Skyline Diffusers Versus QRD Diffusers for Planar Dipole Loudspeakers

TitaniumTroy said:

One train of thought is you want to just diffuse in the horizontal plane

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That's my thinking too. Also, to be clear, 1D and 2D diffusers are both QRD types.

--Ethan

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.

We should remember that diffusion/absorption requirements are different for a multichannel audio and stereo. When I had SoundLabs for stereo the best solution in my room was having RPG abffusors behind them and RPG diffractals in the remaining of the front wall.

While I see absorption as a must for multichannel, I'd argue that it is quite applicable to stereo as well.

Some 2ch SACDs have wonderfully precise soundstages on my current setup, yet I recall those recordings being more 'diffuse' and less focused before the front wall was totally treated.

But I do understand people wanting that huge soundfield a well set up panel speaker can throw. It's just I'm coming around to viewing dipole rear-wave management as a bug, not a feature.

JonFo said:

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

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That's overly categorical. Much depends on whether the floor is carpeted or hardwood, how high the ceiling is and how far the speakers are from the back wall. As with most things in acoustics the answer is that "it depends".

If your speakers are relatively close (4-5ft) to the rear wall something like a RPG BAD Arc panel which is a combination absorber / diffuser is a very good solution. Further away diffusers can work well, either 1D or 2D depending on the desired effect and room configuration.

I was about to start a thread on this very topic, but then I found this existing thread.

My understanding is that QRD diffusers diffuse either in the horizontal plane or in the vertical plane, but not in both planes. My understanding is that skyline diffusers diffuse in both the horizontal plane and in the vertical plane.

What are the pros and cons of these two types of diffusers on the front wall behind planar dipole loudspeakers?

What is your personal experience on this matter in your own system?

TitaniumTroy said:

Anyone had experience using Skyline type diffusors vs QRD diffusors. In regards to taming the back wave of a planer speaker, if so what were the results? I have Magnepan 3.6's with the rear wave firing into the corners of my room. Currently I have three QRD, six inch deep diffusors in each corner, it really improved the imaging overall and vocals, but was curious about the 2D Skyline type of diffusor, for taming the back wave.

One train of thought is you want to just diffuse in the horizontal plane, with planers. Others think it should not matter, and that a 2D diffusor has the advantage of being more efficient.

Troy

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Which did you end up using, Troy?