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Shhh! FOCUS Question What can people do to control noise?

The Problem of Noise Sound tells us a lot about our world. It warns us of danger, lets us hear music, and helps us communicate. But some sounds, such as a leaf blower or an airplane, are simply noise. They are sounds we don't like to hear. Sometimes noise is just annoying, but it can also cause problems. Noise can make it hard to concentrate. Loud noises and long exposure to noise can damage your hearing. Harmful or annoying sounds are called noise pollution.

The Science of Sound To reduce noise, it helps to understand sound. Sound begins when tiny particles called molecules move back and forth, or vibrate. These vibrations make the molecules around an object vibrate and create a sound wave. The wave travels outward until it reaches our ears. Sound waves can travel through gases such as air, and liquids, such as water. Sound waves travel faster through water than through air because liquids have molecules that are closer together. The vibrations can quickly transmit energy to surrounding molecules. Sound waves even travel through solids, such as walls. So how can we stop the noisy sounds?

Mass Reduces Noise When sound waves hit a solid surface, such as a wall, three things can happen. Some of the waves are reflected, like when you hear an echo off a cliff. Other waves are absorbed into the surface and converted to heat energy. Sometimes, the sound waves travel all the way through the wall. Depending on how they are made, walls can allow sound to pass through easily. But walls with a lot of mass, like thick concrete walls, absorb more sound and keep it from passing through. To make walls thicker, builders attach sheets of dense material, such as drywall. If thick walls have cracks, sound will still sneak in. Sealing cracks in walls helps keep sound out.

Space Reduces Noise Another way to reduce noise traveling through walls is to add dead space—a layer of air between two walls. Sound waves travel through the solid surface, and then slow down, bend, or bounce around in the dead space. This loss of energy keeps some waves from getting through to the other side. Lightweight materials, such as foam and insulation, have many air spaces. They slow down sound and reduce the number of sound waves that reflect off the surface, so you hear fewer echoes. Drapes and rugs also reduce echoes and make a room quieter. Math Moment If 30 of a sound outside your room gets absorbed by your wall. 20 gets reflected. and 15 gets stuck in the dead space, how much of the sound reaches your ears?

Not All Sound is Equal Using materials to reduce the transfer of sound is called soundproofing. But even with soundproofing, you may still hear some sounds Sounds with a lower pitch, like a city bus engine, are harder to block than sounds with a higher pitch, like singing birds. The pitch of a sound depends on its frequency, the number of sound waves that pass a point in one second. High-frequency sounds have a high pitch. Low-frequency sounds have a low pitch. High-frequency sound waves can easily be absorbed or reflected. But low-frequency waves can travel far and do not absorb or reflect easily.