Sound Waves

IGCSE Edexcel Physics
3.2, 3.23 Sound as a longitudinal wave, echoes, ultrasound and sonar
Key Concepts: Sound is a longitudinal wave — compressions and rarefactions travel through a medium. Sound cannot travel through a vacuum. Speed of sound in air ≈ 340 m/s. Ultrasound (frequency >20 000 Hz) can be used for medical imaging and sonar. Distance = speed × time (halved for echo calculations).

Section A — Properties of Sound

1. State that sound is a longitudinal wave. Describe compressions and rarefactions. [3]
2. Explain why sound cannot travel through a vacuum, but light can. [2]
3. State how the speed of sound compares in solids, liquids and gases. [2]
4. A loudspeaker produces a note of frequency 170 Hz. Calculate the wavelength in air (speed of sound = 340 m/s). [2]

Section B — Reflection of Sound and Echoes

5. Explain how an echo is produced. [2]
6. A bat emits a sound pulse and hears the echo 0.06 s later. The speed of sound is 340 m/s. Calculate the distance to the object. [3]
7. A ship uses sonar to find the depth of the sea. A pulse returns after 0.4 s. Speed of sound in water = 1500 m/s. Calculate the depth. [3]

Section C — Ultrasound

8. Define ultrasound and state its frequency range. [2]
9. Describe how ultrasound is used to produce an image of a fetus during pregnancy. [3]
10. Give one advantage of using ultrasound rather than X-rays for medical imaging. [1]

Total marks: 23

Mark Scheme

1. Sound is a longitudinal wave [1]; compressions: regions where particles are pushed together (high pressure) [1]; rarefactions: regions where particles are spread apart (low pressure) [1] [3]
2. Sound needs a medium (particles) to travel — it requires vibrating matter [1]; light is an electromagnetic wave and can travel through a vacuum as it does not require a medium [1] [2]
3. Sound travels fastest in solids, slower in liquids, slowest in gases [2]
4. $\lambda = v/f = 340/170 = 2\,\text{m}$ [2]
5. Sound waves reach a surface and are reflected [1]; the reflected waves return to the listener as an echo [1] [2]
6. Total distance = $v \times t = 340 \times 0.06 = 20.4\,\text{m}$ [1]; distance to object = $20.4/2 = 10.2\,\text{m}$ [2] [3]
7. Total distance = $1500 \times 0.4 = 600\,\text{m}$ [1]; depth = $600/2 = 300\,\text{m}$ [2] [3]
8. Ultrasound is sound with frequency above the upper limit of human hearing [1]; above 20 000 Hz (20 kHz) [1] [2]
9. Pulses of ultrasound are directed into the body [1]; they reflect at boundaries between different tissues [1]; the reflected pulses are detected and the time delays are used to build up an image [1] [3]
10. Ultrasound is not ionising radiation (unlike X-rays) so it is safer for the fetus/patient [1]