Propagation of Sound Waves Class 9 ICSE — Physics Chapter 8
Propagation of sound waves class 9 ICSE is explained below section by section. This page follows the Class 9 Physics textbook, chapter 8, keeping the book’s own numbers and titles. Every heading matches the printed chapter, so you can move through both together. One caution: the book prints the number 8.5 twice — once for propagation in a medium and once for the wavelength–velocity–frequency relation. This page keeps both exactly as printed.
Download Propagation of Sound Waves Class 9 ICSE Physics Chapter 8 PDF
Download the Propagation of Sound Waves Class 9 ICSE Physics chapter PDF to revise offline or print for practice. It is the full chapter as printed in the book — 12 pages, 12 sections, 63 questions.
What is inside Chapter 8
12 pages · 12 sections · 2 worked examples · 63 exercise questions
| Section | Title |
|---|---|
| 8.1 | SOUND AND ITS PRODUCTION FROM VIBRATIONS |
| 8.2 | SOUND PROPAGATION REQUIRES A MATERIAL MEDIUM |
| 8.5 | PROPAGATION OF SOUND IN A MEDIUM |
| 8.4 | SOME TERMS RELATED TO WAVE MOTION |
| 8.5 | RELATIONSHIP BETWEEN THE WAVELENGTH, WAVE VELOCITY AND FREQUENCY |
| 8.6 | SPEED OF SOUND IN DIFFERENT MEDIA |
| 8.7 | FACTORS AFFECTING THE SPEED OF SOUND IN A GAS |
| 8.8 | FACTORS NOT AFFECTING THE SPEED OF SOUND IN A GAS |
| 8.9 | COMPARISON OF SPEED OF SOUND WITH SPEED OF LIGHT |
| 8.10 | EXPERIMENTAL DETERMINATION OF SPEED OF SOUND IN AIR |
| 8.11 | INFRASONIC, SONIC AND ULTRASONIC FREQUENCIES |
| 8.12 | ULTRASOUND AND ITS APPLICATIONS |
- EXERCISE 8 (A): 35 questions
- Multiple choice type :: 13 questions
- EXERCISE 8 (B): 12 questions
- Multiple choice type:: 3 questions
8.1 SOUND AND ITS PRODUCTION FROM VIBRATIONS
The chapter opens with a definition to learn exactly: sound is a form of energy that produces the sensation of hearing in our ears. The whole chapter explains how that energy moves from a vibrating body to your ear.
The book proves that a vibrating body is the source through five small experiments. A rubber string plucked between the teeth hums while it vibrates. A paper rider on a stretched wire flies off when the wire vibrates. A tuning fork arm knocks a suspended table-tennis ball to and fro. A drum membrane vibrates, and you can feel it. A plucked sitar string and a blown whistle behave the same way. In every case, sound ceases the moment vibration ceases.
Sound is therefore a form of energy. Mechanical energy starts the vibration. The vibrating body transfers that energy to the medium as waves. The waves reach your ears, vibrate the ear drum, and you perceive sound.
8.2 SOUND PROPAGATION REQUIRES A MATERIAL MEDIUM
The bell jar experiment delivers this section’s conclusion. An electric bell hangs inside an airtight jar connected to a vacuum pump. While the jar holds air, you hear the bell ring. As the pump removes air, the loudness falls. At full vacuum, you still see the hammer striking the gong, but you hear nothing.
The conclusion: a material medium is necessary for the propagation of sound, and sound cannot travel through vacuum. That is why the moon is silent — there is no medium between two people on the surface. Light needs no medium, so sunlight still reaches the moon.
The medium must satisfy three requisites. It must be elastic, so its particles return to their initial positions after displacement. It must have inertia, so its particles can store mechanical energy. It should be frictionless, so energy is not lost as sound propagates through it.
A common exam check: sound fails in vacuum because there are no particles to form compressions and rarefactions. The source is still vibrating. The missing medium stops the sound, not the source.
8.5 PROPAGATION OF SOUND IN A MEDIUM
The book builds a longitudinal wave with a thin metal strip. When the strip pushes right, it crowds the air layers in front of it. That crowded, high-pressure region is a compression C. When the strip pulls left, it leaves a low-pressure gap. That gap is a rarefaction R. One full to-and-fro motion makes one compression plus one rarefaction, which together form one longitudinal wave.
The key fact: particles vibrate about their mean positions but do not travel with the wave. Density and pressure are maximum at compressions and minimum at rarefactions. Sound in air travels as longitudinal waves, and such waves form in solids, liquids and gases.
The pond-stone example shows the other wave type. A dropped stone sends circular ripples outward. A cork floating nearby bobs up and down; it does not move ahead with the ripple. That is a transverse wave, made of crests and troughs. Transverse waves form only in solids and on liquid surfaces. They cannot form inside liquids or in gases.
Wave motion has two characteristics. A wave is produced by a periodic disturbance at a point in the medium. Particles vibrate about their mean positions while energy is transferred with constant speed from one place to another.
8.4 SOME TERMS RELATED TO WAVE MOTION
This section fixes the vocabulary of the chapter. Amplitude a is the maximum displacement of a particle on either side of its mean position; its unit is the metre. Time period T is the time for one complete vibration, in seconds. Frequency f is the number of vibrations per second, in \text{s}^{-1} or hertz (Hz).
The three are linked by f = \frac{1}{T}. Frequency equals the vibration frequency of the source. It does not depend on the amplitude of vibration or on the medium.
Wavelength \lambda is the distance the wave travels in one time period, in metres. In a longitudinal wave, it is the distance between two consecutive compressions or rarefactions. In a transverse wave, it is the distance between two consecutive crests or troughs. Wave velocity V is the distance the wave travels in one second. It is the speed at which energy transfers, not the velocity of a single particle. It stays constant for a given medium.
Two graphs test your reading. A displacement–time graph shows amplitude a and time period T for one particle at a given position. A displacement–distance graph is a snapshot of the wave at one instant; it shows amplitude a and wavelength \lambda.
Worked pattern: a source makes 120 vibrations in 40 s. Frequency f = \frac{120}{40} = 3\ \text{Hz}. Time period T = \frac{1}{f} = \frac{1}{3}\ \text{s}.
8.5 RELATIONSHIP BETWEEN THE WAVELENGTH, WAVE VELOCITY AND FREQUENCY
This second 8.5 derives the central formula of the chapter straight from the definition of wavelength. By definition, \lambda is the distance travelled in one time period T. Distance equals speed times time, so \lambda = V \times T.
Since T = \frac{1}{f}, substitute to get V \times \frac{1}{f} = \lambda. That rearranges to the relation V = f \lambda. Wave velocity equals frequency times wavelength. This formula handles most numericals in this Class 9 ICSE chapter.
Worked example: a wave of frequency 850 Hz has wavelength 0.4 m. Its velocity is V = 850 \times 0.4 = 340\ \mathrm{m\,s^{-1}}. Flip it once: with V = 340\ \mathrm{m\,s^{-1}} and f = 500\ \text{Hz}, the wavelength is \lambda = \frac{340}{500} = 0.68\ \text{m}.
Watch the trap. The formula tempts you into thinking that raising frequency raises speed. It does not. The medium fixes V. When f rises, \lambda shrinks to keep V = f\lambda constant. Section 8.8 states this directly.
8.6 SPEED OF SOUND IN DIFFERENT MEDIA
Sound does not reach you instantly. The hammer is seen striking before the sound arrives. Speed depends on two properties of the medium: elasticity E and density \rho. The starting relation is V = \sqrt{\frac{E}{\rho}}.
Newton’s formula: he assumed propagation is isothermal, so no temperature change occurs. Then E = P, the gas pressure, giving V = \sqrt{\frac{P}{\rho}}. For air at N.T.P. this gives about 279.5\ \mathrm{m\,s^{-1}}, lower than the experimental value of nearly 330\ \mathrm{m\,s^{-1}}.
Laplace’s correction: he treated propagation as adiabatic, with no heat exchange. Then E = \gamma P, where \gamma = C_p/C_v = 1.4 for air. The corrected formula V = \sqrt{\frac{\gamma P}{\rho}} gives about 330.7\ \mathrm{m\,s^{-1}} at N.T.P., matching experiment.
The speed differs with the medium: about 330\ \mathrm{m\,s^{-1}} in air, 1450\ \mathrm{m\,s^{-1}} in water, 5100\ \mathrm{m\,s^{-1}} in steel. Solids are much more elastic, so the order is solid, liquid, gas. Two everyday proofs: a very long struck steel bar yields two sounds, one through steel and one through air, and people press an ear to a railway track to sense an approaching train early.
Worked pattern: a steel rail is 1020 m long. Through steel at 5100\ \mathrm{m\,s^{-1}}, sound takes about 0.2 s. Through air at 330\ \mathrm{m\,s^{-1}}, it takes about 3.1 s. The steel-borne sound arrives first.
8.7 FACTORS AFFECTING THE SPEED OF SOUND IN A GAS
Four factors change the speed. Density: from V = \sqrt{\frac{\gamma P}{\rho}}, we get V \propto \frac{1}{\sqrt{\rho}}. Oxygen is 16 times as dense as hydrogen, so sound in hydrogen is \sqrt{16} = 4 times faster.
Temperature: speed rises with temperature because heated gas is less dense. On the Kelvin scale, V \propto \sqrt{T}. For small rises, the book gives V_t = V_0 + 0.61t, where t is the Celsius rise.
Worked example: at 40^\circ\text{C}, V = 330 + 0.61 \times 40 = 354.4\ \mathrm{m\,s^{-1}}.
Humidity: water vapour is about \frac{5}{8} as dense as dry air. Moist air is less dense, so sound travels faster in humid air than in dry air. Wind: the wind speed adds when wind blows with the sound and subtracts when it blows against it. With still-air speed 330\ \mathrm{m\,s^{-1}} and a 12\ \mathrm{m\,s^{-1}} wind, the speed is 342\ \mathrm{m\,s^{-1}} downwind and 318\ \mathrm{m\,s^{-1}} upwind.
8.8 FACTORS NOT AFFECTING THE SPEED OF SOUND IN A GAS
Three factors leave the speed unchanged. Pressure: in V = \sqrt{\frac{\gamma P}{\rho}}, doubling P doubles \rho as well, because the gas occupies half the volume while its mass stays constant. The ratio P/\rho is unchanged, so V is unchanged.
Amplitude: no effect. Frequency or wavelength: no effect. A loud high-pitched note and a soft low note travel at the same speed in the same air.
The table below separates the factors that change the speed from the factors that do not. Use it as a quick revision sheet for the factor questions.
| Factor | Effect on the speed of sound in a gas |
|---|---|
| Density \rho | Affects — V \propto \frac{1}{\sqrt{\rho}} |
| Temperature T | Affects — V_t = V_0 + 0.61t |
| Humidity | Affects — speed rises as moisture lowers density |
| Wind direction | Affects — V + W with the wind, V - W against it |
| Pressure P | No effect — P/\rho stays constant |
| Amplitude | No effect |
| Frequency or wavelength | No effect |
8.9 COMPARISON OF SPEED OF SOUND WITH SPEED OF LIGHT
The book gives four points of distinction. Light travels in vacuum; sound cannot. Light in air moves at about 3 \times 10^{8}\ \mathrm{m\,s^{-1}}, roughly a million times sound’s 330\ \mathrm{m\,s^{-1}}. Light slows in optically denser media, while sound speeds up in mechanically denser media. Light is a transverse electromagnetic wave; sound in air is a longitudinal mechanical wave.
These differences explain everyday observations. Lightning is seen before thunder is heard, although both start together. The starter’s smoke is seen before the gun’s crack reaches the spectator. A batsman’s stroke is seen before the sound of the hit arrives.
Worked example: thunder is heard 3 s after the flash. With V = 340\ \mathrm{m\,s^{-1}}, the storm is about 340 \times 3 = 1020\ \text{m} away. Light’s own travel time is ignored because light is a million times faster.
8.10 EXPERIMENTAL DETERMINATION OF SPEED OF SOUND IN AIR
The method uses two observers, two guns and two stopwatches. Observers at A and B stand about 1 km apart, facing each other in still air. A fires. B starts the watch on seeing the flash and stops it on hearing the sound, recording time t_1. B then fires, and A records t_2.
The average t = \frac{t_1 + t_2}{2} is the time sound takes to cover the known distance d. The speed is V = \frac{d}{t}. Averaging the two times removes the error caused by wind, which helps sound one way and hinders it the other.
The result is not perfectly accurate. The book lists the limits: personal error of the two observers, and changing temperature and humidity between the two places. The method assumes light’s travel time is negligible.
Worked pattern: d = 1100\ \text{m}, t_1 = 3.30\ \text{s}, t_2 = 3.10\ \text{s}. Then t = 3.20\ \text{s} and V = \frac{1100}{3.20} \approx 343.75\ \mathrm{m\,s^{-1}}, inside the expected 330 to 345 range. For the Class 9 Physics syllabus and examination format, check the official CISCE website.
8.11 INFRASONIC, SONIC AND ULTRASONIC FREQUENCIES
For an average person, the audible range is 20 Hz to 20 kHz. The ear is not equally sensitive across this range. It is most sensitive between 2000 Hz and 3000 Hz, where even a very feeble sound is heard. High-frequency sensitivity falls with age: a child may hear up to about 30 kHz, while an older person often stops around 12 kHz.
The bands are set by the human ear. Below 20 Hz is infrasonic. From 20 Hz to 20 kHz is sonic, or audible. Above 20 kHz is ultrasonic.
Animals extend both ends of the range. Dogs hear up to about 50 kHz, bats to about 100 kHz, and dolphins to about 150 kHz. Elephants and whales produce infrasonic sound below 20 Hz.
Classification pattern: 5 Hz is infrasonic, 500 Hz is audible, 30 kHz is ultrasonic. These labels describe the human ear, not a property of the wave itself.
8.12 ULTRASOUND AND ITS APPLICATIONS
Ultrasound is sound above 20 kHz. It travels in a medium at the same speed as audible sound; in air that is 330\ \mathrm{m\,s^{-1}}. At high frequencies it gains two properties audible sound lacks: very high energy, and high directivity from its small wavelength, so it does not bend appreciably at the edges of an obstacle.
These properties make ultrasound useful. Bats navigate by reflected ultrasound. Industry uses it to drill and cut glass, and to clean watch parts and electronic components in a cleaning solution. Medicine uses it in ultrasonography for organ images, in echo cardiography for heart images, in cataract removal, and in breaking small kidney stones. It also detects cracks and cavities in metals, and SONAR uses it for underwater detection.
Ultrasonic and supersonic are not the same. Ultrasonic describes a frequency above 20 kHz. Supersonic describes an object moving faster than the speed of sound in air, about 330\ \mathrm{m\,s^{-1}}. A 40 kHz whistle is ultrasonic; a jet at 400\ \mathrm{m\,s^{-1}} is supersonic.
What to practise from Propagation of Sound Waves Class 9 ICSE
After reading the sections, work through the question blocks below. The names and counts match the printed chapter.
- Exercise 8(A) — 35 questions covering definitions, short answers and numericals.
- Multiple-choice set after Exercise 8(A) — 13 questions.
- Exercise 8(B) — 12 questions on audible range, ultrasound and its applications.
- Multiple-choice set after Exercise 8(B) — 3 questions.
That makes 63 questions in total across the propagation of sound waves class 9 ICSE chapter. Try the Exercise 8(A) numericals after sections 8.5 and 8.6, and Exercise 8(B) after sections 8.11 and 8.12.
More chapters from this Physics book are on the Class 9 Physics hub.
Propagation of Sound Waves Class 9 ICSE frequently asked questions
How do I revise propagation of sound waves class 9 ICSE for the exam?
Work through the sections in book order. Learn the definitions in 8.4, then apply V = f\lambda on the numericals. Finish with the factors table from 8.7 and 8.8, and the ultrasonic-versus-supersonic distinction from 8.12. Other Class 9 subjects are listed on the Class 9 hub.
Why is lightning seen before the thunder sound is heard in Class 9 ICSE Physics?
Light and sound begin together, but light travels about a million times faster than sound in air. The flash reaches you almost instantly, while the thunder sound takes seconds. The time gap lets you estimate the storm’s distance.
Why does the speed of sound not increase when frequency increases in this ICSE chapter?
The medium fixes the speed of sound, not the source. When frequency rises, wavelength shrinks so that V = f\lambda stays constant. Section 8.8 states this directly: speed is independent of frequency.
What is the difference between ultrasonic and supersonic in Class 9 ICSE Physics?
Ultrasonic names a frequency above 20 kHz. Supersonic names an object moving faster than the speed of sound in air, about 330\ \mathrm{m\,s^{-1}}. One is a property of the wave; the other is a property of motion.
Why does sound travel faster in humid air than in dry air in Class 9 ICSE?
Water vapour is about \frac{5}{8} as dense as dry air. Moist air is therefore less dense, and since V \propto \frac{1}{\sqrt{\rho}}, the speed rises.
Reference: ICSE Class 9 Physics textbook material, chapter 8: Propagation of Sound Waves — CISCE-aligned.
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