How does soundproofing reduce noise?

1. Max is playing loud music. Next door, Pip the kitten is trying to sleep, but the sound comes straight through the thin wall. How can we keep the noise in?
2. Sound is air shaking back and forth. When it hits a thin, light wall, the wall shakes a tiny bit too, and it passes the shaking on to the air on the other side.
3. A thick, heavy wall is much harder to shake. Most of the sound bounces off it and stays in the noisy room. A heavy wall is a good barrier.
4. But look under the door. Sound is moving air, so it slips through any gap, like water through a hole in a bucket. A strip of rubber seals the gap.
5. Soft panels are full of tiny holes and fibers. Sound goes in, rubs against them and fades away. That soaks up echoes, so the noisy room itself gets quieter.
6. Use all three together: a heavy wall to bounce sound back, sealed gaps so it cannot leak, and soft panels to soak it up. Max plays on, and Pip sleeps soundly.

A soundproof wall is sliced open. A sound wave comes in from the left. Tap each layer to see how it makes the wave smaller.

• Soft panel. This foam panel is full of tiny holes. Sound that goes in rubs against the foam and fades, so it does not bounce back into the room.
• Heavy boards. These thick, heavy boards are hard for sound to shake. Most of the sound bounces off them. Two layers are heavier, so they block even more.
• Fluffy filling. The hollow part of the wall is stuffed with fluffy wool. It soaks up sound that gets inside, so the wall does not boom like a drum.
• Air gap. The two halves of the wall do not touch each other. When sound shakes one side, there is no solid path to carry the shaking across.
• Sealed edges. A bead of squishy sealant fills the cracks where the wall meets the floor. With no gaps, sound has no easy way to leak around the wall.

Try a thick wall, soft panels and a sealed gap, one at a time and together. Watch the waves and the meter next door.

Experiment: You build a thick, heavy wall but leave the gap under the door. What will it sound like next door?

1. Switch on the thick wall and look at the meter next door.
2. Now seal the gap under the door. What happens?
3. Add the soft panels and compare the waves in the noisy room.

What we found: A heavy wall alone hardly helps while there is a gap, because the sound simply goes through the gap. Seal the gap and the heavy wall can do its job. Soft panels make the noisy room itself quieter.

Turn on JavaScript to try it yourself.

You can afford only two fixes. Choose the two that make it quiet enough for Pip to sleep, then press Test.

Sound is air shaking back and forth. To keep a noise from getting from one room to another, you have to stop that shaking on its way. Builders use three different tools, and each one does a different job.

The first tool is a barrier: something heavy and solid, like a brick wall or thick boards. Sound has to shake a wall to get through it, and heavy things are hard to shake. So most of the sound bounces back. The second tool is an absorber: something soft and full of tiny holes, like foam, thick curtains or carpet. Sound goes in, rubs against the fibers and fades away. Absorbers stop echoes and make a noisy room calmer, but they are too light to block sound on their own.

The third tool is the one people forget: sealing gaps. Sound travels wherever air can go. A crack under a door, a keyhole or an open vent lets sound pour through, like water through a hole in a bucket. A heavy wall with a gap is a bucket with a hole.

The bucket idea is not perfect. Water cannot get through a solid bucket at all, but a little sound always gets through a solid wall, because the wall itself shakes. That is why the best quiet rooms use all three tools together, and sometimes two walls with a gap of air between them.

Key words

Barrier

Something heavy and solid that sound bounces off.

Absorber

A soft material full of tiny holes that soaks up sound.

Decibel (dB)

The unit for how loud a sound is.

Transmission loss

How many decibels of sound are lost on the way through a wall.

Echo

Sound that bounces off a hard surface and comes back.

Check your thinking

1. A heavy wall has a gap under its door. What happens to the sound?

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   It leaks through the gap. Sound goes wherever air can go, so the gap lets it pour through.

2. Why does a heavy wall block sound better than a light one?

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   Heavy things are harder for sound to shake. Sound must shake the wall to get through. A heavy wall barely moves, so most sound bounces off.

3. What do soft foam panels on the walls of a music room do best?

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   Soak up echoes inside the room. Soft panels soak up sound so it does not bounce around. They are too light to stop sound going through a wall.

4. You double the mass of a sealed wall. About how much more sound does it block?

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   About 6 dB more. The mass law gives roughly 6 dB of extra loss for each doubling of mass per square meter.

5. Your neighbor plays music. Which part of it are you most likely to hear through the wall?

   Show the answer

   The deep, low thump. Low sounds are the hardest to block, so the deep thump gets through when the rest does not.