Practice questions · Speed and echoes

Mind the factor of two.

Seven original Cambridge-style questions. They cover the audible range, the order of the speed of sound in solids, liquids and gases, and echo calculations where the round trip is easy to forget.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Before you start an echo calculation

The sound goes there and back.

Audible range: about 20 Hz to 20 000 Hz for humans.
Speed order: fastest in solids, then liquids, then gases. In air, about 330 to 340 m/s.
Echo equation: 2d = v t. The 2 is the round trip. Divide by 2 to get the one-way distance.

[2 marks]

State the approximate range of frequencies that a healthy human ear can detect.

From about 20 Hz to about 20 000 Hz (20 kHz). ✓✓

[3 marks]

Sound travels at different speeds in solids, liquids, and gases.

(a) Place these three states in order of the speed of sound, from fastest to slowest. [1] (b) Explain, in terms of the particles, why sound travels fastest in a solid. [2]

(a) Solid (fastest), then liquid, then gas (slowest). ✓

(b) In a solid the particles are very close together and tightly linked. ✓ So a vibration is passed from one particle to the next very quickly. ✓

Calculation

[3 marks]

A person stands in a valley and shouts. They hear the echo from a rock face 1.5 s later. The speed of sound in air is 340 m/s. Calculate the distance to the rock face.

Total path: 2d = v t

2d = 340 x 1.5 = 510 m

One way: d = 510 / 2

d = 255 m

The sound travels 510 m in total, so the rock face is half that away.

Calculation

[3 marks]

To measure the speed of sound, a student stands 660 m from a large cliff, claps once, and times the echo at 4.0 s. Calculate the speed of sound from this experiment.

Round trip: distance = 2 x 660 = 1320 m

v = 2d / t = 1320 / 4.0

v = 330 m/s

A sensible value for the speed of sound in air.

Analysis

[2 marks]

In an echo question, a student writes d = v t and calculates the distance to a wall without dividing by two. Explain the error and its effect on the answer.

The timed sound travels to the wall and back, so v t is the total there-and-back distance, which is 2d. ✓
Leaving out the division by two gives a distance twice as large as the real one. ✓

Analysis

[2 marks]

A student says that sound, like light, must slow down when it passes from air into a solid such as steel, because the solid is denser. Explain why this is wrong.

Sound actually travels faster in a solid than in air, not slower. ✓
The particles in a solid are close together and tightly bonded, so they pass the vibration on to each other much more quickly than the widely spaced particles in air. ✓

Light and sound behave very differently when entering a denser medium.

Calculation

[3 marks]

A survey ship sends a pulse of sound straight down to the seabed and detects the reflected pulse 0.40 s later. The speed of sound in seawater is 1500 m/s. Calculate the depth of the water.

Round trip: 2d = v t = 1500 x 0.40 = 600 m

d = 600 / 2

d = 300 m

Depth is half the total path, because the pulse goes down and comes back.

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.