Today we’re going to talk about ideal acoustic diffuser placement positioning. I’ve been getting a lot of emails from people with questions about this and there appears to be some confusion so let’s look at some definitions and applications and I think we can get to the bottom of this. We always work with quadratic diffusion and everybody that looks at our website and talks to us through email and telephone realize that we like quadratic because it’s consistent, predictable and has a measured frequency response. So it’s a great acoustical tool but it’s widely misunderstood.

So with a quadratic diffuser we have a series of wells and each of these wells is based on one quarter wavelength rule and each of the well widths is half wave length rule so you get a tool that’s really versatile in distribution and frequency response. So we like this particular technology and support it tremendously because it’s a great tool to have in your acoustical palette if you will.

That said there are quadratic diffusers that need to be positioned correctly in order to achieve a sound field.

How many sound fields do we have?

We have three. Length, width and depth okay. So with a vertically positioned diffuser in your room, sound spreads out in a horizontal fan like array from the diffuser. So we get this horizontal spreading of sound with the diffuser.

There’s an inverse relationship to the position of the quadratic diffuser and the sound that it spreads out. A vertically positioned diffuser spreads sound out in a horizontal fan like array. A horizontal positioned diffuser spreads sound out in a vertical array. So we get two dimensions of sound by the way we position the diffusers and that’s the predictability and consistency of diffusion. Where to use what, positions, where in the room is a function of room usage.

What is your room usage?

Home theater, control and listening rooms are all a little bit different in their approach. Let’s take the control room and look at that. With a control room we have a couple issues that we need to address but the consistent issue that we need to address in our control room is always the rear wall. The rear wall is always an issue here.

So how are we going to deal with the rear wall? Well a popular method in today’s studios is to use diffusion and quadratic diffusion is a method of choice. So we know that with vertically positioned diffusers we will be spreading sound out in this horizontal dimension. Years back they used both vertically and horizontally positioned diffusers to give two dimensions of sound field. That kind of thinking now is gone by the wayside and I believe the two dimensions of diffusion add a little bit more confusion than we really want so it’s now vertically positioned diffusers with a horizontal array.

A home theater room and listening room are a little bit different.

With a home theater room, we have all these multiple sources. We have side, rear channels and all kinds of sound fields generated by the front channels. We have a whole set of sound field energy directed by the side channels and we have a whole series of energy directed by the rear channels. So each one of those sound fields has to have a particular technology employed to maximize those three sound fields and it’s always a combination of diffusion and absorption in a home theater room that achieves the best results. Diffusion on the rear wall and ceiling are common applications for quadratic diffusion in home theaters.

For critical listening rooms we employ a little bit of a different approach. Its more similar to the control room scenario than the home theater listening room but in a critical listening room we have our two channels and we have our seated position. Diffusion on the rear wall is popular, diffusion on the front wall is popular and then absorption technology on the side walls to minimize the time signature of the reflections.

What does diffusion on the front and rear wall do in a critical listening room?

It gets rid of, acoustically, in our brain, the boundaries. It makes the room appear larger to our brains. So diffusion takes away this reflection from the boundary surfaces so our brain can’t localize that there’s really a wall there. It’ll give you the impression that there’s more distance there. So diffusion on the front and rear wall in a listening room is advisable. That gives you more of a spacious feeling, more area if you will and diffusion also adds a lot more definition to the signal.

Poor diffusion is probably at ninety-nine percent of the rooms I see. When I put diffusion in it and the customer hears the difference they wondered how they lived without it.

So how to position a diffuser?

It really depends on your room usage. Is it home theater, control or listening room? Each has its own special applications because each sound field that you’re trying to create is different. Vertical and horizontal diffusion placement with quadratic is a great way to achieve it. You also have to keep in mind the diffuser to speaker distance.

You have to realize that the prime number sequence that you choose for your diffusers, has enough space from the seated distance for the wave form to fully form before it reaches the listening position and how do we determine that? Well we went over that in past videos but a good summary is take the lowest frequency of the diffuser and its quarter wave length, take it times four and add fifty percent of that distance. If you have that distance then you can use that particular prime number sequence.

We have seven, eleven, thirteen, seventeen, nineteen so as you go higher up the scale you need more distance between the diffuser and the listening position. This is what confuses some people. It’s the listening position distance from the diffuser to the listening position that’s critical. You must have a certain amount of distance for the frequencies to fully form with the diffuser.

In Summary

So if you have any questions about that just send me your information to dennis@acousticfields.com and I’ll be more than happy to tell you what prime number of diffusion will work for that particular situation. We offer the seven, eleven and thirteen on our website. You can build it yourself provided you have the proper distance. Just drop me a line or call me on 520โ€“392โ€“9486 and I’ll be more than happy to tell you the correct distances. Or complete the free room analysis form here and I’ll take a look for you.

Thank you
Dennis

Dennis Foley

Author Dennis Foley

I am an acoustic engineer with over 30 yearsโ€™ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

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Join the discussion 2 Comments

  • Tony says:

    Hi Dennis, with your diffuser – the first one I saw looked like the depth varied across the reflection board ( like a convex instead of flat across).
    I had a vision issue ! But would this cover more frequencies or not?
    It puts the quarter rule behind I am guessing, but have probably seen similar on plastic crap speaker horns…pls let me know what you think.

    • T, The frequency response of a diffuser is determined by a formula too complex for discussion in this format. The depth of each well is at a quarter wavelength and the width of each well is at a half wavelength.

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