Why Can You Hear Through a Wall but Not See Through It?

When a wave reaches the edge of a material, three things can happen to it. You met them for light in Grade 8 Grade 8: Light: some light is reflected, some absorbed, some passed through. The same three are true of every wave.

• Reflected: the wave bounces back off the surface, like a ball off a wall. An echo is reflected sound; your face in a mirror is reflected light.
• Absorbed: the wave goes into the material and its energy stays there, mostly as heat. A black pot on a fire absorbs light and warms up.
• Transmitted: the wave passes through and carries on out the other side. Light through a window is transmitted.

Usually all three happen at once, in different amounts. And because energy can’t appear or disappear, the three always add up to all the energy that arrived:

energy arriving = energy reflected + energy absorbed + energy transmitted

This is our model: a simple picture of what happens, which lets us explain and predict. Figure 10.1 shows it for two real cases.

Here is the most important part of the model. What a material does to a wave depends on the material, and on the kind of wave. A material can be clear for one wave and a solid wall for another.

You already know a material that does this. Your phone works inside a brick house. The phone talks to the tower with radio waves, which are electromagnetic waves like light Grade 9: Electromagnetic spectrum. They pass through the brick wall that stops light completely, weakened but not stopped. So a brick wall is “transparent”, at least partly, to the waves your phone uses.

Material	Visible light	Radio waves (phone, Wi-Fi)	Sound
window glass	mostly through	mostly through	mostly reflected; some through
brick wall	reflected and absorbed; none through	weakened, but enough gets through	almost all reflected; a tiny part through
metal sheet, foil	reflected	reflected	mostly reflected; some through
your hand	red light a little way through; the rest absorbed or reflected	partly through, partly absorbed	partly through

Glass is another example. It lets through visible light, but not all the waves near it:

• Ultraviolet: ordinary window glass absorbs almost all of the ultraviolet that causes sunburn (UVB). So you hardly ever get sunburnt through a closed window, though glass lets through some of the longer ultraviolet Grade 9: Ultraviolet rays.
• Infrared: glass absorbs the long infrared that warm things give out. A thermal camera, which sees this infrared, can’t see through a window at all. To it the glass is a solid sheet, which shows only its own warmth and, like a dim mirror, a faint reflection of you.
• X-rays pass through skin and muscle easily, but much more of them is absorbed by bone. That is how an X-ray photo shows your bones Grade 9: X-ray imaging.

What makes a wave different from another of the same kind? Its frequency (how many times it vibrates each second) and so its wavelength. Radio waves, light and X-rays are the same kind of wave with very different frequencies. Deep and high sounds are the same kind of wave with different frequencies too. So the full model says: what a material does to a wave depends on the material and on the wave’s frequency.

Everything is made of tiny particles: atoms, and molecules made of atoms. When light enters a material, it meets these particles. In many materials, like brick, wood and your skin, the particles take in the light’s energy. The light is absorbed, and its energy becomes heat. In brick this happens in a very thin layer, less than a millimeter deep. Not a trace of light reaches the far side of a wall.

Some of the light that arrives is reflected instead, scattered in every direction off the rough surface Grade 8: Light. That is the light you see the wall by. A dark wall absorbs most of it; a white wall reflects most of it. You saw this in Grade 8: black things absorb and get hot, white and shiny things reflect Grade 8: The color of an object and radiation.

Glass and water are different. Their particles can’t take in the energy of visible light, so the light passes them by. That is what makes them transparent. But glass’s particles can take in ultraviolet and long infrared, which is why glass stops those.

Colored glass

A piece of red glass, or red cellophane, is clear to red light but not to other colors. White light is all the colors mixed together. The red glass absorbs the blue, green and yellow parts, and transmits only the red. So the light that comes out is red. A filter is a sheet that lets some colors through and absorbs the rest.

A filter doesn’t add color to light. It only takes colors away. A red filter followed by a green one lets through nothing, because the green filter absorbs all the red light that the red filter let through. A yellow filter lets through red and green light (together they look yellow), so a yellow filter followed by a red one still lets red light through.

In Grades 6 to 8 you met light that is reflected and absorbed. There is one more thing that happens when light is transmitted from one material into another: it bends.

Put a pencil in a glass of water and look from the side. The pencil seems broken at the surface. Light from the part under water bends as it comes out of the water into the air, so it reaches your eye from a different direction.

Why does light bend? Light travels at different speeds in different materials: about 300,000 km every second in air, about 225,000 km/s in water and about 200,000 km/s in glass. When a beam enters water at a slant, one side of the beam reaches the water first and slows down first, while the other side is still in the faster air. So the beam swings round, like a row of marching students when one end of the row walks onto soft sand first. This bending of light as it passes from one material into another is called refraction. You will study it fully in Grade 10 Grade 10: Refraction of light.

At the surface, not all the light goes in. Some is reflected. How much depends on the angle.

Shining nearly straight down, only about 2 % of the light is reflected by water, and 4 % by glass. The rest goes in, and bends only a little. But when the beam only skims the surface, much more is reflected, and what goes in bends a lot.

That is why a lake looks like a mirror when you look across it. Light from the far shore and the sky reaches the far water at a low angle, so a large part of it is reflected towards you. Look straight down into the same lake from a boat, and little is reflected: you see into the water, and in a clear lake, down to the bottom.

Now for sound. You learned in Grade 8 that sound travels as a wave in a material: each particle pushes the next one Grade 8: How sound travels. In Grade 9 you heard a fingernail scratch through a table: sound travels very well inside a solid Grade 9: Mechanical waves. The hard part is getting from the air into the wall, and out again.

Here is what happens. The sound in your neighbor’s room pushes and pulls on the wall, many times a second. The wall is heavy and stiff, so it hardly moves: most of the sound bounces back into their room. That is the reflection you met as echoes in Grade 9 Grade 9: Reflection of sound. But the wall does move, a tiny amount, far less than the width of a hair. As it shakes, its other face pushes and pulls on the air in your room, and that makes a new, much weaker sound wave on your side. So sound gets through a wall by shaking it.

Two things decide how much the wall shakes:

• How heavy the wall is. A heavier wall is harder to shake, so less sound gets through. This is why a brick wall stops more sound than a wooden door, and a thick wall more than a thin one.
• The pitch of the sound. A low, deep sound pushes the wall slowly. Each push lasts long enough to get the heavy wall moving. A high sound changes direction so fast that the wall hardly has time to move before the push reverses. So low sounds get through far better than high ones.

That is the drum and the singer. The drum’s boom is low, and gets through. The words of a song are carried mostly by higher sounds, and they are stopped. A simple rule describes both: double the weight of the wall, or double the frequency of the sound, and only about a quarter as much sound energy gets through.

The numbers come from the simple rule above, which engineers call the mass law. Real walls let through a little more, because of their stiffness, and because sound sneaks through any gap, under the door or round a window frame. But the pattern is right: heavier walls and higher pitches let less through.

How can 1 part in 100,000 still be heard? Because your ears are amazing. You learned in Grade 8 that each 10 dB means ten times the sound energy Grade 8: Intensity of sound. Your ears hear sounds from 0 dB up to about 120 dB: the loudest sound you can stand has a million million times the energy of the faintest you can hear. A wall that lets through 1 part in 100,000 makes a sound 50 dB quieter. Music at 90 dB next door reaches you at about 40 dB: as loud as a quiet library, and easy to hear at night.

Soft things absorb sound

So far the wall has mostly reflected the sound. Soft, porous things, like curtains, carpets, cushions, foam and clothes, are different: they absorb sound. The air pushes into their tiny holes and fibers, rubs against them, and the sound’s energy becomes a little heat. That is why an empty room with bare walls echoes, but the same room full of furniture and people doesn’t Grade 9: Acoustic protection.

But a curtain is light and full of holes, so it lets plenty of sound through as well. To keep sound out, you need something heavy and airtight, like a wall. To stop sound echoing inside a room, you need something soft. The two jobs need different materials.

Here is the last puzzle. Sound needs a material, such as air, water or a wall, to travel. Light doesn’t: sunlight crosses 150 million kilometers of empty space Grade 8: Mechanical and electromagnetic waves. Why the difference?

A sound wave is particles pushing their neighbors. Where there are no particles, there is nothing to push, and sound can’t go on.

A light wave is something quite different. It isn’t particles moving. It is a ripple in electric and magnetic fields. You met fields in How Can Something Push Without Touching?: the space round a magnet or a charge, where it pushes and pulls on other things. A magnet pulls a pin across empty space, with nothing in between. Fields don’t need any material: they can fill empty space.

In a light wave, the electric field gets stronger and weaker, and points one way, then the other, over and over. A changing electric field makes a magnetic field, and a changing magnetic field makes an electric field. So each part of the wave makes the next, and the ripple carries itself along, with nothing to push. That is why light is called an electromagnetic wave, and why it can cross empty space.

For the deeper story, how scientists worked out that light is an electromagnetic wave, and why it also behaves like a stream of particles, go further still to Is Light a Wave or a Stream of Particles? in Going Further 9–12. It also explains the other reason you can hear round a corner but not see round it: sound waves spread out round edges far more than light.

• How Can Something Push Without Touching? The electric and magnetic fields that carry light across empty space, and how they push.
• Is the Earth Really Getting Warmer? Carbon dioxide in the air is transparent to sunlight but absorbs the infrared the Earth gives out: the same model, at the scale of the whole planet.
• Going further still: Is Light a Wave or a Stream of Particles? in Going Further 9–12: how light was shown to be a wave, how it bends round edges, and why it also arrives in packets. And Is Phone Radiation Dangerous? for what happens when your body absorbs radio waves, light and X-rays.