Can you explain more about an ideal QD diffuser well depth

In a recent hangout I was asked “I have two polystyrene diffusers which I took from a studio that closed. How can I determine the lowest frequency that I can place it at, the right distance? Now I think you answered this in the video about sound diffuser placement but again I can’t quite understand what you say in there. Are you saying depth of the well is one quarter of the wavelength through the wave it refers to and width half of the wavelength of the wave it refers to? But (I’m) all well depths with the same width, if that makes sense.”

It does and yes all well widths are the same with quadratic diffusers. He mentioned a diffuser by the material it’s made from. Polystyrene, that doesn’t tell us what kind of diffuser it is. Is it a one-dimensional diffuser? Is it a two-dimensional diffuser? Is it a quadratic diffuser? Is it a sound-redirection device and not a diffuser? If it has wells and it’s quadratic we know the wells are a quarter wavelength so whatever the dimension of the deepest well is or whatever that dimension is, multiply it by 4 to get the lowest frequency that the diffuser will diffuse.

The well width has to do with the higher frequencies but we won’t go into that now. I should probably do a video on how the vertical and the horizontal wells work together. But let’s just stay focused on the depth and so if it’s quadratic, just take the deepest dimension and multiply it by 4 and that’s your lowest frequency that the diffuser will work at. So that’s what you need to do and then adjust the room and plan accordingly for the listener to diffuser distance.