. Sometimes you can get away with this with dipoles if they're low, but remember the sonar effect -- they can interfere with the imaging of any speaker. They're better put on the side or at the rear of the room.

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