Chapter 12 — Sound
Welcome to HSLC Guru! This page provides a complete guide to Class 9 Science Chapter 12 — Sound for the Assam State School Education Board (ASSEB). Sound is one of the most fascinating phenomena in physics — it is the form of energy that allows us to communicate, listen to music, and sense our surroundings. In this chapter, we explore how sound is produced, how it travels through different media, the characteristics that distinguish one sound from another, the reflection of sound waves, the working principle of SONAR, and the applications of ultrasound. Carefully prepared Textbook Question Answers, Additional MCQs, Fill in the Blanks, True/False questions, a Glossary, and a Formula Table will help you achieve top marks in your HSLC examination.
Summary of the Chapter
Sound is a form of energy that produces the sensation of hearing in our ears. It is produced by vibrating bodies — when an object vibrates, it disturbs the particles of the surrounding medium, and this disturbance propagates as a wave. Examples of vibrating bodies include the strings of a guitar, the membrane of a drum, the tines of a tuning fork, and the vocal cords in our throat. Sound waves are longitudinal waves, meaning the particles of the medium oscillate back and forth in the same direction in which the wave travels. As the wave moves through the medium, regions of high pressure called compressions and regions of low pressure called rarefactions are formed alternately. Sound requires a material medium (solid, liquid, or gas) for its propagation; it cannot travel through vacuum, as proved by the bell-jar experiment.
Every sound wave has certain characteristics. The wavelength (λ) is the distance between two consecutive compressions or two consecutive rarefactions. The frequency (f) is the number of complete oscillations made in one second, measured in hertz (Hz). The time period (T) is the time taken for one complete oscillation, related to frequency by T = 1/f. The amplitude is the maximum displacement of particles from their mean position. The pitch of a sound depends on its frequency — higher frequency means higher pitch. The loudness depends on the amplitude — larger amplitude means louder sound. The quality (timbre) distinguishes between two sounds of the same pitch and loudness produced by different sources. The fundamental wave equation linking these quantities is v = f × λ, where v is the speed of sound. The speed of sound varies with the medium — it is fastest in solids, slower in liquids, and slowest in gases. In air at 25°C, the speed of sound is approximately 346 m/s, while in water it is about 1500 m/s, and in iron it is around 5950 m/s.
When sound waves strike a hard surface, they undergo reflection following the same laws as the reflection of light. An echo is the repetition of the original sound due to reflection from a distant obstacle. To hear a distinct echo, the time gap between the original sound and the reflected sound must be at least 0.1 second. Since the speed of sound in air at 25°C is 346 m/s, the minimum distance of the reflecting surface must be 17.2 m. Reverberation is the persistence of sound caused by repeated reflections in large enclosed spaces such as auditoriums and concert halls; it is reduced by using sound-absorbing materials like curtains, carpets, fibreboard, and rough plaster. SONAR (Sound Navigation And Ranging) uses ultrasonic waves to determine the depth of the sea or to detect underwater objects like submarines, icebergs, and shoals of fish — a principle similar to echolocation used by bats and dolphins.
Humans can hear sound waves with frequencies between 20 Hz and 20,000 Hz; this is called the audible range. Sound below 20 Hz is infrasound, while sound above 20,000 Hz is ultrasound. Ultrasonic waves have several important applications: medical imaging (ultrasonography to view internal organs and the foetus), non-destructive testing (NDT) to detect cracks in metal blocks, cleaning of delicate objects like jewellery and electronic parts, breaking kidney stones (lithotripsy), and echocardiography. The human ear has three parts — the outer ear (pinna and ear canal) which collects sound, the middle ear with three small bones (hammer, anvil, and stirrup) that amplify vibrations from the eardrum, and the inner ear (cochlea) which converts pressure variations into nerve impulses sent to the brain via the auditory nerve.
Textbook Question Answers
1-Mark Questions
Q1. How is sound produced?
Answer: Sound is produced by vibrating bodies. When an object vibrates, it sets the surrounding medium particles into vibration, producing sound waves.
Q2. What is the SI unit of frequency?
Answer: The SI unit of frequency is the hertz (Hz). One hertz equals one oscillation per second.
Q3. Which type of wave is a sound wave?
Answer: A sound wave is a longitudinal mechanical wave in which particles of the medium vibrate in the same direction as the wave travels.
Q4. What is the audible frequency range for human beings?
Answer: The audible range for humans is from 20 Hz to 20,000 Hz (20 kHz).
Q5. Define wavelength.
Answer: Wavelength is the distance between two consecutive compressions or two consecutive rarefactions of a sound wave. It is denoted by λ (lambda) and measured in metres.
Q6. Can sound travel through vacuum?
Answer: No, sound cannot travel through vacuum because it requires a material medium (solid, liquid, or gas) for its propagation.
Q7. What is meant by ultrasound?
Answer: Sound waves with frequencies above 20,000 Hz, which cannot be heard by humans, are called ultrasound or ultrasonic waves.
Q8. Define amplitude of a sound wave.
Answer: Amplitude is the maximum displacement of the particles of the medium from their mean (rest) position when the wave passes through them. It determines the loudness of the sound.
Q9. What is the full form of SONAR?
Answer: SONAR stands for SOund Navigation And Ranging. It is used to detect underwater objects and measure ocean depth.
Q10. Name the part of the ear that converts vibrations into nerve impulses.
Answer: The cochlea in the inner ear converts pressure vibrations into electrical nerve impulses, which are then transmitted to the brain via the auditory nerve.
2 to 3-Mark Questions
Q1. Distinguish between longitudinal and transverse waves with an example each.
Answer:
| Longitudinal Waves | Transverse Waves |
|---|---|
| Particles vibrate parallel to wave direction. | Particles vibrate perpendicular to wave direction. |
| Consist of compressions and rarefactions. | Consist of crests and troughs. |
| Example: Sound waves in air. | Example: Light waves, water surface waves. |
Q2. Why do we hear thunder some time after we see lightning, even though both are produced together?
Answer: Lightning and thunder are produced at the same instant. However, the speed of light (3 × 10⁸ m/s) is enormously greater than the speed of sound (≈346 m/s in air at 25°C). Hence, light from the lightning reaches our eyes almost instantly, but the sound of thunder takes additional time to travel the same distance. As a result, we hear thunder after seeing the flash.
Q3. Explain how an echo is formed. State the conditions required to hear a clear echo.
Answer: An echo is the repetition of an original sound caused by reflection from a distant obstacle. Conditions for a clear echo:
- The minimum time gap between the original and reflected sound must be at least 0.1 second (the persistence of hearing).
- The minimum distance of the reflecting surface must be 17.2 m (at 25°C, since 346 × 0.1 ÷ 2 = 17.3 m, taken as 17.2 m).
- The reflecting surface must be large enough and rigid (such as a cliff or wall).
- The intensity of the original sound must be sufficiently high.
Q4. Differentiate between pitch and loudness.
| Pitch | Loudness |
|---|---|
| Depends on the frequency of the sound wave. | Depends on the amplitude of the sound wave. |
| Higher frequency = higher pitch (shrill sound). | Higher amplitude = louder sound. |
| Helps distinguish between male and female voices. | Measured in decibels (dB). |
Q5. What is reverberation? How can it be reduced in an auditorium?
Answer: Reverberation is the persistence of sound in a large enclosed space due to multiple reflections from the walls, ceiling, and floor. Excessive reverberation makes speech and music unclear. It can be reduced by:
- Covering the walls and ceiling with sound-absorbing materials such as fibreboard, compressed fibreboard, or rough plaster.
- Hanging heavy curtains and laying carpets on the floor.
- Using cushioned upholstered seats which absorb sound.
- Designing curved reflecting surfaces to direct sound uniformly.
Q6. Why is the speed of sound greater in solids than in gases?
Answer: The speed of sound depends on the elasticity and density of the medium. In solids, particles are tightly packed and the elastic constants are very large, so they transmit vibrations rapidly from one particle to the next. In gases, particles are far apart and weakly bonded, so vibrations travel slowly. Therefore, the speed of sound is greatest in solids (e.g., 5950 m/s in iron), intermediate in liquids (e.g., 1500 m/s in water), and least in gases (e.g., 346 m/s in air at 25°C).
5 to 6-Mark Questions (with Numericals)
Q1. Describe the structure and working of the human ear with a labelled diagram explanation.
Answer: The human ear is a remarkable organ that detects sound waves and converts them into nerve signals. It has three main parts:
- Outer Ear (Pinna and Auditory Canal): The pinna collects sound waves from the surroundings and channels them through the auditory canal to the eardrum (tympanic membrane). The eardrum is a thin elastic membrane that vibrates when sound waves strike it.
- Middle Ear: Contains three small bones — the hammer (malleus), anvil (incus), and stirrup (stapes). These bones amplify the vibrations from the eardrum (about 22 times) and transmit them to the oval window of the inner ear.
- Inner Ear: Contains the cochlea, a spiral fluid-filled tube lined with sensory hair cells. As the oval window vibrates, pressure waves travel through the cochlear fluid, stimulating the hair cells. These cells convert the mechanical vibrations into electrical nerve impulses, which are sent to the brain through the auditory nerve. The brain interprets these impulses as sound.
Q2. Explain the working principle of SONAR. A SONAR device on a submarine sends out a signal and receives an echo 5 seconds later. If the speed of sound in salt water is 1531 m/s, calculate the distance of the object from the submarine.
Answer: SONAR (Sound Navigation And Ranging) is a device that uses ultrasonic waves to detect the distance, direction, and speed of underwater objects such as submarines, shoals of fish, sunken ships, and icebergs. It consists of a transmitter that emits ultrasonic pulses and a receiver (detector) that detects the reflected echoes. The transmitter sends ultrasonic waves into the sea; when the waves strike an object, they are reflected and detected by the receiver. The time interval between transmission and reception is recorded electronically. Using the formula 2d = v × t, the distance can be calculated.
Numerical:
Given: time t = 5 s, speed v = 1531 m/s.
Total distance travelled by signal = v × t = 1531 × 5 = 7655 m.
Distance of object from submarine = 7655 ÷ 2 = 3827.5 m.
Q3. What are the characteristics of a sound wave? A source vibrates 1500 times in 6 seconds. The speed of sound in air is 340 m/s. Calculate (i) frequency, (ii) time period, and (iii) wavelength of the wave produced.
Answer: The main characteristics of a sound wave are:
- Wavelength (λ): Distance between two consecutive compressions or rarefactions.
- Frequency (f): Number of oscillations per second; determines pitch.
- Time Period (T): Time taken for one complete oscillation; T = 1/f.
- Amplitude: Maximum displacement of a particle from its mean position; determines loudness.
- Pitch: Subjective sensation of frequency.
- Loudness: Subjective sensation depending on amplitude and intensity.
- Quality (Timbre): Property by which two sounds of the same pitch and loudness are distinguished.
Numerical:
(i) Frequency f = number of vibrations / time = 1500 / 6 = 250 Hz.
(ii) Time period T = 1/f = 1/250 = 0.004 s.
(iii) Wavelength λ = v/f = 340/250 = 1.36 m.
Q4. List five important applications of ultrasound. Why is ultrasound preferred over ordinary sound for these applications?
Answer: Five important applications of ultrasound:
- Medical Imaging (Ultrasonography): Used to obtain images of internal organs like the liver, kidney, gallbladder, and the developing foetus in pregnant women.
- Echocardiography: Reflected ultrasound waves form images of the heart to detect abnormalities.
- Lithotripsy: Ultrasound is used to break kidney stones into small pieces that can pass out with urine.
- Non-Destructive Testing (NDT): Detects cracks and flaws in metal blocks, ship hulls, and aircraft components without damaging them.
- Cleaning: Ultrasound waves are used to clean delicate parts like jewellery, watches, and electronic components placed in a cleaning solution.
Ultrasound is preferred because: (a) it has very high frequency and short wavelength, allowing precise focusing; (b) it can travel along well-defined paths; (c) it does not get easily scattered or absorbed; and (d) it can penetrate matter without significantly losing energy.
Q5. A person standing in a valley between two cliffs claps loudly and hears two echoes — one after 1.5 s and another after 2.5 s. The speed of sound in air is 340 m/s. Find the distances of the two cliffs from the person and the distance between the two cliffs.
Answer:
For the first echo (t₁ = 1.5 s):
Distance to first cliff d₁ = (v × t₁) / 2 = (340 × 1.5) / 2 = 510 / 2 = 255 m.
For the second echo (t₂ = 2.5 s):
Distance to second cliff d₂ = (v × t₂) / 2 = (340 × 2.5) / 2 = 850 / 2 = 425 m.
Distance between the two cliffs = d₁ + d₂ = 255 + 425 = 680 m.
Additional Multiple Choice Questions (MCQs)
Q1. Sound waves are:
(a) Transverse waves (b) Longitudinal waves (c) Electromagnetic waves (d) Light waves
Answer: (b) Longitudinal waves.
Q2. The speed of sound is maximum in:
(a) Air (b) Water (c) Iron (d) Vacuum
Answer: (c) Iron.
Q3. The frequency range audible to human beings is:
(a) 0–20 Hz (b) 20–20,000 Hz (c) Above 20,000 Hz (d) Below 20 Hz
Answer: (b) 20–20,000 Hz.
Q4. The pitch of a sound depends on its:
(a) Amplitude (b) Frequency (c) Speed (d) Quality
Answer: (b) Frequency.
Q5. The minimum distance required to hear a distinct echo at 25°C is:
(a) 10 m (b) 17.2 m (c) 34 m (d) 50 m
Answer: (b) 17.2 m.
Q6. SONAR uses:
(a) Infrasonic waves (b) Ultrasonic waves (c) Radio waves (d) Light waves
Answer: (b) Ultrasonic waves.
Q7. Loudness of a sound depends on:
(a) Frequency (b) Wavelength (c) Amplitude (d) Time period
Answer: (c) Amplitude.
Q8. The relation between speed (v), frequency (f), and wavelength (λ) is:
(a) v = f/λ (b) v = λ/f (c) v = f × λ (d) v = f + λ
Answer: (c) v = f × λ.
Q9. Bats use which technique to detect obstacles?
(a) Smell (b) Light (c) Echolocation (d) Radar
Answer: (c) Echolocation.
Q10. The three small bones of the middle ear are:
(a) Pinna, eardrum, cochlea (b) Hammer, anvil, stirrup (c) Femur, tibia, fibula (d) Malleus, cochlea, pinna
Answer: (b) Hammer, anvil, stirrup.
Fill in the Blanks
Q1. Sound is produced by ____________ bodies.
Answer: vibrating.
Q2. Sound waves cannot travel through ____________.
Answer: vacuum.
Q3. The SI unit of frequency is ____________.
Answer: hertz (Hz).
Q4. Sound waves of frequency above 20,000 Hz are called ____________.
Answer: ultrasound.
Q5. The persistence of sound in a closed space due to repeated reflection is called ____________.
Answer: reverberation.
True or False
Q1. Sound travels faster in air than in water.
Answer: False. Sound travels faster in water than in air.
Q2. The pitch of a sound depends on its frequency.
Answer: True.
Q3. Sound waves are transverse waves.
Answer: False. Sound waves are longitudinal waves.
Q4. Bats produce ultrasonic waves to navigate and find food.
Answer: True.
Q5. Echo is heard only when the reflecting surface is less than 17.2 m away.
Answer: False. Echo is heard only when the reflecting surface is at least 17.2 m away.
Glossary
| Term | Meaning |
|---|---|
| Sound | A form of energy that produces the sensation of hearing. |
| Longitudinal Wave | Wave in which particles vibrate parallel to the direction of propagation. |
| Compression | Region of high pressure and density in a sound wave. |
| Rarefaction | Region of low pressure and density in a sound wave. |
| Wavelength (λ) | Distance between two consecutive compressions or rarefactions. |
| Frequency (f) | Number of oscillations per second; measured in hertz. |
| Time Period (T) | Time for one complete oscillation; T = 1/f. |
| Amplitude | Maximum displacement of particles from mean position. |
| Pitch | Subjective sensation of frequency — sharpness of a sound. |
| Loudness | Subjective sensation depending on amplitude and intensity. |
| Quality (Timbre) | Characteristic that distinguishes two sounds of equal pitch and loudness. |
| Echo | Repetition of a sound due to reflection from a distant obstacle. |
| Reverberation | Persistence of sound due to repeated reflections in a closed space. |
| Ultrasound | Sound waves of frequency above 20,000 Hz. |
| Infrasound | Sound waves of frequency below 20 Hz. |
| SONAR | Sound Navigation And Ranging — uses ultrasound to detect underwater objects. |
| Audible Range | 20 Hz to 20,000 Hz for humans. |
| Cochlea | Spiral organ of the inner ear that converts vibrations to nerve impulses. |
Formula Table
| Quantity | Formula | SI Unit |
|---|---|---|
| Wave Speed | v = f × λ | m/s |
| Frequency | f = 1 / T | hertz (Hz) |
| Time Period | T = 1 / f | second (s) |
| Wavelength | λ = v / f | metre (m) |
| Distance from Reflector (Echo / SONAR) | d = (v × t) / 2 | metre (m) |
| Number of Vibrations | n = f × t | — |
| Speed of Sound in Air (25°C) | v ≈ 346 m/s | m/s |
| Minimum Distance for Echo | d = (v × 0.1) / 2 ≈ 17.2 m | metre (m) |
Stay connected with HSLC Guru for more chapter-wise notes, solved question papers, and practice resources for the ASSEB Class 9 Science curriculum. Practise the numericals regularly and revise the glossary terms before the examination to score full marks in Chapter 12 — Sound.