As a follow up to the Quadratic diffuser placement video I made recently I was asked whether six-foot from the listening position or six-foot from the sound source was the ideal position. Well the diffuser is a tool to minimize reflections at the listening position, by definition that’s what it is. So that said the listening position is our central focus when we consider how many diffusers to use, where to place them and what particular prime number for them to be at.

There are three variables we have to look at, there’s more but we’ve got to have those three satisfied or it doesn’t make any sense to go to the rest because each successive requirement builds on those three. So bottom line is we have to assess the distance first from the listening position to the intended diffuser installation. What wall surface are we putting it on? Let’s for example site the rear wall.

Let’s assume our listening position is six feet from the rear wall as that’s a good starting number. Now that said what’s the highest number of prime we can use? Let’s assume we’ve got plenty of space. What’s the highest number prime we can get on that rear wall knowing that we’re six feet away from the listening position? The prime number sequence dictates that, the lowest frequency in the diffuser tells us we need so much distance.

So I think from past videos we use the QD 13 as the example. The low frequency on a QD 13 is two hundred seventy-five cycles which is about a four foot wave. Divide eleven thirty-two by two seventy five, let’s call it four feet rough numbers and then in our practice, in our design, in our recommendations we always add another fifty percent to that distance for certainty.

Add 50% to the diffuser distance

That’s a subjective call on my part based on all the experience that 25-years I’ve had with diffusion and so far it has served us really well. So we have four plus two so we have six feet. So prime thirteen would be a good diffusion sequence to use for that six foot distance.

Now let’s take that information and go one step farther. The QD-13 is twelve and a half inches deep. So you have twelve and a half inches to give up on the rear wall? Because you’ve got to give up that twelve and a half then you’ve got to have six more feet to the listening position. So we’re asking for seven feet from the rear wall. Do you have it?

These are issues that you have to calculate and figure out. So you could go with an eleven because that’s less, you could go with the seven but depending on room usage you may not want either one of those. So there’s a lot of variables that have to be considered but just looking at that distance example, six-feet is a good distance for prime thirteen quadratic residue diffuser.

In Summary

I hope that explanation on positioning helped. Our QD 13 is available here, our QD 11 kit is available here and our QD 7 kit is available here. If you would like me to take a look at your room layout and advise the correct quadratic diffuser placement in your room then please fill in this form and I’ll be happy to advise


Dennis Foley

Author Dennis Foley

I am an acoustic engineer with over 30 years’ 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

  • Steve Hege says:

    Hi Dennis:
    On my rear wall, I have steps that are 3′ wide that go up about 28″ high. I’m wanting to put a couple of QRD-11’s on the rear wall, directly behind my listening chair. Due to the steps, this will place them 28″ above floor height. How will this affect the diffusion?

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