NCERT Class 9 Science Chapter 12: Sound - A Detailed Guide to Sound Waves and Propagation

Introduction to Sound

Sound plays a vital role in our daily lives. It allows us to communicate with one another, enjoy music, and stay aware of our surroundings. From the gentle rustle of leaves to the roaring sound of a jet engine, sound is everywhere. In physics, sound is defined as a form of energy that produces the sensation of hearing in our ears. Just like heat, light, and electrical energy, sound is subject to the law of conservation of energy—it can neither be created nor destroyed, but can only be transformed from one form to another.

Understanding sound involves exploring how it is produced, how it travels through different mediums, how we perceive it, and how it can be reflected or harnessed for advanced technologies like SONAR and medical imaging. This comprehensive guide covers every concept outlined in NCERT Class 9 Science Chapter 12: Sound, designed strictly according to the CBSE curriculum.

Production of Sound

Sound is produced by vibrating objects. Vibration refers to a rapid to-and-fro or back-and-forth motion of an object about its central equilibrium position. Whenever an object vibrates, it imparts kinetic energy to the surrounding air particles, creating sound waves.

Examples of Sound Production

• Stretched Rubber Band: Plucking a stretched rubber band causes it to vibrate rapidly and produce sound.
• Tuning Fork: Striking a tuning fork against a rubber pad causes its prongs to vibrate, producing a clear, continuous musical tone.
• Human Voice: In human beings, sound is produced by the vibration of the vocal cords located in the voice box or larynx.
• Musical Instruments: In a guitar or violin, vibrating strings produce sound; in a drum, a vibrating membrane creates sound; and in a flute, a vibrating air column generates sound.

Propagation of Sound

Once sound is produced by a vibrating body, it needs to travel from the source to the listener. The substance or material through which sound travels is called a medium. Mediums can be solid, liquid, or gas.

When an object vibrates, it sets the particles of the medium around it into vibration. These particles do not travel all the way from the vibrating object to the ear. Instead, a particle of the medium in contact with the vibrating object is displaced from its equilibrium position. It exerts a force on the adjacent particle, displacing it as well. The original particle then returns to its initial position. This continuous process creates a periodic disturbance that travels through the medium, known as a wave.

Sound Needs a Medium to Travel (Bell Jar Experiment)

Sound is a mechanical wave, which means it requires a material medium (solid, liquid, or gas) for its propagation. Sound cannot travel through a vacuum because there are no particles to transmit the disturbance.

Experiment: To demonstrate this, an electric bell is suspended inside an airtight glass bell jar connected to a vacuum pump. When the switch is pressed, the bell rings and can be clearly heard. As the vacuum pump evacuates air from the jar, the sound becomes fainter and fainter. When the air is completely pumped out, no sound can be heard, even though the hammer can still be seen striking the bell. This proves that sound requires a material medium and cannot propagate in a vacuum.

Sound Waves are Longitudinal Waves

Waves can be broadly classified based on the direction of particle vibration relative to the direction of wave propagation:

• Longitudinal Waves: Waves in which the individual particles of the medium vibrate parallel to the direction of propagation of the wave. Sound waves in air and other fluids are longitudinal waves.
• Transverse Waves: Waves in which the particles of the medium vibrate perpendicular to the direction of wave propagation (e.g., light waves, or waves on a plucked guitar string).

When a sound wave travels through air, it creates alternate regions of high density (high pressure) and low density (low pressure):

• Compression (C): A region where particles are crowded together, resulting in high pressure and high density.
• Rarefaction (R): A region where particles are spread apart, resulting in low pressure and low density.

Characteristics of a Sound Wave

A sound wave can be graphically represented as a sine wave showing density or pressure variations as a function of distance or time. Key characteristics include:

The mathematical relationship between speed (v), frequency (ν), and wavelength (λ) is given by the wave formula:

v = ν × λ

Also, since frequency is the reciprocal of time period (ν = 1/T), the speed can be written as:

v = λ / T

Pitch and Loudness

• Pitch: Pitch is the brain's interpretation of the frequency of a sound wave. High-frequency sound waves correspond to high pitch (shrill sound, e.g., a whistle or female voice). Low-frequency sound waves correspond to low pitch (deep sound, e.g., a drum or male voice).
• Loudness: Loudness depends on the amplitude of the vibrating body. A larger amplitude produces a louder sound because it carries more energy. Loudness is proportional to the square of the amplitude of vibration.
• Quality or Timbre: Quality is the characteristic of sound that enables us to distinguish between two sounds having the same pitch and loudness, produced by different sources (e.g., guitar vs. piano).

Speed of Sound in Different Media

The speed of sound depends on the physical properties of the medium through which it travels, mainly its elasticity, density, and temperature.

• State of Matter: Sound travels fastest in solids, slower in liquids, and slowest in gases. This is because particles in solids are packed closely together, allowing vibrations to transfer rapidly.
• Temperature: The speed of sound increases with an increase in the temperature of the medium. For instance, the speed of sound in dry air at 0°C is about 331 m/s, whereas at 22°C it increases to approximately 344 m/s.

Reflection of Sound

Just like light, sound reflects off hard solid or liquid surfaces. Reflection of sound obeys the same laws as the reflection of light:

• The angle of incidence is equal to the angle of reflection.
• The incident sound wave, the reflected sound wave, and the normal to the reflecting surface at the point of incidence all lie in the same plane.

Echo

An echo is the repetition of sound caused by the reflection of sound waves from an obstacle back to the listener. Human memory retains the sensation of sound for about 0.1 seconds (persistence of hearing).

To hear a distinct echo, the reflected sound must reach the ear at least 0.1 s after the original sound.

• Taking the speed of sound in air at 22°C as 344 m/s:
• Distance traveled by sound in 0.1 s = Speed × Time = 344 m/s × 0.1 s = 34.4 metres.
• Since this total distance includes going to the obstacle and coming back, the minimum distance between the source of sound and the reflecting obstacle must be half of 34.4 m, which is 17.2 metres.

Reverberation

In large rooms or auditoriums, sound reflects repeatedly off walls, ceiling, and floor. The persistence of sound due to multiple reflections is called reverberation. Excessive reverberation creates unwanted noise and makes speech unintelligible.

Methods to Reduce Reverberation:

• Covering ceilings and walls with sound-absorbent materials like compressed fibreboard, rough plaster, or acoustic tiles.
• Using heavy curtains on windows and doors.
• Providing upholstered seats in auditoriums to absorb sound.

Applications of Reflection of Sound

• Megaphone / Loudspeaker: Tubes shaped like cones reflect sound repeatedly in a forward direction without letting it spread in all directions.
• Stethoscope: A medical instrument used by doctors to hear heart and lung sounds. Sound travels through the tube via multiple reflections.
• Curved Ceilings: Ceilings of concert halls and cinema halls are curved so that sound after reflection reaches all corners of the hall evenly.

Range of Hearing

The human ear can perceive sounds within a specific range of frequencies known as the audible range.

• Audible Range for Humans: Roughly 20 Hz to 20,000 Hz (20 kHz). Children below 5 years and animals like dogs can hear up to 25 kHz. As people grow older, their ears become less sensitive to higher frequencies.
• Infrasonic Sound (Infrasound): Sound waves with frequencies lower than 20 Hz. Examples: Rhinoceroses communicate using infrasound (around 5 Hz); whales and elephants also produce infrasound. Earthquakes generate low-frequency infrasound before main shockwaves start.
• Ultrasonic Sound (Ultrasound): Sound waves with frequencies higher than 20,000 Hz (20 kHz). Examples: Bats, porpoises, and dolphins emit ultrasound for navigation and hunting.

Applications of Ultrasound

• Cleaning Delicate Parts: Objects with hard-to-reach places (spiral tubes, electronic components) are placed in a cleaning solution and subjected to ultrasound waves, causing dirt particles to detach.
• Flaw Detection in Metals: Ultrasound is passed through metal blocks. If cracks or voids are present, ultrasound reflects back, signaling a defect.
• Echocardiography: Ultrasonic waves are reflected from various parts of the heart to form images of the heart.
• Ultrasonography: Medical technique using ultrasound to image internal organs (liver, kidneys, uterus) and monitor fetal development during pregnancy.
• Kidney Stone Treatment: High-intensity ultrasound breaks kidney stones into fine grains, which pass out with urine.

SONAR (Sound Navigation and Ranging)

SONAR is a device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects such as icebergs, sunken ships, submarine trenches, and enemy submarines.

Working Principle:

• SONAR consists of a transmitter and a detector fitted at the bottom of a ship.
• The transmitter produces ultrasound pulses that travel through sea water, strike objects on the seabed, and reflect back to the detector.
• The detector converts ultrasound signals into electrical signals for analysis.

If the time interval between transmission and detection is t and speed of sound in sea water is v, total distance covered is 2d = v × t. Hence, distance to object d is:

d = (v × t) / 2

This method is known as echo-ranging.

Structure of the Human Ear

The human ear converts pressure variations in air into electrical signals that travel to the brain via the auditory nerve. The ear consists of three main parts:

1. Outer Ear

• Pinna: Collects sound waves from surroundings and directs them into the auditory canal.
• Auditory Canal: Leads sound waves inward to the eardrum.
• Tympanic Membrane (Eardrum): A thin stretched membrane at the end of the auditory canal. When compressions reach it, it moves inward; when rarefactions reach it, it moves outward, causing it to vibrate.

2. Middle Ear

• Contains three interconnected tiny bones: Hammer (Malleus), Anvil (Incus), and Stirrup (Stapes).
• These bones amplify the mechanical vibrations received from the eardrum several times and transmit them to the inner ear.

3. Inner Ear

• Cochlea: A coiled, liquid-filled tube that converts pressure variations/vibrations into electrical signals using tiny hair cells.
• Auditory Nerve: Transmits electrical signals from the cochlea to the brain, which interprets them as sound.

Important Questions and Answers

Q1: What is wavelength, frequency, time period, and amplitude of a sound wave?

Answer:

• Wavelength (λ): Distance between two consecutive compressions or rarefactions. Unit: metre (m).
• Frequency (ν): Number of complete oscillations per second. Unit: hertz (Hz).
• Time Period (T): Time taken to complete one full oscillation. Unit: second (s).
• Amplitude (A): Maximum displacement of medium particles from equilibrium position. Unit: metre (m).

Q2: A sound wave has a frequency of 2 kHz and wavelength 35 cm. How long will it take to travel 1.5 km?

Answer:

Given:
Frequency (ν) = 2 kHz = 2000 Hz
Wavelength (λ) = 35 cm = 0.35 m
Distance (d) = 1.5 km = 1500 m

Step 1: Calculate wave speed (v):
v = ν × λ = 2000 × 0.35 = 700 m/s

Step 2: Calculate time (t):
t = Distance / Speed = 1500 / 700 = 2.14 seconds

Q3: Explain how an echo is produced. What is the minimum distance required to hear a distinct echo in air at 22°C?

Answer: An echo is produced when a sound wave strikes a hard reflecting surface and returns to the listener's ear. Because human hearing retains sound for 0.1 seconds, the reflected sound must arrive after 0.1 s. At 22°C, speed of sound in air is 344 m/s. In 0.1 s, sound travels 34.4 m total. Therefore, the minimum distance to the obstacle is 34.4 / 2 = 17.2 metres.

Q4: What is SONAR? Explain its working principle with an application.

Answer: SONAR stands for Sound Navigation and Ranging. It uses ultrasonic waves to locate objects underwater. A transmitter sends out ultrasound pulses that reflect off objects like sunken ships or ocean beds. A detector records the reflected waves and calculates distance using d = (v × t) / 2. SONAR is widely used to map the ocean floor and detect underwater hazards.

Chapter Summary

• Sound is a form of energy produced by vibrating objects and propagated as longitudinal mechanical waves.
• Sound requires a material medium (solid, liquid, or gas) to travel; it cannot travel through a vacuum.
• Sound waves propagate through alternate regions of compression (high density/pressure) and rarefaction (low density/pressure).
• The key characteristics of sound waves are wavelength (λ), frequency (ν), time period (T), amplitude (A), and wave speed (v = ν λ).
• Sound travels fastest in solids, slower in liquids, and slowest in gases. Speed increases with temperature.
• An echo is a reflected sound wave. Minimum distance to hear an echo in air at 22°C is 17.2 metres.
• Audible frequency range for humans is 20 Hz to 20,000 Hz. Infrasound is below 20 Hz; ultrasound is above 20 kHz.
• Ultrasound has major applications in flaw detection, echocardiography, ultrasonography, and SONAR technology.
• The human ear consists of outer ear (pinna, eardrum), middle ear (hammer, anvil, stirrup), and inner ear (cochlea, auditory nerve).