Pearson Edexcel International GCSE in Physics · 4PH1

Sound Waves

What is actually travelling when you hear something, what you can and cannot hear, and how to measure how fast it goes.

Topic 3 · Waves — one of 10 lessons in this topic, and one of 65 in Physics.

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

  • 3.23 — Sound waves as longitudinal waves that can be reflected and refracted
  • 3.24 — The frequency range for human hearing (bold P statement — Paper 2 only)
  • 3.25 — Practical: investigate the speed of sound in air (bold P statement — Paper 2 only) (required practical)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A loudspeaker cone moves out and in, thousands of times a second. Moving out, it pushes the air molecules in front of it closer together — a COMPRESSION, a region of slightly higher pressure. Moving back, it leaves them further apart — a RAREFACTION, of slightly lower pressure. Those regions travel outwards through the air, and a sound wave is nothing more than that moving pattern of pressure.

This makes sound LONGITUDINAL: the molecules vibrate back and forth ALONG the direction the wave travels, not across it. Each molecule jostles its neighbours and returns to where it started, so no air travels from the speaker to your ear. What arrives is the pattern, and the energy it carries — which is enough to move your eardrum, and in a loud enough sound to rattle a window.

Because the molecules must pass the disturbance on, sound needs a MEDIUM and cannot travel through a vacuum. This was settled in 1660 with an air pump: a bell ringing inside a glass vessel grows fainter as the air is drawn out, until it can plainly be seen ringing and cannot be heard at all. Light passes through the same vacuum without difficulty, which is the clearest single demonstration that the two are not the same kind of wave.

How fast sound goes, and what changes it

  1. In AIR at room temperature, sound travels at about 340 m/s — roughly a million times slower than light, which is why you see lightning before you hear thunder.
  2. Sound travels FASTER in liquids than in gases, and faster still in SOLIDS, because the particles are closer together and pass the disturbance on more quickly. In water it is about 1500 m/s and in steel about 5000 m/s.
  3. It travels faster in WARMER air, because the molecules are moving faster and collide more often.
  4. It does NOT depend on the frequency: every note travels at the same speed, which is why a distant band does not arrive scrambled.
  5. Like every wave it can be REFLECTED — an echo from a cliff or a wall — and REFRACTED, bending as it passes between layers of air at different temperatures, which is why sound carries unusually far across water on a still morning.

Think of it like a crowd being shoved from behind

Picture a dense crowd in a corridor. Someone shoves the person at one end; that person stumbles into the next, who stumbles into the next, and a squeeze travels the length of the corridor while everybody ends up roughly where they began. That travelling squeeze is a compression, and the gap left behind each person as they recover is a rarefaction. Notice three things this explains at once. The crowd has to be there — an empty corridor transmits nothing, which is why sound needs a medium. A denser crowd passes the shove on faster, which is why sound is quicker in solids. And nobody has travelled the length of the corridor, which is why no air moves from the speaker to your ear.

Human hearing covers roughly 20 Hz to 20 000 Hz — from a rumble you feel as much as hear to a whine most adults have already lost. The upper limit falls with age, so a teenager can usually hear several kilohertz higher than a middle-aged adult. Below 20 Hz is INFRASOUND, produced by earthquakes and by some large animals over great distances; above 20 000 Hz is ULTRASOUND, which bats use to navigate and which is used for medical scanning and for cleaning delicate objects.

The echo method for the speed of sound

A student stands 85 m from a large wall and claps. The time between the clap and the echo is measured as 0.50 s. Find the speed of sound.

  1. The sound travels to the wall AND back, so the distance covered is 2 × 85 = 170 m.
  2. speed = distance ÷ time = 170 ÷ 0.50.
  3. = 340 m/s.
  4. The doubling is the whole difficulty of the method: forgetting it halves the answer.

Answer: 340 m/s — and an answer of 170 m/s means the return journey was left out.

That experiment has a real weakness, and knowing it is worth marks. Human reaction time is around 0.2 s, and the interval being measured is only about half a second, so the uncertainty is enormous. The standard improvement is to clap repeatedly IN TIME with the returning echoes and time twenty or thirty intervals at once, then divide — because the reaction-time error is then spread across all of them rather than affecting each. Better still, use two microphones a measured distance apart connected to an electronic timer, which removes human reaction from the measurement entirely.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Sound wave

A longitudinal wave consisting of compressions and rarefactions travelling through a medium. The particles vibrate along the direction of travel and do not move with the wave.

The words this topic is marked on

Compression
A region where the particles are pushed closer together, at slightly higher pressure.
Rarefaction
A region where the particles are further apart, at slightly lower pressure.
Longitudinal
Describes a wave in which the particles vibrate along the direction the wave travels.
Infrasound
Sound below about 20 Hz, too low for human hearing.
Ultrasound
Sound above about 20 000 Hz, too high for human hearing; used in scanning and cleaning.

The facts about the speed of sound that get asked

  • About 340 m/s in AIR at room temperature — roughly a million times slower than light
  • FASTER in liquids than gases, and faster still in SOLIDS, because the particles are closer together
  • FASTER in warmer air
  • INDEPENDENT of frequency — every note travels at the same speed
  • Sound CANNOT travel through a vacuum, because there are no particles to pass the disturbance on

Required practical 3.25 — investigating the speed of sound in air

  1. Measure the distance from a clapping position to a large flat wall with a tape measure, using at least 50 m.
  2. Clap once and listen for the echo, then begin clapping in time with the returning echoes so that each clap coincides with the echo of the one before.
  3. Once the clapping is steady, start a stopwatch on one clap and count 20 further claps, stopping the watch on the twentieth.
  4. Divide the total time by 20 to find the time for one clap-to-echo interval.
  5. The distance travelled in that time is twice the distance to the wall, so calculate speed = 2 × distance ÷ time.
  6. Repeat the whole procedure three times and take a mean.

Variables

Independent (changed) — The distance from the wall, in metres
Dependent (measured) — The time for the sound to travel to the wall and back, in seconds

Control variableWhy it must be held constant
The same walla soft surface gives a weak echo that is hard to hear
Air temperaturesound travels faster in warmer air
Background noiseother sounds make the echo hard to identify

Sources of error

TypeWhat goes wrongWhat to do
JudgementDeciding exactly when the echo returns is difficult, and different people hear it slightly differently.Clap in time with the echoes rather than judging one.
RandomHuman reaction time of about 0.2 s scatters the stopwatch readings badly over a half-second interval.Time 20 intervals at once and divide by 20.
SystematicMeasuring to the wrong point on the wall makes every distance, and so every speed, wrong by the same proportion.Measure to the reflecting face of the wall.

The echo calculation — the step everyone forgets

  1. Note that the sound travels to the reflecting surface AND BACK.
  2. So the distance travelled is TWICE the distance to the wall.
  3. Use speed = distance ÷ time with that doubled distance.
  4. Check the answer is near 340 m/s; exactly half of that means the doubling was left out.

Why sound needs air and light does not

A. SOUND is a vibration OF the particles of a medium, so with no particles there is nothing to vibrate. In a vacuum a ringing bell makes no sound at all.

B. LIGHT is an oscillation of electric and magnetic fields and needs no particles, which is why the same bell can still be seen ringing through the evacuated jar.

Model answer [3 marks]

A student stands 60 m from a wall and hears the echo of a clap 0.35 s later. Calculate the speed of sound. [3]

The sound travels to the wall and back, so the distance covered is 2 × 60 = 120 m. Using speed = distance ÷ time, the speed is 120 ÷ 0.35 = 343 m/s.

Model answer [4 marks]

Explain why a bell ringing inside a jar cannot be heard once the air has been pumped out, although it can still be seen ringing. [4]

Sound is a longitudinal wave that travels as a series of compressions and rarefactions of the particles of a medium, so it needs particles to pass the vibration from one to the next. When the air is pumped out there are almost no particles left in the jar, so the vibration of the bell cannot be transmitted across the space and no sound reaches the observer. Light is an electromagnetic wave, which is an oscillation of electric and magnetic fields and does not require a medium, so it travels through the vacuum unaffected and the bell can still be seen to be ringing.

Not this: Sound travels faster in air than in water, because air is thinner and easier to move through.

This: Sound travels FASTER when the particles are closer together, because each passes the disturbance to the next more quickly. It is about 340 m/s in air, 1500 m/s in water and 5000 m/s in steel.

Mark-losing trap. In an echo question the sound travels there AND BACK — double the distance.

Mark-losing trap. Sound is LONGITUDINAL and cannot travel through a vacuum. Both facts are commonly asked.

Mark-losing trap. Sound is FASTER in solids than in liquids, and faster in liquids than in gases.

Mark-losing trap. Human hearing is about 20 Hz to 20 000 Hz. Learn both ends.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

  1. Grade 6 · State [1 mark] — What type of wave is sound?
  2. Grade 7 · State [2 marks] — What is the approximate range of frequencies that a young human being can hear?
  3. Grade 8 · Calculate [3 marks] — A student stands 75 m from a large wall. The time between a clap and its echo is 0.44 s. Calculate the speed of sound.
  4. Grade 9 · Explain [5 marks] — Select every statement that belongs in a full-mark explanation of why a bell ringing inside a jar cannot be heard once the air is pumped out, although it can still be seen ringing.
  5. 9+ · Analyse [6 marks] — A student measures the speed of sound by clapping once, starting a stopwatch, and stopping it on the echo. They get 290 m/s and conclude that the accepted value of 340 m/s must be wrong. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who took the air away and listened, and the one who first calculated the speed of sound and got it wrong. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Robert Boyle — the one who took the air away and listened

The central claim of this page — that sound needs a medium — was established by Boyle in 1660 with an air pump built by his assistant Robert Hooke. A watch with an alarm, and later a bell, was suspended inside a glass receiver, and as the air was drawn out the sound faded until nothing could be heard, though the striker could plainly be seen moving. Light crossed the same emptied space without difficulty. It is one of the cleanest experiments in the history of physics, and he published the whole procedure, including the trials that did not work.

  • “What did you hear as the air was drawn out of the receiver?”
  • “How did you know the bell was still ringing if you could not hear it?”
  • “What did the candle and the bird tell you about air?”
  • “Why did you publish the experiments that failed?”
  • “What part did Hooke play in building the pump?”

Isaac Newton — the one who first calculated the speed of sound and got it wrong

In the Principia Newton did something nobody had attempted: he derived the speed of sound from the properties of air itself, rather than measuring it. The result came out around fifteen per cent below the measured value, and he knew it. His treatment assumed the compressions happen slowly enough for the air to stay at a constant temperature, and they do not — a compression warms the air slightly, which stiffens it and speeds the wave up. Laplace supplied the correction a century later. It is a rare and instructive case of a great physicist producing a wrong number honestly and leaving the discrepancy on the page.

  • “How can the speed of sound be calculated rather than measured?”
  • “Why did your figure come out lower than the measured value?”
  • “What did you do about the discrepancy you could not explain?”
  • “What properties of the air decide how fast sound travels through it?”
  • “Is a calculation that disagrees with measurement worth publishing?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Robert Boyle in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course