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Can sound absorption or diffusion at least diminish the SBIE in a set-up that is limited by where the speakers can be placed?

Dennis Foley February 16, 2015 No Comments
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In a recent Google Hangout I was asked “Can sound absorption or diffusion at least diminish the SBIE in a set-up that is limited by where the speakers can be placed?”

This is an issue that a lot of people simply don’t understand correctly so let’s spend a little time going over the detail. When you take a sound generating device, I think this helps people think about things a little bit better, forget that it’s a speaker, just think of it as a sound and energy-producing device. We have lots of those in our lives. Our cell phones are energy-producing devices when they ring. Our cars are energy-producing devices when we put the key in the ignition and we start it. The engine produces noise.

So a speaker is just a sound generating device. Whenever you take this sound generating device, you put it next to a boundary surface like a wall, a whole plethora of things change in the laws of physics. Pressure increases because you have a boundary surface and a sound generating device and some of these speakers have 8 inch, 10 inch, 15 inch, 18 inch drivers. That’s a huge sound generating device. Compare that to your speaker size of your phone which is about an inch or a half inch.

So whenever you take a sound generating device and you put it next to a wall, in a room or too close to the wall in a room, you create two more issues that are major issues, than you would if you put the speaker in the center of the room. When you put a sound generating device close to a wall, you create more pressure and its artificial pressure. It’s not created by the speaker and it’s not created by the wall, it’s created by the speaker being too close to the wall.

Room Sound

So that’s room sound and you can get plus 3DB and we know 1DB, half DB, quarter DB is audible. So you get a plus 3DB punch in whatever frequency, the speaker and the wall decide based on the laws of physics to created at. Usually it’s in the lower registers.

What’s the second issue? Reflections. When you put a speaker next to a wall surface, you create a whole host of reflections that are now time signatured based on the distance the speaker is from the wall. And reflections are 50% of what we try to manage in room acoustics. If you don’t get the time signature of those reflections correct, you just shoot yourself in the foot.

So you can’t have a speaker too close to a boundary surface. What is the best distance? unknown. It varies from speaker size to room size to room volume. I can’t give you a one-size-fits-all answer. Some rooms you can get away with a little bit closer to the wall because the speakers are smaller, because the low frequency driver is small. And the room volume can handle the excess energy. Put a small speaker in the big room, just off the top of my head probably can move it closer to the wall, put a small speaker in a small room farther away from the wall so there is no one-size-fits-all. But two issues, pressure and reflections, those are the two issues we fight every day in room acoustics.

We don’t want to add more issues to the issues we already have to deal with in a room. And here’s what happens, it doesn’t even sound like a speaker when you keep it close to the wall. It’s a whole new sound. Just try it, put your speaker next to a wall and listen. Move it away from the wall and listen. All that speaker boundary interference effect, all that comb filtering, that’s what you’re hearing. You’re probably hearing more distortion than you are sound. I guarantee you the manufacture of that speaker, if he’s alive, he would be freaking out if you have it next to a wall. If he’s dead he would be rolling over in his grave.

AD: Just as an addition to that, some setups will put some speakers actually in the walls, why do they do that?

Dennis: Well, they do that to save space. They do that to use the wall to give more energy output and it’s just well, since we’re talking about sound quality, it’s the difference between listening like in a tunnel and listening outside. That’s called coloration. Coloration is a big part of what we hear in our rooms. We have the room coloration. We have the speaker coloration. We have the amplifier coloration. We have the cable coloration. We have the acoustical treatment coloration. We even have the coloration from the sound bouncing off your brain in your head into your ears. It’s this constant coloration management that we have to address in order to seek the truth, the true sound in the recording.

So let’s don’t make our job more difficult. Let’s make our job easier and minimize speaker boundary interference effect, comb filtering, let’s recognize these room distortions for what they are, listen for them, and eliminate them. And let’s eliminate them through proper setup and acoustical treatment. There’s a lot of beauty in that music and you’re not hearing it. And when you do hear it, your life will change. The quest that you’re seeking for quality sound and emotional connection to your music, you will get closer to your goal. Don’t sacrifice convenience for sound quality, just don’t do it. It’s disrespectful to the sound quality, it’s disrespectful to the music and it’s disrespectful to yourself. Because you’re trying so hard to hear and all you’re doing is creating more barriers.

It’s like you’re driving on the freeway because it’s faster and then all of a sudden there’s three or four accidents in front of you and you’d have been better to take the side roads. So you just keep on putting barriers and things in your way and we have to remove those barriers, we have to get rid of those barriers.

Remember, free spacing listening. The best sound in the world is sitting out into grassy meadow surrounded by mountains, that’s our barrier, no wind, no sound and we’re listening to our system out there in the grass. That’s the best sound. We only have one boundary surface. What is it? I asked that the other day to a guy he said the sky, well, I’m done, sorry, LOL. It’s the earth, not the sky. I thought he said space at the beginning and that’s a vacuum so I would really be done after that.

In Summary

To learn more about room acoustics and how drums interact with your room, please sign up to download our free ebooks and video series on room acoustics here. And please let me know if you have any questions at any time.

Thanks
Dennis Foley

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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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