C. Matter and Energy · Light and Sound

How Sound Travels: Echoes, Absorbing and Changing Sound

C2.4 C2.6 A1.3 A3.2About 40 minutes

Learning goals

I can explain that sound travels as a wave through a medium: a gas, a liquid or a solid.
I can explain why there is no sound in outer space.
I can explain that sound can be reflected (echo) or absorbed.
I can describe ways sound can be modified: made louder, quieter or changed in pitch.
I can use the design process to build and test a sound shield.

Words to know

medium
The material a sound wave travels through: a gas (like air), a liquid (like water) or a solid (like wood).

echo
A sound that reflects (bounces) off a hard surface and comes back to you.

absorb (sound)
To soak up sound energy, so less sound bounces back or passes through.

modify
To change. Sound can be modified to be louder, quieter or a different pitch.

amplify
To make a sound louder.

soundproof
Built to stop sound from getting in or out.

Hello... hello... hello!

Have you ever shouted in an empty gym or under a bridge and heard your voice come back? That is an echo. And have you noticed how quiet everything sounds after a fresh snowfall? Both are clues about how sound travels and what happens when it hits things.

Sound travels as a wave through a medium

When something vibrates, it pushes on the tiny bits (particles) of the material around it. Those bits bump into the bits next to them, and those bump the next ones, and so on. The vibration is passed along like a wave, like when fans at a hockey game stand up and sit down one after the other. The sound wave carries energy, but the air itself does not travel all the way to your ear.

Sound needs a material to travel through. This material is called a medium. Sound can travel through:

gases, like air
liquids, like water (whales and dolphins call to each other underwater)
solids, like wood, metal or the ground (put your ear on a table while someone taps the other end!)

In outer space there is no air, so there is nothing to carry the sound. Space is silent. Astronauts use radios to talk to each other.

The drum vibrates. The vibrations spread out through the air as a sound wave and reach the listener.

How fast is sound?

Sound is fast, but light is much faster.

In air, sound travels about 340 metres every second.
In water, it travels more than four times faster than in air.
In solids like steel, it is faster still.

That is why, in a thunderstorm, you see the lightning before you hear the thunder. The light reaches you almost instantly. The sound takes about 3 seconds to travel 1 kilometre.

What can happen to sound when it hits something?

1. Sound can be reflected. Sound bounces off hard, smooth surfaces like walls, floors, rock cliffs and ice. A sound that bounces back to you is an echo. Empty rooms and gyms are echoey.

2. Sound can be absorbed. Soft materials with lots of tiny holes or air spaces soak up sound energy. Carpets, curtains, couches, foam, winter coats and fresh snow absorb sound. That is why a room sounds quieter once furniture is in it, and why the world sounds hushed after a snowfall.

3. Sound can be modified (changed).

Made louder (amplified): cupping your hands around your mouth, a megaphone, a microphone and speakers.
Made quieter: earmuffs, earplugs, closing a window, noise walls along highways.
Changed in pitch: tightening a guitar string, pressing a finger on it, or blowing across a bottle with more or less water.

A trail through a forest of spruce trees covered in thick fresh snow. — Fresh snow is full of tiny air spaces that absorb sound. That is why a snowy forest seems so quiet. AI-generated image

Sound reflects off a hard surface like a cliff and travels back to you. A sound that comes back is an echo.

Material	What it mostly does to sound	Example
hard, smooth wall	reflects it	echo in a gym
carpet or curtains	absorbs it	quieter bedroom
fresh snow	absorbs it	quiet winter morning
megaphone	modifies it: louder	coach at a track meet
earmuffs	modifies it: quieter	using a snow blower

Sound and materials

Just for fun

Why are echoes terrible at keeping secrets?

They repeat everything you say!

Worked example

Let's use sound to find out how far away a storm is

Kayla sees a flash of lightning. She counts slowly: "one-one-thousand, two-one-thousand..." She hears the thunder when she reaches 9 seconds. About how far away is the storm?

Light reaches us almost instantly, so the flash shows when the lightning happened.
Sound travels about 1 kilometre every 3 seconds.
Make groups of 3 seconds: 9 ÷ 3 = 3 groups.
Each group means 1 kilometre, so the storm is about 3 kilometres away.

The storm is about 3 km away. If the time between the flash and the thunder gets shorter, the storm is getting closer: time to go inside! ("When thunder roars, go indoors.")

Kayla's count on a number line. Every 3 seconds of waiting is about 1 km, so 9 seconds is 3 groups, or about 3 km.

Hands-on activity

Design challenge: build a sound shield

Problem: A baby is sleeping, but a timer (or phone alarm) is beeping. Design a sound shield that makes the beeping as quiet as possible.

You need: a phone or kitchen timer that beeps, a shoebox with a lid, test materials (a towel, cotton balls, bubble wrap, newspaper, aluminum foil, egg cartons, a wool sock), a measuring tape, and a listener.

Ask: How will we know if a shield works? Plan to measure: walk away from the beeping box until you can't hear it any more, and measure that distance. Shorter distance = better shield.
Test with no shield: put the beeping timer in the empty box with the lid on. Measure the distance.
Plan: predict which materials will absorb sound best. (Hint: soft, with air spaces.)
Build: line the box with one material. Keep the same timer, same volume, same box and same listener.
Test: measure the distance again. Repeat with other materials. Record in a table.
Improve: combine the best two materials and test again.
Share: make a bar graph of your results, one bar per material.

String telephone (bonus): poke a small hole in the bottom of two paper cups (a grown-up helps), thread a long string through and knot it inside each cup. Pull the string tight and talk into one cup while a friend listens. Sound travels through the solid string!

What to look for: soft materials like towels, socks and cotton balls usually work best because they absorb sound; foil usually does little. The bar graph should show the shortest bars for the best shields.

Safety: keep the alarm volume comfortable. A grown-up pokes the holes for the string telephone.

Your turn

What is a medium?

Name the three kinds of media sound can travel through, with one example of each.

Why is there no sound in outer space?

What is an echo?

Why does a room with carpet and curtains sound quieter than an empty room?

Name one way to make a sound louder and one way to make it quieter.

Do you see lightning first or hear thunder first? Why?

More practice: warm-up

Can sound travel through water?

Does a hard wall reflect or absorb sound?

Does a thick blanket reflect or absorb sound?

What does amplify mean?

Is sound faster or slower than light?

More practice: on your own

You count 6 seconds between lightning and thunder. About how far away is the storm?

Why does the world sound so quiet after a fresh snowfall?

Why do some highways have tall walls beside them near houses?

You put your ear on a table and a friend taps the other end. Why can you hear it so well?

In the sound shield test, why must you use the same timer at the same volume every time?

Challenge

A music room has lots of echoes. Suggest two changes to fix it, and explain using the words reflect and absorb.

Sound travels about 340 metres each second in air. About how far does it travel in 3 seconds? Does that match the 3-second rule?

In the sound shield test, foam made the beeping fade at 2 m, a towel at 3 m, foil at 8 m and no shield at 9 m. Rank the shields from best to worst and explain.

Check your understanding

Quiz

Sound travels as...

Which can sound not travel through?

An echo happens when sound is...

Which material absorbs sound best?

You count 12 seconds between lightning and thunder. About how far away is the storm?

A megaphone modifies sound by making it...

For parents and teachers

Big idea: Ontario Grade 4 expectation C2.4 asks children to describe properties of sound, including that sound travels through a medium as a wave and that sound can be absorbed or reflected and modified. C2.6 is about how materials interact with sound. The sound shield is an engineering design challenge (A1.3), and graphing results and dividing by 3 for the thunder rule connect science with math (A3.2).

How to help: Next thunderstorm, count the seconds together from indoors. In different rooms (bathroom, bedroom, garage), clap once and ask "Echoey or quiet? Why?"

Common difficulties:

Thinking sound can travel through empty space. Movies often show loud explosions in space; real space is silent.
Thinking air travels from the source to your ear. Only the vibration is passed along.
Mixing up reflect and absorb. Hard and smooth = reflects; soft with air spaces = absorbs.

Safety: keep the alarm at a comfortable volume, and help with poking holes in cups. Thunderstorm rule: "When thunder roars, go indoors."