
You know how the sound from that TV in your den comes right through the bedroom wall and you can hear every word? Or the annoyance you feel when your neighbor’s lawn maintenance crew comes every Saturday afternoon right when you want to take a nap? How about the sound of that interminable music in Energy Vanguard’s automated answering system? Well, I can’t help you with that last one (do we even have music?), but let’s talk about soundproofing a wall. And just to be clear, I’m talking about sound transmission through a wall, not the sound that bounces around within a room.
The complexity of sound transmission
Sound is a wave. Unlike light or magnetism, this kind of wave needs physical matter, a medium, through which to travel. That property of sound waves, of course, led to the brilliant tagline for the 1979 movie, Alien: “In space, no one can hear you scream.” There’s nothing to vibrate and carry the sound waves in space; hence, no sound transmission.
So, the best way to reduce sound transmission would be to live in a vacuum. You’d have a short life but you wouldn’t be bothered by noise. In addition to being dead, you’d also be broke because turning your house into a vacuum chamber wouldn’t be cheap. Therefore, I’m going to take a leap and assume the vacuum technique won’t work for you.
With air in our homes, we can hear said scream through the wall—or the blender, the dog, or the rototiller. Sound is a wave that vibrates matter between the source and the sensor, your ear. Most of the sound we want to reduce begins its path in the air. Some, though, is from an impact, like banging your head against the wall.
Then there’s the issue of frequency. High-pitched sounds, like whistling, have a high frequency of vibration. Low-pitched sounds, like Johnny Cash singing A Boy Named Sue, have a low frequency. Sounds of different frequencies behave differently, especially when traveling through solid materials.
And that brings us to the basic techniques for reducing sound transmission through walls:
- Breaking the pathways through solids
- Adding mass
- Breaking the pathways through air
Before we move on, though, let me throw one more thing out here. The BS* + Beer Show did an episode on the science of a quiet home and one panelist made an important distinction: Sound is any vibration that can be heard; noise is unwanted sound. (There’s also a lot of other good information in that show, so watch it to learn more.)
Breaking the pathways through solids
To reduce sound transmission through the solid parts of a wall, we can apply one of the same tactics used in reducing heat flow through walls. For heat flow, we want to eliminate thermal bridges. Those are the pathways with low thermal resistance that rob heat from a house in winter or allow it to enter the house in summer.
Likewise, we want to eliminate acoustic bridges to reduce noise. When a sound hits the outer surface of a wall (e.g., drywall), the sound wave in the air can get the molecules in the solids oscillating, too. Then the wave moves through the solids as long as there’s a continuous path. Break that pathway, and you help reduce the transmission of sound.
One way to do that is by building two walls with a gap between them. When the sound hits drywall on one side, it goes into the studs of the first wall. Then it hits the air gap. To continue, the sound wave would have to go back into the air. Then back to solid in the second wall. Each step can reduce the amplitude of the sound waves, thus making it quieter on the other side.
Rather than building two separate walls, you could do a staggered stud wall (photo below). That’s really just one wall with a separate set of studs for each side of the wall. That breaks the continuous path between studs but not between the top and bottom plates of the wall.
The way I chose to do it in my basement renovation was to use two sheets of drywall with a soundproofing material called Green Glue between them. The Green Glue is an acoustical material that makes it difficult for sound waves to continue from the first sheet of drywall to the second.
Yet another way to break—or reduce—the acoustic pathway is to use resilient channels, which are long metal pieces that attach perpendicular to the studs to decouple the drywall from the studs.
Adding mass
Walls with more mass are also good for reducing sound transmission. Think about how well you can hear through a thick concrete wall, for example. One way to add mass to wood-framed walls is with a product called mass-loaded vinyl. Note that it can be difficult to work with, and vinyl isn’t the most environmentally sound material.
The way I did it in my house was to use two layers of thick drywall. The standard drywall thickness is 1/2 in. When you’re adding mass this way for soundproofing, 5/8-in.-thick drywall is most commonly used. The short video above shows how I did that.
Breaking the pathways through air
Sounds that begin in the air in one room have the easiest time infecting the next room over when they have an unbroken pathway through the air. You’re familiar with this technique if you’ve ever closed a door to keep noise out of the room you’re in.
But sound waves don’t need a full door. Even small holes on one side of a wall can let sound waves into the space between. Then if there are holes on the other side, the sound can continue right on through. That’s why airtight homes are also quieter homes.
The photo above shows a standard plastic electrical junction box. You can see light coming through the wall around the perimeter of the box. It also can come straight through the box. Those holes the wires go through have big spaces around the edges.
You can use regular air-sealing spray foam to seal up those holes. A better way to reduce sound transmission, though, is to use acoustical putty pads. The video above shows how that works.
Rating wall assemblies for noise-reducing quality
When you’re trying to decide which wall assemblies to use to keep things quiet, there are a couple of numbers you need to know about. The first one is called the Sound Transmission Class, or STC. It’s a rating for how well an assembly reduces the transmission of airborne sound. The higher the number, the better it will be at keeping things quiet.
A standard 2×4 wood-framed wall with 1/2-in. drywall on each side and no cavity insulation has an STC rating of 34. Moving to double 5/8-in. drywall on one side raises the STC into the 40s. Keeping the double drywall on one side and then putting fiberglass or mineral wool insulation in the stud cavities and putty pads on the junction boxes raises it to 50 or higher. And that’s considered a pretty quiet wall!
But there’s a lot of complexity here. You still could have a noise problem even with a high STC rating. That’s because of the role of frequency. If the noise you’re trying to limit has a significant amount of low frequencies, it can still come through a wall with a high STC rating. So if you live near an airport or have a budding drummer in the house, going with more mass in the wall will help more than just a high STC rating.
The other rating you’ll see mentioned in the soundproofing world is the Impact Insulation Class, or IIC. That person banging their head on the wall in the other room makes impacts. The same happens when someone walks with hard soled shoes on the hardwood floor above you. As with STC, the IIC rating is better when the number is higher. It’s usually applied to floors above occupied spaces.
So, the two ratings cover airborne transmission of sound (STC) and impacts that travel through solid materials (IIC). For walls in homes, the main one to know is STC. But there’s one more set of initials that we need to mention here…
Indoor Environmental Quality
Most people know about IAQ, indoor air quality, but not nearly as many are familiar with IEQ, indoor environmental quality. When you’re designing, building, or remodeling a home, it should be top of mind.
As you can see from the graphic above, sound is an important part of indoor environmental quality. So learn what you can about reducing noise transmission through walls, floors, and ceilings, and you’ll make people happier when they benefit from your knowledge. Now, if only we could find a way to improve automated answering services that foster peace and tranquility, the world will really be a better place.
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Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel. Images courtesy of the author.
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