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Engine fan noise test in a NASA anechoic chamber.

Soundproofing

by Chris Woodford. Last updated: January 13, 2014.

Sound is wonderful—think of Beethoven or birdsong. But sound we don't want to hear, in the wrong place, at the wrong time, is simply noise: a nuisance that can make life stressful and work, study, or sleep impossible. If you're plagued by a noise problem, the simplest approach is to kill the sound at its source, but sometimes that's just not an option. If you live near a construction site, a noisy bar or nightclub, or you have an elderly, forgetful neighbor who plays the TV through your wall at full blast, getting the volume turned down may be very hard work. Maybe you have the reverse problem: perhaps you have a noisy occupation or hobby—you might be a practicing musician or a DJ—and you want to spare the people around you from suffering the sounds you make. Either way, your thoughts have probably turned to soundproofing. Just what is it and how does it work? Let's take a closer look!

Photo: Acoustic instruments are tested in soundproof rooms called anechoic chambers, lined with sound-absorbent materials such as these triangles of foam. In this photo, an airplane engine's inlet fan is being tested for noise emissions in an anechoic chamber at NASA. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).

The science of soundproofing

Testing an antenna in a NASA anechoic chamber.

There are times when it really helps to think like a scientist, and tackling problems with noise is definitely one of them. Understanding the science of sound is the best way to set about reducing it.

Sound is a kind of energy that's produced when things vibrate. The energy has to go somewhere, so it travels outward, away from the sound source, making objects and the air all around us vibrate in sympathy until what's left of the energy reaches our ears. Inside our ears, the air vibrates too, banging on our ear drums, stimulating tiny hair cells deep inside our heads, and registering the sounds in our brains. In short, sound starts life at a source, travels through one or more media, enters our ears, and lights up our brains—and if you want to stop it in its tracks you have to interrupt that chain of events somewhere along the route.

Photo: Anechoic chambers are also used to test antennas. Photo by courtesy of NASA Langley Research Center (NASA-LaRC).

Understanding how sound waves travel through air and solid materials is the key to stopping it, but that's easier said than done. One reason we struggle with soundproofing is that we confuse sound with light. Although both are kinds of energy that travel in wave form, light waves have much shorter wavelengths than sounds and are far easier to block out: it's much simpler to make your house pitch black than completely quiet. Unlike nanoscopic light waves, long-wavelength sounds can bend (diffract) round corners and wriggle through the tiniest cracks and openings. More importantly, while light waves pass through only a handful of solid materials (such as transparent plastic and glass), sound energy will happily storm through most solids and emerge almost as loud the other side. For example, sound travels through (solid) steel about 15 times faster than through (gaseous) air. When engineers build tunnels, they sometimes bang on the metal walls to communicate with colleagues because that's the quickest, most efficient way of transmitting sound!

Two kinds of noise-reduction earplugs.

The three easiest ways to stop sound are to turn off the source, increase your distance from it (walk out of that noisy bar), or stop the sound waves from entering your ears (cover your ears or wear earplugs at the rock concert). As we've already seen, the first of these is often impossible: if you're living near an airport, the airplanes aren't going to stop flying just for you! Earplugs (widely available from drug stores or online for just a few dollars) and noise-canceling headphones are probably the most effective option if your objective is quiet work or study or traveling in peace on an airplane or train—but they're not always suitable ways of reducing sound at home. If you're a musician and you want to keep traffic noise out of your room while you record an LP, you need to block incoming sounds in more drastic ways.

Photo: Why go to the bother of soundproofing your room if soundproofing your head will do the job just as well? Ear plugs are the cheapest, simplest way of getting peace and quiet. Here we have two of the more common kinds. On top, there's a cheap, comfortable, disposable foam earplug (suitable for working or sleeping). Underneath, there's a heavy-duty, washable and reusable airplane-style plug with ridges that bunch up inside your ears to make an effective suction seal. If you need "ear defenders" for industrial-strength ear protection, check the packaging to make sure you're getting the right ones.

Stopping sound in its tracks

Suppose you're sitting comfortably in a room in your house, hoping to record some music, but wondering how to block out traffic noise from outdoors. Think about the sound waves coming into your room: they travel through the outside air, hit the walls and windows of your home, and make those solid materials vibrate. The energy is transmitted right through the solid glass, wood, concrete, or stone and makes the air vibrate again on the other side. That's how sounds from outside get inside. You can probably see that you have several different ways to solve the problem, but they're not really alternatives—they're solutions you can combine: you can reduce incoming noise by blocking any direct air pathways that allow sound to travel from the outside to the inside; you can absorb or dampen sound energy coming through the walls; or you can physically "decouple" the inside of the room from the outside world.

Noise reduction

The first and simplest step is to reduce noise by blocking off the paths sound is likely to take into your room. Obvious things like extra layers of glazing help, but only if they're tightly sealed around the edges. Double-glazed windows with a tiny air gap aren't going to help much at all if they're made of wood and the opening part of the window doesn't seal properly into the frame. If air can get in, sound can get in too so installing good seals, gaskets, and caulks around doors and windows is extremely important. Even things like ducts and channels for cables or electrical outlets provide access points for sound. Sorting out drafts and leaks in your home to improve heat insulation is a different thing from soundproofing. The benefits of doing one will also improve the other, but the objectives are different; heat insulation materials generally improve soundproofing, but don't always work as well as materials designed specifically for sound insulation.

Absorbing and dampening

A small piece of mass-loaded vinyl being held in someone's hand.

There are two slightly different techniques at work here, but they typically go hand in hand. Absorbing means using rubbery materials that soak up incoming sound energy so there's less to transmit onwards into a room, whereas dampening means using a solid, acoustically "dead" wall that doesn't readily vibrate. In practice, dampening and absorbing might mean fitting solid, extra-thick doors (rather than hollow ones), or heavy double doors separated by an air gap. Or it might mean constructing a building with massive walls (made of dense, heavy materials such as lead or concrete) with large air gaps in between. Absorbing by itself could mean adding materials between walls that soak up vibrations with such things as fiberglass, neoprene rubber, viscoelastic foam, or MLV (Mass-Loaded Vinyl).

Sound can still pass between insulated rooms through the floors and ceiling.

Photo: Left: Mass-loaded vinyl is a simple plastic (such as PVC) with ceramic material added to give it extra weight and improved soundproofing properties. Typically, it weighs a hefty 5–10 kg per square meter (1–2 lbs per square foot). You can buy it on rolls from DIY stores and specialist soundproofing companies.

Decoupling

In theory, the perfect way to soundproof a room is to build a smaller room inside it and stop sounds traveling from one to the other. This is sometimes called a "room within a room" or acoustic decoupling. Each room is made from heavy, solid materials but the two rooms cannot be touching one another directly or sound will pass through. Instead, the inner room is typically supported by small clips (such as RSIC™ Resilient Sound Isolation Clips or WhisperClips) and the walls are lined with sound-absorbing material.

Artwork: Right: If you want to keep sound out of a building, heavy concrete walls separated with an air gap are one approach you might take. Suppose the noise is outside (1) and you're on the inside (2). The concrete walls and air gap (3) will dramatically reduce any direct transmission of noise. But sound will still travel through the floor (4) and the ceiling (5), reducing the gains you make. For really effective soundproofing, you need to consider all the paths by which sound might travel from source to listener.

Measuring sound insulation

A Quest sound pressure level meter (decibel meter).

How can you compare the amount of sound insulation you get from different materials? There are several different measurements you'll come across.

Sound Transmission Class (STC)

In the United States, a common way of comparing sound insulation in buildings is using a measurement called STC (Sound Transmission Class), which describes how well or badly sound waves (broadly in the range of normal human voices, 125–4000 Hz) travel through ceilings and walls. A very bad partition wall through which you could hear more or less everything would score about 20–25, while a luxury hotel wall that blocks out virtually everything would notch up about 60. Most domestic walls rate somewhere in the middle from about 30–45. You can improve the STC of a partition wall by building it from a more dense material (sound insulation improves by about 5 decibels for every doubling of mass), by adding an air gap, or by adding sound absorbing material.

Photo: Measuring noise with a sound pressure level (SPL) meter. Photo by James R. Evans courtesy of US Navy.

Sound Reduction Index (SRI)

In countries outside the USA, SRI (Sound Reduction Index) is a more common measurement. Typically, companies offering soundproofing products will suggest they can achieve an improvement of so many decibels (dB) sound reduction or SRI. Everyday materials have widely differing SRIs. A thin plane of glass would achieve about 20–25 dB, light concrete slabs would be about 40 dB, while two brick walls separated by a large air cavity would cut noise by 60–75 dB. Like STC, SRI measurements are highly dependent on sound frequencies: a material that gives a considerable improvement in sound insulation for human speech (cutting out conversation from your neighbors upstairs) is likely to be much less effective at cutting lower sound frequencies (so you may still hear the deep bass of their stereo).

NRC values for typical everyday materials showing examples of good and bad sound reduction.

Noise Reduction Coefficient (NRC)

While STC and SRC indicate how well noise passes through different materials, NRC (Noise Reduction Coefficient) measures how well materials stop sound from reflecting (how much sound they can absorb). The NRC is the percentage of sound that a surface absorbs (in other words, hits a surface and doesn't reflect back again into the room). So a carpet on rubber underlay could easily have an NRC of about 0.4 (it absorbs 40 percent of the sound hitting it and 60 percent bounces back), while a glass window might score only about 0.05 (it reflects 95 percent of the sound hitting it straight back again).

Chart: Typical values of NRC (noise reduction coefficient) for some common materials. Note that NRC values do vary substantially according to whether materials are, for example, painted or coated with other materials, the frequency of the sound, and so on. Even so, this chart shows exactly what we'd expect: harder materials reflect sound more (and reduce it less) than softer materials. It's worth noting that people absorb sound very well. This is a major factor in the design of concert halls and their seats, which are usually designed to sound similar whether they're occupied or empty.

Simple tips for soundproofing a room

An airplane under test in a giant anechoic chamber.

If you just want to make your home a little quieter, you won't want to go to extreme lengths like building yourself an anechoic chamber or a room within a room; if all you're doing is the crossword or the knitting, you probably don't need a padded cell! Making an ordinary room quieter involves a two-pronged attack on noise through a combination of noise reduction and noise absorption.

Photo: Some people go to extreme lengths to soundproof rooms, but that doesn't mean you have to! Here's the world's biggest anechoic chamber at Edwards Air Force Base, California. The blue triangles you can see are energy-absorbing foam wedges similar to the ones in our upper pictures. Photo by Thomas Powell courtesy of U.S. Air Force.

Noise reduction

The first, obvious step is to tackle the routes by which noise enters your room. Doors and windows are likely your biggest problem, so check those out first: make sure they're shut tight and locked, check the seals, install draftproofing or caulking, and use draft excluders. Double or triple glazing can be a big help if your problem is something like airport or highway noise, but make sure there's a big air gap and proper seals. Do you have an open chimney you're not using? If it's safe to do so, block it up. If soundproofing is your mission, you really need to go around your room (or rooms) systematically, identifying every possible access point where sound can get in and doing whatever you can to block that path. But in your quest to block out sound, don't forget that blocking your ears (with earplugs) may be far more effective—especially if the noise is only a temporary nuisance.

Noise absorption

Once you've reduced incoming sound as much as you can, you could try altering the interior of your room so sound waves are absorbed rather than reflected by materials inside. Carpets work better than wooden floors, but rugs can work a treat too. Soft furnishings such as wall hangings (tapestries or blankets), sofas and cushions soak up sound very effectively (that's why rooms sound so different when you've emptied them to decorate or move house). Curtains are good at absorbing sounds from either inside or outside, but make sure they're dense and heavy, reach right down to the floor, and seal well all round. Thermal curtains linings and blackout curtains (designed to stop heat loss and make rooms darker at night) can significantly improve sound insulation for problems like traffic and street noise.

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Text copyright © Chris Woodford 2009, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009) Soundproofing. Retrieved from http://www.explainthatstuff.com/soundproofing.html. [Accessed (Insert date here)]

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