Written by 5:00 am Room Acoustics

4 Acoustic Myths That Should Go Into the Garbage Can

Dennis Foley explains why certain common beliefs about room acoustics are dead wrong.

As a professional acoustician, I fight acoustical treatment misrepresentations every day. To be honest, though, misrepresentation is too kind of a word — lies might be more appropriate. Most of the people who contact us are making some of the following mistakes because they’ve been, quite honestly, misled by acoustic product manufacturers. Let’s address some of the issues and see what we can figure out.

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Acoustic Myth 1: Put Carpet on the Wall

Carpet is like every other sound-absorbing material in the world. It has what’s called a sound absorption coefficient, meaning how much this particular material absorbs at what frequency. It has a rate and a level of absorption. Is carpet sound absorbing technology? Yes. Is it the correct sound absorption technology for a listening room? No.

For the purposes of this discussion, let’s confine all our comments today to two-channel listening.

You have carpet and you’ve heard it is a sound-absorbing technology so you decide to put it up on your walls. Sidewalls are the most important reflections. The energy from the lateral reflections at the listening position (shown in the image above) combines with a direct signal from your loudspeaker to create a stereo soundstage with width, depth and height . To get this right, you have to balance the direct and reflected energy from the sidewalls, using the proper rates and levels of absorption, and blend that reflected time signature with the direct sound. There is no one-size-fits-all.

With recent tests we have been running, we’re even finding that all the positions along the sidewalls work better at the listening position and achieve a better balance between direct and reflected energy if the rates and levels of absorption are varied along the sidewalls. Let’s leave that for another video, but the bottom line here is that if you put carpet on your sidewalls to manage the reflections, you’re reducing the time signature of the reflection, so it is less than the time signature of the direct energy assuming the carpet is thick enough to have adequate absorption. Will carpet absorb? Sure. Will the absorption be at the right rate and level to balance the direct and reflected energy time signatures at the listening position? Never and I mean never.

Carpet can be cheap, but you get what you pay for. You want to seek technologies that have the proper rates and levels of absorption for sidewall reflections. Because even a 1- or 2-millisecond delay between the time signature of the reflection and the direct energy from the speaker can cause the image of your soundstage to shift. That’s not what you want. You want your image to stay in the middle, between the speakers. You want your speakers to disappear, so that all you hear is music. I don’t want to know that it’s being produced by speakers. I want to be taken out of the front row and brought onto the stage and given an instrument to play. That’s how you become connected to your music, that’s how you become involved, that’s how you get the room out of the way, so it’s just you and your music.

Keep the carpet for the floor. The floor area has certain absorption requirements, but they’re not as critical as the sidewall. As a general rule, keep the padding a half-inch and the carpet an inch. Put it on the floor, walk on it, but don’t put it on the sidewalls.

Acoustic Myth 2: Egg Cartons Are Diffusers

Let’s talk about egg cartons. Thankfully I don’t see too much of this anymore although I still will receive photos of people’s rooms using them. An egg carton is a device to safely store and transport a delicate instrument or foods like  eggs. It’s not an acoustical tool. It’s definitely not a diffuser.

Diffusion is based on quarter wavelength and half wavelength rules that specify distances, depths and lengths. An egg carton is all the same size, all the same length. Sometimes different grades of eggs produce larger cartons but the differences are minimal when it comes to diffusion. This is a common area of people’s lack of knowledge.

You see a flat surface, you see an egg carton, you realize that the egg cartons has all kinds of indentations and wells and dips and all these things. You think that would be better than a flat surface. So you put it on the wall and you think will that help. I even have people tell me that egg cartons are great barrier technologies. Really? Cover your room with egg cartons and ask your neighbor if he can hear your low end. It’s just nonsense, so let’s try to stay in reality with this stuff.

Room acoustics, your music, your emotional connectivity is too important to be relegated to a device that’s used to transport eggs. Designing room acoustics products — tools to achieve your musical goals — requires lots of research and development. I spent $2 million developing my company’s foam and carbon technology. Should I have used carpet and egg cartons instead? Well, I could’ve but I didn’t want to be part of the problem. I wanted to come up with solutions.

Acoustic Myth 3: Foam Is a Bass Trap

Here’s another common misconception with foam. I just get so tired of hearing this and I hear it every day. I do video after video after video explaining why foam isn’t a bass trap, and yet I still hear this.

Foam is not a bass trap. Let’s do some defining here, so we can understand why. First off, there’s no such thing as a bass trap. Wavelengths that are 30- and 40-cycle waves that are around 35, 45 feet long, are not trapped by anything. You’re not going to contain them. All you can do is manage them. Maybe you can clip the head and the tail of the wave and reduce the amplitude that way. How are you going to do that? Through absorption. Is foam going to be able to grab a big part of the head and the tail of a 30, 40 foot long wave? Absolutely not.

Here’s how I think about foam and low-frequency pressure. Foam is holding up a feather to stop a hurricane or a tornado. You run out into the field you see a tornado coming and you’re going to protect your house with a feather? That’s what foam is to low-frequency pressure. I understand the need to achieve acoustical results without spending a fortune. But there are ways to use science and physics and technology to do that without spending a lot of money. You want to use real measured technology with real measured performance — not foam. You need pressure-activated, applied scientific technologies for this specific problem.

Acoustic Myth 4: Building Insulation Is a Low-Frequency Absorption Device

Here’s another one along with the foam line, namely that building insulation is a low-frequency absorption device. Nothing could be further from the truth unless you want to use 10 to 14 feet of the stuff. The technical and general category for this kind of material is called a limp mass material. Rock wool, Corning 703, fiberglass and mineral wool are all examples of limp mass materials. Whenever you see the word limp, in front of the word mass, please do not associate it with low-frequency waves.

Why? Because low-frequency waves are anything but limp and do require extensive mass to treat. Low frequency energy is the bull in the china shop. You want to manage it, you want to get it out of the china shop as quick as you can, so it won’t do any damage.

That’s one good thing about low-frequency energy in rooms: When you have a room that has a size and a volume, you know how big your bull is and how big of a door you need in the room to get rid of it. The hard thing is coming up with the money and the space to protect the china shop from the bull because a lot of times it won’t leave. It’s this delicate balancing act between space and money and using the right technology.

Every day I see people spend thousands and thousands of dollars, using foams as bass traps, using carpet on the wall, using egg cartons for diffusion, and then I ask why they’re calling me. They’ll say “because I don’t like what I hear,” so that tells you right away that these are not viable solutions.

If it sounds too good to be true, it probably is and the truth is when you put it in your room and it doesn’t work, you’ll hear why straight away. Be very, very careful out there and just use a little common sense. If you can’t figure something out just let me know. I’m sure I have run across it in my day-to-day activities or past R&D work and I can help you.

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