Propagation of Sound Waves
ICSE Class 9 · Physics · Chapter 8
What this chapter covers
- Wave motion
- Transverse and longitudinal waves
- Nature of sound waves
- Terms used to describe a wave
- Speed of sound in different media
- Speed of sound compared with speed of light
- Infrasonic, sonic and ultrasonic frequencies
Key points
- A wave carries energy from place to place; the particles of the medium only vibrate about their mean positions.
- Mechanical waves need a medium that has inertia and elasticity.
- Transverse waves (crests and troughs) have particles vibrating perpendicular to the wave; longitudinal waves (compressions and rarefactions) have particles vibrating along it.
- Sound is produced by vibrating bodies and travels in air as a longitudinal wave; it cannot travel through a vacuum.
- Amplitude (m), time period (s), frequency (Hz), wavelength (m) and velocity (m s-1) describe a wave; f = 1/T.
- Wave equation: V = fλ.
- Sound is fastest in solids, slower in liquids and slowest in gases (about 340 m s-1 in air, 1500 m s-1 in water, about 5000 m s-1 in steel).
- The speed of sound in air increases with temperature and humidity; it does not depend on pressure at constant temperature, nor on frequency or amplitude.
- Light (about 3 × 108 m s-1) is far faster than sound, so lightning is seen before thunder is heard; distance = speed of sound × time gap.
- Infrasonic: below 20 Hz; sonic: 20 Hz–20 kHz; ultrasonic: above 20 kHz.
- Ultrasound is used in medical imaging, SONAR, cleaning, crack detection and breaking kidney stones.
- Ultrasonic and infrasonic describe frequency; supersonic and subsonic describe the speed of a body.
Questions answered
What is a wave? Does a wave carry matter?
A wave is a disturbance that travels through a medium carrying energy. It does not carry the particles of the medium along with it.
Distinguish between transverse and longitudinal waves, with one example of each.
In a transverse wave particles vibrate perpendicular to the direction of the wave (a wave on a string); in a longitudinal wave they vibrate along it (sound in air).
Why can transverse mechanical waves not travel through a gas?
A gas has no rigidity to pull back a layer displaced sideways, so it cannot carry transverse vibrations.
Explain why sound in air is a longitudinal wave.
The air molecules vibrate back and forth along the direction in which the sound travels, forming compressions and rarefactions.
Describe an experiment to show that sound needs a medium.
A ringing electric bell in a bell jar becomes fainter as the air is pumped out and is almost inaudible when the jar is nearly empty of air, though the hammer is seen striking.
Define wavelength and frequency, and state their SI units.
Wavelength is the distance between two consecutive particles in the same state of vibration (metre). Frequency is the number of vibrations per second (hertz).
Derive the relation V = fλ.
In one time period T the wave advances by one wavelength λ, so V = λ/T. Since f = 1/T, V = fλ.
A wave has a frequency of 50 Hz and a wavelength of 6.8 m. What is its speed?
V = 50 × 6.8 = 340 m s-1.
Past board exam questions
ICSE 2019 sample paper · 2 marks
A sound wave of frequency 500Hz and wavelength 0.66m is travelling in a medium. Calculate the velocity of the wave in the medium.
Show answer
v = fλ = 500 × 0.66 = 330 m s⁻¹.
ICSE 2019 sample paper · 2 marks
Sound waves A & B are travelling in two different media. Find which wave will be travelling faster, when: (i) A is travelling in water and B is travelling in CO₂. (ii) A is travelling in CO₂ and B is travelling in hydrogen Also support your answers with reasons.
Show answer
(i) Wave A travels faster. Sound travels faster in liquids (water) than in gases (CO₂) because liquids are far more elastic (less compressible).
(ii) Wave B travels faster. In gases the speed of sound is inversely proportional to the square root of the density; hydrogen is much less dense than CO₂, so sound travels faster in hydrogen.
ICSE 2019 sample paper · 3 marks
(i) Differentiate between the terms supersonic and ultrasonic. (ii) State two uses of ultrasound.
Show answer
(i) Supersonic refers to a speed greater than the speed of sound in air (e.g. a supersonic jet); ultrasonic refers to sound of frequency above 20 kHz, beyond the upper limit of human hearing.
(ii) Uses of ultrasound: 1. Sonar — measuring the depth of the sea and locating submarines or shoals of fish. 2. Medical imaging (ultrasonography) of internal organs. (Also: cleaning of fine machine parts, detecting flaws in metals.)
ICSE 2019 sample paper · 3 marks
State three factors on which the speed of sound depends.
Show answer
1. The density of the medium (in gases, speed ∝ 1/√density).
2. The temperature of the medium (speed increases with rise in temperature).
3. The humidity of the air (speed increases with humidity).
(Also the elasticity of the medium and wind direction.)
ICSE 2019 sample paper · 4 marks
The speed of sound in air is 320 ms⁻¹ and in water it is 1600ms⁻¹. It takes 2.5 s for sound to reach a certain distance from the source placed in air. (i) Find this distance. (ii) How much time will it take for sound to travel the same distance when the source is in water?
Show answer
(i) Distance = speed × time = 320 × 2.5 = 800 m.
(ii) Time = distance ÷ speed = 800 ÷ 1600 = 0.5 s.