Making a PRD Diffuser

[Miguemiguemigue]

Hi! I have made a PRD Difusser (I've seen some posts about this type of diffuser) and I only want to share the way I do, if that can be useful for someone.
I used an excellent calculator from this site http://www.oliverprime.com/prd/.
You can enter the desired data in the calculator and It generates a map with the division of the diffuser and the sizes of the wells.
I needed a wooden plank, wood strips, glue for wood and too much patience!! I share with you a video that I made with all the process:

http://www.youtube.com/watch?v=V9jbjIt2FzQ

I encourage anyone who can serve this type of diffuser, to make himself. This diffuser have his efficiency in 3 octaves (like QRD Difusser).This is a cheap solution and it can improve the quality of your control room (If you need to increase the TR60, or solve reflections problems).

Surely there is someone who knows more than me about this type of diffusers, never hurts to learn!
Regards.

[Soundman2020]

Hi "Miguemiguemigue". Welcome!

Thanks for the link. Very useful.

I encourage anyone who can serve this type of diffuser, to make himself.

What is the low cutoff frequency for the one you show in that picture? What are the dimensions of the room it is in? Are you aware that this type of diffuser is not suitable for small rooms, due to the lobing?

This is a cheap solution and it can improve the quality of your control room (If you need to increase the TR60, or solve reflections problems).

A diffuser cannot increase RT60 times, and small rooms don't actually have statistical reverberant fields, so RT60 is not a valid term for typical home studios.

In other words, that device is only suitable for large rooms, where it can be far enough away from the mix position and other critical listening positions as to not cause problems. For example, in the photo there is a sofa just a few centimeters away from the diffuser: it would be impossible to hear the mix accurately with a good sense of space, good definition, clarity, imaging and balance while sitting on that sofa. The lobing artifacts that close up would seriously distort the sound field, especially considering that the left and right ears of someone sitting on that sofa would be hearing entirely different timing and phasing, due to being so close to very different parts of the diffuser.

The sofa should be moved at least 3m forward, and maybe more, depending on the lower cutoff frequency.

So that device looks nice, and would work very well, provided that the room is large enough, and that the design philosophy of the room requires diffusion on the rear wall.


- Stuart -

[Miguemiguemigue]

What is the low cutoff frequency for the one you show in that picture? What are the dimensions of the room it is in? Are you aware that this type of diffuser is not suitable for small rooms, due to the lobing?

For this one the range is 850Hz - 3800Hz, this is the one I made. My room have a small dimensions 4x7x2.50, it´s a personal studio for mixing and sound design and I don't usually have company, so I'm afraid that my listening point is the only affected by the diffuser. Although this diffuser is apropriate for large rooms, can it be useful in a small room where only can be one listening point? I already knew that the listening must be at least the wavelenght of the minimum diffusion frequency, and I respect this fact.

A diffuser cannot increase RT60 times, and small rooms don't actually have statistical reverberant fields, so RT60 is not a valid term for typical home studios.

I didn´t know that RT60 cannot be applied to small room, What is the reason?
Thanks for the information!

[Soundman2020]

I already knew that the listening must be at least the wavelenght of the minimum diffusion frequency,

Actually, the listening distance must be a minimum of three times the wavelength of HALF the lowest diffusion frequency, and at least ten feet (roughly 3m) regardless. The reason for using half of the lowest frequency is because numeric-sequence based diffusers still exhibit scattering to approximately one octave below the cut-off frequency. In your case, with a cut-off of 850 Hz, there would still be scattering at 425 Hz, where the wavelength is 809cm. Three times that is about 2.4m, so the sofa and mix position should be at least 2.4 from the front face of the diffuser. But that is still less than the overall general rule of ten feet, so they should be at least 3m away.

Although this diffuser is apropriate for large rooms, can it be useful in a small room where only can be one listening point?

It can be useful, provided that the listening point is further away than the minimum distance, and provided that the overall philosophy of the room design requires diffusion. For example, in a classical (but outdated) LEDE room, that diffuser would be good on the back wall. But for the modern variations on RFZ design, where the front of the room is hard and the rear is absorptive, that diffuser would not be good. It would be no use at all in an MR philosophy, but might be good in a NE or even CID design. Etc. It all depends on the basic design philosophy, and also on what other treatment is already in the room.

I didn´t know that RT60 cannot be applied to small room, What is the reason?

Basically, in order to have a statistically valid reverberant field, the room needs to support several overlapping modes for each note on the keyboard. It is more complicated than that, but that's the general idea. Small rooms have very few modes in the low end of the spectrum, large gaps between them, and almost no overlap, so the Schroeder frequency is high. So there is no real reverberant field: just individual modes. And since RT60 measures the decay of the reverberant field in a room, it cannot be used if there is no reverberant field! So what most people are actually measuring in a small room, is simple modal decay, which is still useful, of course, but is not the same thing as true RT60. Rooms need to be pretty big before you can get a statistical reverberant field at all frequencies.

- Stuart -

[Deesubsee]

Basically, in order to have a statistically valid reverberant field, the room needs to support several overlapping modes for each note on the keyboard. It is more complicated than that, but that's the general idea. Small rooms have very few modes in the low end of the spectrum, large gaps between them, and almost no overlap, so the Schroeder frequency is high. So there is no real reverberant field: just individual modes. And since RT60 measures the decay of the reverberant field in a room, it cannot be used if there is no reverberant field! So what most people are actually measuring in a small room, is simple modal decay, which is still useful, of course, but is not the same thing as true RT60. Rooms need to be pretty big before you can get a statistical reverberant field at all frequencies.

- Stuart -

+1

So many people attempt to ignore the reality of the physics.