KS3 · Physics

Speed of sound in air, water, solids

Flash... then, seconds later, BOOM. Why do your eyes and ears disagree? And how can a dolphin judge distance by timing its own click coming back?

Physics · Particle model

What decides how fast sound travels?

Sound is a vibration handed on from particle to particle, and the material it travels through is called the medium. Pick air, water or steel and compare how close the particles sit, and how fast the vibration gets handed on.

Selected state

Gas: air

Spacing

Far apart compared with a liquid or a solid

Motion

Each particle only vibrates back and forth. It is not carried forwards.

Speed of sound near the Earth's surface: about 340 m/s. Picture the vibration being handed on across big gaps: with the particles so far apart, each hand-on takes longer. That makes air the slowest of the three.

Physics · Sound

Which of these sounds right to you?

Four students are arguing about how sound travels.

Which one is closest to what you think right now?
How sure are you?

Physics · Sound

Thunder, with a stopwatch

A lightning strike is 1,000 m away. You see the flash. How many seconds later do you hear the thunder?

Your estimate

5s

0s10s

Two ways to measure the speed of sound in air

One-way methodvsEcho method

Both end with speed = distance ÷ time. Watch which distance goes in.

Focus

Distance used in the sum

One-way method

The distance you measured with a trundle wheel or long tape measure

Echo method

Double the distance to the wall, because the sound goes there and back

The insight

This is the difference that catches people out. In an echo the sound makes a round trip, so measure the gap to the wall and then double it.

What starts and stops the timer

One-way method

Starts when you see the blocks of wood hit (or a flag lowered). Stops when you hear the sound.

Echo method

Starts when the blocks are hit. Stops when you hear the echo.

Where you need to be

One-way method

The timer needs to be over about 150 m away

Echo method

In front of a large, flat, hard outdoor wall

Accuracy

One-way method

Often less accurate than the echo method

Echo method

Often more accurate than the one-way method

Ways to reduce errors (the same for both)

One-way method

Longer distances and times; film the stopwatch and watch in slow motion; repeat and calculate a mean; make the sound short and sharp

Echo method

Longer distances and times; film the stopwatch and watch in slow motion; repeat and calculate a mean; make the sound short and sharp

Worked example: the echo method

Problem

You stand 180 m from a large, flat, hard wall and clap two blocks of wood together. You time the echo and get 1.09 s. What is the speed of sound in air?

Physics · Echoes

Spot the slip in the SONAR answer

A boat sends a SONAR pulse straight down. The echo from the sea floor comes back after 0.7 s. Sound travels at 1,500 m/s in water. How deep is the water?

A student's answer — which line goes wrong?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why the material decides how fast sound travels, and how a delayed echo turns time into distance

What you need to know

  • Sound travels through a medium (the material it passes through) as a vibration handed on from particle to particle. Each particle only vibrates back and forth.
  • The medium decides the speed of sound, not the frequency. Particles that are closer together and held more firmly pass the vibration on faster, so sound is fastest in solids and slowest in gases.
  • speed = distance ÷ time, and distance = speed × time. With an echo the sound goes there and back, so the distance it travels is double the gap to the wall.

The big picture

Sound is a vibration handed on from particle to particle, and the material it travels through (the medium) decides how fast it goes: about 340 m/s in air, about 1,500 m/s in water and nearly 6,000 m/s in steel. A higher frequency does not make sound faster. With speed = distance ÷ time you can measure the speed of sound, work out how far away thunder is, and find distances by echo, remembering that an echo goes there and back.

Key points

1The speed of sound is the distance a sound pulse travels each second, in metres per second (m/s).
2About 340 m/s in air, about 1,500 m/s in water and nearly 6,000 m/s in steel: the closer and more firmly held the particles, the faster the vibration is handed on.
3All frequencies travel at the same speed in one medium. A higher frequency means more pulses each second, not a faster wave.
4Sound takes time to travel, so thunder arrives after the flash. Light is about a million times faster than sound.
5To measure the speed of sound, use long distances, repeat and take a mean, and remember the doubled distance in the echo method.
6To find a distance by echo (SONAR, echolocation), work out speed × time, then halve it.

Worked example

Problem

A class uses the one-way method. The timer stands 200 m from the blocks and measures 0.59 s between seeing them hit and hearing the sound. What speed of sound do they get?

⚠ Watch out

Using the wrong distance in an echo sum. The sound travels to the wall and back, so the distance it travels is double the gap to the wall. And when you are asked how far away the object is, halve the distance travelled.

🧠

Memory hook

A line of people passing a nudge: shoulder to shoulder, it races along; spread across a field, each must reach for the next. Closer and firmer means faster. Echoes: there and back, so halve it.

✓

Check yourself

Rank air, water and steel for the speed of sound and give the particle reason. Then: why can't a high-frequency sound beat a low one across a hall?

Flashcards

(15)
What is the speed of sound?
The distance a sound wave pulse travels each second, measured in metres per second (m/s).
What is a medium?
The material that a wave travels through. For sound, that could be air, water or steel.
What do the particles of a medium do as sound passes through?
They vibrate back and forth. They are not carried forwards. The pulse moves on, the particles do not travel with it.
Two sounds with different frequencies travel through the same medium. Which is faster?
Neither. They travel at the same speed. A higher frequency means more pulses each second, not a faster wave. The medium decides the speed.
Roughly how fast does sound travel in air, water and steel?
Air: about 340 m/s (near the Earth's surface). Water: about 1,500 m/s. Steel: nearly 6,000 m/s.
Why is sound faster in liquids and solids than in gases?
The particles are much closer together, so the vibration is passed from particle to particle more quickly.
Why is sound faster still in a solid than in a liquid?
Strong forces hold the particles of a solid firmly together.
Write the speed equation, and the rearrangement for distance.
speed = distance ÷ time, and distance = speed × time.
How long does sound take to travel 1 m, 100 m and 340 m in air?
About 0.003 s, 0.3 s and 1 s.
Why do you see lightning before you hear the thunder?
Light travels through air about a million times faster than sound. You hear a sound when its waves arrive at your ears, which takes longer.
What is an echo?
A sound wave that reflects from a hard surface, travels back and is heard again.
In the echo method, which distance goes into speed = distance ÷ time?
Double the distance to the wall, because the sound goes there and back.
Why does the one-way method need the timer to be over about 150 m away?
Over shorter distances, reaction time makes the timing errors too large.
Name ways to reduce errors when measuring the speed of sound.
Measure longer distances and times (errors become a smaller proportion). Film the stopwatch and watch in slow motion. Repeat and calculate a mean. Make the sound short and sharp.
How do SONAR and dolphin echolocation find a distance?
Send out a sound and time the echo. The longer it takes to return, the further away the object. Distance travelled = speed × time, and the object is half that distance away.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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