I've tried and measured all kinds of products with my MartinLogan set, and my conclusions based on setting up the most accurate multichannel audio / movie config was as follows:
- Skyline diffusion (reflect highs in a randomized vertical and horizontal plane) should never be used behind a planar, as you just wind up spraying sound off all surfaces. This is the antithesis of controlled directivity, and destroys many of the benefits of a linesource speaker
- 1D diffusors, like the RealTraps Abfusor (reflects in the horizontal plane) work much better than a flat wall at 'spreading out' the rear wave, and limiting the amount reflected back at the speaker (more of an issue with ESLs than maggies)
That said, for front speakers, neither is recommended IMHO. As all that out-of-phase energy is being redirected into the room, causing comb filtering and affecting the imaging in substantial ways. It might sound 'better' than the truly awful sound of an untreated wall, but it is still sub-optimal.
For instance, I use the Abfusors directly behind my rear speakers to generate a wide, diffuse soundfield which is appropriate for rear speakers doing movies that more than 1 person is listening to.
Now, I do use 4 Skyline diffusers on the rear wall of the room, these are 6’ away from the nearest listener and provide a nice ‘envelopment’ in stereo modes, as the reflections from the rear of the room are not as localizable.
For front dipolar speakers, I have only one recommendation: Absorption, lot's of it.
My favorite product for that is the Realtraps Minitrap HF, this model fully absorbs high frequencies as it does not have a reflective element in it, yet still absorbs well into the mid-bass. The damped mid-bass reduces the cancellation normally encountered in that range, really improving the impact planars can have, and the high frequency absorption mitigates the comb-filtering and the image smearing.
I covered my entire front wall with MiniTraps, and my sidewalls behind the speakers (and actually all along the next 14' of side wall) are also absorptive.
See my
thread about acoustic treatments over on the ML forum for more details, and if you follow a link to the original acoustics thread, you can see plenty of actual impulse response measurements made demonstrating the various impacts of bare wall, 1D diffusion, absorption, etc. It was quite the epiphany to discover the significant impact managing the rear wave a dipole can have.
The final result is extremely satisfying, with a very wide soundstage and pin-point imaging in multichannel audio. It's often counter-intuitive for planar owners to consider significant dampening, but when done right, you can get impressive results.
