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How Much Acoustic Foam Do I Need?

There comes a day in the life of certain special people when they realize the need for acoustic treatment. When I first got into this, I thought the apple spacers from my produce receiver job would work. Well, they didn’t. Later, once I got my little home studio setup, I knew I needed treatment so I asked myself, “How much acoustic foam do I need?”

Let me put this as succinctly as possible: acoustic foam is probably the worst option for its cost-to-performance ratio. Fluffy fiberglass, Ultratouch, or even rigid mineral wool is as effective – if not more so – and will cost much less. How much treatment you need depends on the application. For improving speech clarity and reducing flutter echo, a checkerboard pattern of 2″ rigid panels will work wonders. However, if you have a home studio or intend to improve bass performance then using very thick fluffy – greater than 2 feet thick in some cases – is what the doctor ordered.  All the following advice will apply to foam as well, however.

Upon First Reflection

First we’ll talk about early reflections, of only because the concept uses the word ‘early’. Early, also known as first, reflections are damaging to your listening experience. Whether they come from nearby walls, your desk, or any other hard surface, these carbon copies erase clarity. Depending on where you are, the first sound could come from any number of places.

Let’s say you’re in a restaurant. While you might not be consciously aware of all the sounds happening, there are the cooks making noise with their yelling and clanging pans, there are guests around you scraping their plates and clinking glasses and then there’s your conversation – lost in the stew of sound. If it was just these sounds, it would be bad enough. In truth, the entire environment is conspiring against you as an echo chamber keeping all those distracting noises around for as long as possible.

Enter acoustic treatment. Like a sonic competitive eater, it eats up these noises as soon as they reach it. By doing this, these noises don’t continue to bounce around and interfere with your chat. Even your voice itself will sound more clear because, upon reaching the treatment, it stays there and never comes back – just like Vegas.

Apply Yourself

Luckily, improving speech clarity is easy and cheap. All you need is about 2″ thick of absorbent material – like foam or mineral wool – and you can apply that in a 2×2′ checkboard grid. Surely you’ve seen this type of application in the video of one of your favorite YouTubers. You can stick them flat on the wall or ceiling.

Checkmate, Early Reflections. Oh, that’s chess. Same board.

While it’s hard to say precisely how much you’ll need, more is almost always better. Start with one wall at ear level and proceed from there. Remember, though, that the closer the surface, the stronger the reflection. So, if you want it to work, don’t put it up on a 15′ ceiling if your diners are near solid walls.

Home studios may have slightly different needs. Studios generally have two kinds of spaces: control rooms and live rooms. In a live room, this same kind of checkerboard pattern can help with flutter echo and early reflections. However, in the control room, you have to go about things slightly differently.

Same Song, Different Pattern

If you do have a home studio then your listening position is where you spend the most time. Using a checkerboard pattern of 2″ thick panels isn’t going to do the job. Why? Because you have two basic fixed sound sources – your loudspeakers – and one generally static listening position. Plus, you’re listening critically to lower frequencies than you would when someone is just talking.

For your needs, rigid absorption from 4-8″ thickness is best. You can also use pink fluffy beyond 8″ but it’ll take up more space. How you find where to put it is: be at your listening position and have a friend slide a mirror over your ceiling and side walls. Wherever you can see your speakers is where you need the put your treatment. In practice, this tends to be half way between you but make sure to look for both speakers.

By doing this, you create what’s known as a Reflection Free Zone, RFZ for short, and you get better clarity and stereo imaging as well as a more accurate frequency response. If you’re into graphs, I’m borrowing an image from Sound On Sound. In it, you can see that the treatment kills the sonic reflections that would ordinarily blend with your direct signal.

acoustic-foam-reflection-free-zone-response
Impulse response inside a Reflection Free Zone. Credit: Cox and D’Antonio, Acoustic Absorbers And Diffusers, Spon Press, 2009.

What About That Bass?

For studios of all kinds, bass is usually a problem. Whether it’s a dance studio, home studio, or professional studio, you’re going to need a lot of treatment. How much? It depends on several factors.

The harder your walls are, the more they reflect sound and the more the room functions as an echo chamber. If your walls are soft, the sound leaks out and you get a shorter decay. Even if, however, your walls were made of paper, that bass is going to bounce off of something and come back to haunt you.

To keep it simple, even several well-placed 4-8″ rigid panels will practically solve your needs. If you want to get more extreme then super chunks made of pink fluffy or even rigid absorption will power down your modal issues. At first, you want to focus on corner placement – all 12 of them in a rectangular room. Later, or especially once you’ve taken an acoustic measurement or three, you can more truly ‘tune’ your room by placing absorbers on surfaces which correlate to your worst modal issues.

In my room, I have 20 4″ mineral wool absorbers and 9 4’hx31″w super chunks made from 2′ pink fluffy R-19 cut diagonally and stacked. With this much treatment, I can achieve the target I’ve set for myself: 20dB of decay within 150ms above 63Hz.

Of course, decay is only part of a room’s acoustic response – arguably the more important part and the component with which acoustic treatment helps the most. The other factor is your sound pressure level response, SPL for short. According to Ethan Winer, you want that to fall within plus or minus 10dB from your measurement level.  If you can do better than that – great!

This isn’t an article about acoustic analysis but, for now, when you take a measurement make sure not to smooth your bass range below 300Hz or so. For the range above that, you can use whatever smoothing level is useful for you. By doing this, you’ll be able to accurately assess your bass and understand your higher range without being distracted by comb filtering artifacts.

Last Note

Hopefully, by now, you understand that acoustic problems come in two basic flavors: reflections and bass modes. Reflections are what make everything unclear up top and room modes just sit around and muddy up your bass. Whether you’re in a room primarily intended for social interaction or education or a recording studio, you can use relatively thin acoustic absorbers – foam or otherwise – to soak up extra sound energy in your higher frequency ranges.

Even so, big low frequencies require big treatment. If you ever see someone who has ‘treated their studio’ and all you see is thin foam, feel free to laugh because now you know better. To tackle bass, you need 4-8″ of rigid absorption or 8″+ of pink fluffy. Remember that no matter what kind of absorption you use, you need to take acoustic measurements of your room and compare it to the targets I’ve given you.

Checkerboard patterns clarify speech and tame flutter echoes. Precisely placed thicker panels will be perfect for creating a reflection-free zone in a studio. Big absorbers in corners, and also gapped in front of other large surfaces, will tame modal ringing which will help you hear the bass range evenly and with maximum punch.

I hope this article was helpful. Really, no one can tell you precisely how much absorption you need for a given space without more information. At least with what you’ve just read, you know how to proceed and when to stop.

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