Appropriate Diffuser Type vs. Location

Appropriate Diffuser Type vs. Location

By 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.

© Norman Varney

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.

© Norman Varney

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.

© Norman Varney

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.

To carry on this conversation with me, Bob Katz and rest of the Digido Fora team head over to

At your service,

Norman Varney

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