PhysicsICSE Class 9 · Propagation of Sound Waves

Speed of Sound in Different Media and Thunder-and-Lightning Distance Problems (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

Sound travels fastest in solids, slower in liquids and slowest in gases (about 5000 m s⁻¹ in steel, 1500 m s⁻¹ in water, 340 m s⁻¹ in air). In air its speed increases with temperature and humidity but does not depend on pressure at constant temperature, amplitude or frequency. Light is almost a million times faster, so the distance of lightning is about 340 m s⁻¹ times the gap before thunder.

Sound is fastest in solids and slowest in gases; light reaches you almost instantly, so the flash-to-thunder gap t gives distance d = 340 × t.

Illustration drawn for QuestionBuddy.

Understand Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems

Speed in different media. The disturbance is passed on from particle to particle. In solids the particles are tightly bound, so the disturbance is passed on quickly; in gases they are far apart and it is slow. So

$V_{so l i d s} > V_{l i q u i d s} > V_{g a ses} .$

Typical values: steel about 5000 m s⁻¹, water about 1500 m s⁻¹, air about 340 m s⁻¹ at room temperature.

Factors affecting the speed of sound in air.

Temperature: speed increases when the temperature rises.
Humidity: speed increases with humidity (moist air is less dense).
Medium: depends on its nature (density and elasticity).
Pressure: no effect at constant temperature.
Amplitude, frequency, wavelength: no effect; all notes travel at the same speed in the same medium, which is why a band is heard in tune from far away.

Sound and light. Light travels at about $3 \times 1 0^{8}$ m s⁻¹, nearly a million times faster than sound in air. So:

lightning is seen before thunder is heard, though both happen together;
the distance of the flash is $d = V_{so u n d} \times t$ , where $t$ is the gap between seeing the flash and hearing the thunder (the travel time of light is negligible);
the smoke of a starter’s gun is seen before its bang is heard far away.

Two-observer method. If two observers at different distances from a source both time the sound, the speed is $V = \frac{d _{2} - d _{1}}{t _{2} - t _{1}}$ . The reaction time of starting the clock cancels in the difference.

Two-path problems. When the same sound reaches an observer through two media (air and a rail or pipe), the time gap is $Δ t = \frac{d}{V _{ai r}} - \frac{d}{V _{so l i d}}$ , and the sound through the solid arrives first.

Free-fall plus sound. A stone dropped into a well: total time $= \frac{2 h}{g} + \frac{h}{V}$ . Ignoring the sound travel time gives an over-estimate of the depth.

Common error. Do not divide by 2 for thunder: the sound travels only one way (echo problems are the ones with "out and back").

Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems: formulas and key facts

Distance of a flash: $d = V \times t$; V ≈ 340 m s⁻¹ in air; t = flash-to-thunder gap.
Two observers: $V = \frac{d _{2} - d _{1}}{t _{2} - t _{1}}$; Removes the starting-time error.
Two media: $Δ t = \frac{d}{V _{ai r}} - \frac{d}{V _{so l i d}}$; Solid first.
Speed ratio of light and sound: $\frac{3 \times 1 0 ^{8}}{340} \approx 9 \times 1 0^{5}$; Nearly a million.

How to solve Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems questions

Draw the path of the sound and mark whether it goes one way or out and back.

1Convert minutes and kilometres to seconds and metres.
2Use $d = V t$ for a one-way trip.
3For two media compute each travel time and subtract.
4Use the speed of the medium named in the question (no values not given in the question).
5Check the order of arrival: faster medium first.

Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Counting after the flash

Suppose you count 9 seconds between a flash of lightning and the thunder, taking the speed of sound as 340 m s⁻¹.

How far away is the storm?

Try it first, then show the solutionSolution

$d = 340 \times 9 = 3060$ m.

Answer: About 3.06 km.

Memory hook: Each 3 seconds is about a kilometre.

Tricks to remember Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems

Short memory aids. Say each one aloud once, then use it on a question the same day.

Solid, liquid, gas: fast, medium, slow
The closer the particles, the faster the sound.
Use it when: Ordering speeds.
Light first, sound later
Lightning flash is seen first; thunder arrives after d/V.
Use it when: Thunder questions.
Hot and humid is faster
Warmer or moister air carries sound faster.
Use it when: Factor questions.

Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Fastest medium · 1 mark · MCQBoard level

Sound travels fastest through:
(a) air
(b) water
(c) steel
(d) a vacuum

Try it first, then show the solutionSolution

Solids carry sound fastest; there is no sound in a vacuum.

Answer: (c) steel

Example 2: A distant storm · 2 marks · Short answerBoard level

The thunder is heard 9 s after the lightning is seen. If the speed of sound in air is 340 m s⁻¹, how far is the lightning?

Try it first, then show the solutionSolution

Light takes negligible time, so $d = V t$ . (1)
$d = 340 \times 9 = 3060$ m $\approx 3.06$ km. (1)

Answer: 3060 m

Example 3: Rail and air · 3 marks · Short answerHarder board level

A boy at one end of a long straight railway line hears the sound of a hammer hitting the rail at the other end, 1020 m away, once through the rail (speed 5100 m s⁻¹ in the rail, assumed) and again through the air (340 m s⁻¹). Find the two times and the gap between the two sounds.

Try it first, then show the solutionSolution

Air: $t = \frac{1020}{340} = 3$ s. (1)
Rail: $t = \frac{1020}{5100} = 0.2$ s. (1)
Gap $= 3 - 0.2 = 2.8$ s, rail sound first. (1)

Answer: 3 s; 0.2 s; 2.8 s

Example 4: Light and sound · 3 marks · Short answerHarder board level

A flash is seen and the thunder is heard 5 s later (sound speed 340 m s⁻¹, light $3 \times 1 0^{8}$ m s⁻¹). Find the distance, the time light takes, and the ratio of the speeds.

Try it first, then show the solutionSolution

$d = 340 \times 5 = 1700$ m. (1)
Light: $t = \frac{1700}{3 \times 1 0 ^{8}} \approx 5.7 \times 1 0^{- 6}$ s, negligible. (1)
Ratio $= \frac{3 \times 1 0 ^{8}}{340} \approx 8.8 \times 1 0^{5}$ , nearly a million. (1)

Answer: 1700 m; 5.7 μs; about 8.8 × 10⁵

Example 5: Stone in a well · 4 marks · Case-basedHOTS / competency

A stone is dropped into a well 20 m deep ( $g = 10$ m s⁻², speed of sound 340 m s⁻¹).
(a) Find the time for the stone to reach the water.
(b) Find the time for the sound of the splash to come back up.
(c) Find the time from release to hearing the splash.
(d) A student ignores the sound travel time and calculates the depth from the total time using $h = \frac{1}{2} g t^{2}$ . What depth does she get, and what is the percentage error?

Try it first, then show the solutionSolution

(a) $t_{1} = \frac{2 \times 20}{10} = 2$ s. (1)
(b) $t_{2} = \frac{20}{340} \approx 0.059$ s. (1)
(c) Total $\approx 2.059$ s. (½)
(d) $h = 5 \times 2.05 9^{2} \approx 21.2$ m; error $\approx \frac{1.2}{20} \times 100 \approx 6%$ . (1½)

Answer: (a) 2 s (b) 0.059 s (c) 2.059 s (d) 21.2 m; about 6% too large

Where students lose marks in Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems

The slips examiners see most often, and what to write instead.

Mistake: Distance of lightning halved
Thunder travels one way.
Wrong working
d = 340 × 9 / 2
Right working
d = 340 × 9
Do this: Halve only for echoes.
Usually costs: The answer.
Mistake: Speed of sound said to depend on frequency
It depends on the medium (and temperature).
Wrong working
Higher notes travel faster.
Right working
All notes travel at the same speed in a medium.
Do this: Remember the band-in-tune argument.
Usually costs: 1 mark.
Mistake: Order of speeds reversed
Solids carry sound fastest.
Wrong working
Air > water > steel
Right working
Steel > water > air
Do this: Particles closer, speed greater.
Usually costs: 1 mark.

Exam tips

Quote 340 m s⁻¹ (air), 1500 m s⁻¹ (water), 5000 m s⁻¹ (steel) as approximate values only.
State that the travel time of light is negligible.
For two media always compute each time separately.
Give two factors that do affect and two that do not affect the speed of sound.

Practice questions on Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The speed of sound is least in:
(a) iron
(b) water
(c) air
(d) steel
Hint
Gases are slowest.
Answer
(c) air
Q2 · 1 mark · Very short answerBoard level
Write the relation between the frequency and the time period of a wave.
Hint
Reciprocals.
Answer
$f = \frac{1}{T}$ .
Q3 · 2 marks · Short answerHarder board level
Sound travels in sea water at 1500 m s⁻¹. How far does it travel in 0.8 s?
Hint
d = Vt.
Answer
$d = 1500 \times 0.8 = 1200$ m.
Q4 · 3 marks · Short answerHarder board level
A boy hears thunder 7.5 s after a flash (sound speed 340 m s⁻¹). Forty seconds later another flash is followed by thunder after 4.5 s. Find the two distances and the speed with which the storm is approaching (assume it moves along the line towards him).
Hint
Two distances, then change over time.
Answer
$d_{1} = 340 \times 7.5 = 2550$ m; $d_{2} = 340 \times 4.5 = 1530$ m. Moved $1020$ m in 40 s: speed $= 25.5$ m s⁻¹.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): The speed of sound in air is greater on a hot day than on a cold day.
Reason (R): Sound is a transverse wave.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Which is a wrong fact?
Answer
(c)
Q6 · 3 marks · Short answerHOTS / competency
A gun is fired. Observer A is 680 m from it and B is 1020 m from it in the same straight line. A hears the bang 2.0 s after the flash and B 3.0 s after. Find the speed of sound from these readings, and explain why this method is better than dividing 680 m by 2.0 s if the timekeepers start their watches 0.1 s late.
Hint
Use the differences.
Answer
$V = \frac{1020 - 680}{3.0 - 2.0} = 340$ m s⁻¹. A late start adds the same error to both times, so it cancels in the difference but not in 680/2.0.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Order of speeds of sound?Show answer
Solids > liquids > gases.
2.Effect of temperature on speed of sound in air?Show answer
Increases it.
3.Does pressure change the speed (constant temperature)?Show answer
No.
4.Speed of light?Show answer
About 3 × 10⁸ m s⁻¹.
5.Distance of lightning?Show answer
d = V × time gap.

Speed of Sound in Different Media, Factors and Thunder-and-Lightning Problems: frequently asked questions

In which medium does sound travel fastest?

In solids, then liquids, then gases; for example steel about 5000 m s⁻¹, water about 1500 m s⁻¹, air about 340 m s⁻¹.

How do you find how far lightning struck?

Multiply the speed of sound (about 340 m s⁻¹) by the gap in seconds between seeing the flash and hearing the thunder.

What factors affect the speed of sound in air?

Temperature and humidity (both increase it) and the nature of the medium; pressure at constant temperature, amplitude and frequency have no effect.