[Norman Varney]

There is often confusion regarding diffuser performance, and therefore, its placement within the space. In particular, number-theory diffusers are frequently placed where they cannot function as designed. Number theory diffusers include QRD, MLS and PRD, where non number-theory diffusers are called geometric diffusers and include polycylindrical, pyramidal, semi-spherical, etc. Either type may include 1 or 2 dimensional scattering off its surface and back into the room. In control (or playback) rooms, the problem lies when number theory type diffusers are located at room boundary surfaces which are perpendicular to the sound source and the sound receiver.

 

The way number theory diffusers work requires that the sound waves impinge on the device at or near normal incidence. The receiver must also be in the same relationship to the device. When the device is located, say 90 degrees or perpendicular to the source or receiver, it will cast a shadow upon itself, and sound waves cannot enter the wells. If the waves cannot enter, they cannot be scattered.  We are talking high frequency ray-acoustics here, not wave acoustics, for 99% of such diffusers- where the wavelengths are short. Therefore, in this range you can imagine sound propagating much like light does, where if you shine a flashlight directly at the diffuser, you can see the light reflecting back into your eyes, but as you start moving off-center, the scattering of light dies and is replaced by shadows cast by the device itself.

The ideal place for number-theory type diffusers is in front and behind the sound source, and the receiver. Placing them on side walls or the ceiling offers little value.

[Bob Katz]

Good to know, Norm. What about a series of diffusers along the back wall of a control room? Is there any advantage or disadvantage of using number theory diffusers in the line of incidence and switching to, say, polys at the sides?

[Norman Varney]

I’m going to cover the ‘mostly disadvantages’ of number theory diffusers over geometric. To answer your question, in general, either are fine on the back wall. Both require avoiding more than three repeats in order to eliminate lobing. There are a number of ways to do this, but it can be as simple as spacing groups of three.

[Bob Katz]

Norman’s written a blog about this subject, here

[Bob Katz]

Thanks, Norman.

[Bob Katz]

Norman’s out of town today and he sent me this great little image explanation of how a geometric diffuser works, comparing the physical acoustic wavelengths to the dimensions of the physical diffuser and how they interact in the real world.

[Emmanuel Sammut]

Hello, Norman and Bob, again very interesting information.

I have often heard that the use of diffusers has no interest in small rooms less than 50m3. Is it still a legend or if not can you give us the real reason.

THANKS

[Bob Katz]

Norman’s at a high fi convention. Your question is up his alley! Will be back soon to respond.

[Norman Varney]

There is no reason why diffusers  can’t apply to small rooms that I can think of. Again, it must be of appropriate type for the application. In small rooms adding diffusion can make the room seem larger. In future posts I’ll cover reasons to use diffusion for both performance spaces and playback rooms.

[Emmanuel Sammut]

Thank you for the information, waiting for the next article.

[Norman Varney]

Note how different wavelengths are affected by the size relationship of the device under test. In this instance we have a pyramidal diffuser that is about 12” wide. At 100Hz. the sound wave does not even see it and is totally unaffected because it is about 11.3 feet long. At 1kHz. (about 11.3 inches) it is affected by all faces of the pyramid. At 10kHz. (1.13”) it is only affected by the facing surface of the oncoming sound wave.

[Bob Katz]

Norman is still away from being able to post and he asked me to add this piece of useful info that helps to explain his diagram:

When it comes to absorption and diffusion products, it is important to understand their relationship to frequency wavelengths. Let’s look at a geometric diffuser and compare the physical frequency wavelengths to the physical device under test, and see how they interact in the world. Note how different wavelengths are affected by the size relationship of the device under test. In this instance we have a pyramidal diffuser that is about 12” wide. At 100Hz. the sound wave does not even see it and is totally unaffedcted because it is about 11.3 feet long. At 1kHz. (about 11.3 inches) it is affected by all faces of the pyramid. At 10kHz. (1.13”) it is only affected by the facing surface of the oncoming sound wave.