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Last week I was asked whether diffusion enhances stereo or makes it sound more like mono? The answer is no. Let’s define mono. Mono is a single source not stereo although I guess you could have two mono speakers. But let’s just take one speaker as mono stands for ‘one’. The question is implying that diffusion will focus things and you’ll get a real strong center image and good definition. Well that’s not what diffusion does.

That’s the job and responsibility of absorption and minimizing the time signature of reflections and getting that right ratio between direct energy from your speakers and reflected energy from your sidewalls and other surfaces of the room. There is a correct balance there for you, maybe not for me, maybe not for your buddy but for you.

Now we don’t have a big window to work with as it’s in milliseconds. It’s probably a ten-millisecond window but it can make a huge difference in the presentation you achieve in front of you between your two speakers. That’s why rate and level of absorption is so critical on those sidewall reflections and that’s why our foam is at the rate and level that it is, for that reason, namely those sidewall reflections.

You have to send them back to the listening position not only reduced in time but in the right amounts. So it’s the time and the amount of energy that the listener gets. It’s not a hundred percent, you don’t have to have a hundred percent absorption and technically destroy energy converting it to another form ‘heat’. That’s kind of destroying it as you ain’t getting it back.

So it’s the amount and the level and the rate that you send it back at the listening position that is important. Diffusion takes to definition and the mono focus and puts air around it and it’s really non-localizing in a way. So it’s the job of absorption to provide us with the definition, it’s the job of diffusion to add clarity to that definition. That’s a pretty good way to say it and diffusion will do that. What diffusion will do is take everything that you like about your musical presentation and magnify it. Now it will also take everything you don’t like and magnify it, it will not discriminate.

The job of diffusion is to add clarity to the definition

If your modal issues fall within the frequency response of the diffuser that ain’t good. That’s why the frequency response has to be adjusted and chosen for the room location which is based on dimensions and volume. It’s not easy as you have to consider a lot of things. Good sound is a lot of little things done right. And then at the end maybe you never get to the end, maybe you don’t but it’s the journey of getting there.

It’s a lot of little things done right, so diffusion can only magnify acoustically what you’ve done right through absorption. So, diffusion’s great if you put it in your room and you’re not liking what you’re hearing then call me because now we need to do some tuning. It’s not that you don’t like what you’re hearing it’s just that you’re hearing everything that’s not supposed to be in there. That’s what you don’t like and you’re hearing it for the first time because diffusion is putting you on notice that it’s still there. Because remember it kind of amplifies that. So you have to be a little careful. But to put it in, you’ll quickly determine what you don’t like. It’ll jump right out at you.

But the job of getting what you like, the definition, the separation, the depths, sound stage, clarity, whatever those issues are, really the job of absorption, not just any absorption, rate and level, that has a huge impact on sound quality.

Some acoustic companies will tell you “Oh just put this up on this sidewall, put some stuff here”. Nonsense! What’s that stuff do? How does it absorb energy? How much of this does it absorb? How much of that frequency does it absorb? Because guess what? All those things are important and to dismiss them by saying “Ah I just put this up because it absorbs energy” that’s not the answer.

I want to know how much it absorbs so I have more control with this acoustical tool at the listening position. Remember the goal is the listening position. I’d blow the house up if I could and get rid of it. Get rid of the room and just have the stereo sitting there on the concrete slab. But that’s not going to happen so you’ve got to use absorption to get what you want and that implies you know what you want and most of us do.

Let’s put it this way, most of us know what we don’t want. Well diffusion will exhibit all of the things you don’t want so it’s a good diagnostic tool to use. Try to get everything that you want through the use of absorption and then expand that quality and “qualities” throughout your room with diffusion.

In Summary

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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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