Chapter 14 — Sound
Welcome to HSLC Guru! In this lesson we explore Class 8 General Science Chapter 14 — Sound as prescribed by the Assam State School Education Board (ASSEB). You will learn how sound is produced, how it travels, what its characteristics are, how the human ear detects it, and the harmful effects of noise pollution. Complete textbook questions, additional MCQs, fill-in-the-blanks, true/false items and a glossary are provided to help you score full marks.
Summary
Production of sound: Sound is produced by vibrating bodies. When an object vibrates back and forth, it disturbs the surrounding air particles and creates sound waves that reach our ears. Examples include the vibration of a stretched rubber band, a tuning fork struck against a rubber pad, the membrane of a drum, and the strings of a guitar or violin. If we touch a ringing bell, we can feel the vibration; the moment the vibration stops, the sound also stops. In humans, sound is produced by the voice box or larynx, situated at the upper end of the windpipe (trachea). Inside the larynx are two thin elastic vocal cords stretched across the air passage. When air from the lungs passes through the narrow gap between them, the cords vibrate and produce sound. Muscles attached to the vocal cords change their tension and length, allowing us to produce voices of different pitch — high in children and women, lower in men.
Propagation of sound: Sound needs a material medium (solid, liquid or gas) to travel — it cannot travel through a vacuum. This is proved by the bell-jar experiment: as air is pumped out of a jar containing a ringing electric bell, the sound becomes fainter and finally stops, although the hammer is still seen striking the gong. Sound travels in the form of longitudinal waves, in which particles of the medium vibrate back and forth in the same direction in which the wave moves, creating regions of compression (high pressure) and rarefaction (low pressure). The speed of sound is greatest in solids, less in liquids and least in gases. In air at 20 °C, sound travels at about 343 m/s.
Characteristics of sound: A sound wave is described by its amplitude (maximum displacement of a particle from its mean position — measured in metres), frequency (number of vibrations per second — measured in hertz, Hz), time period (time for one complete oscillation — T = 1/f) and wavelength (distance between two consecutive compressions). Loudness depends on amplitude — larger amplitude means louder sound; it is measured in decibels (dB). Pitch depends on frequency — higher frequency gives a shriller (higher-pitched) sound. Quality or timbre distinguishes two sounds of the same pitch and loudness produced by different sources, e.g., a flute and a violin. The audible range of human hearing is between 20 Hz and 20,000 Hz. Sounds above 20,000 Hz are called ultrasound (heard by bats, dogs, dolphins) and sounds below 20 Hz are called infrasound (produced by earthquakes, whales, elephants).
Music, noise and hearing: Pleasant sounds with regular vibrations are called music, while unpleasant sounds with irregular vibrations are called noise. Excessive or unwanted sound in the environment is termed noise pollution. Major sources include vehicles, loudspeakers, factory machinery, crackers and household appliances. Noise pollution causes lack of sleep, headache, hypertension, anxiety, hearing impairment and even permanent deafness. It can be controlled by planting trees along roads, using silencers, banning loud horns near hospitals and schools, soundproofing rooms, and switching off appliances when not in use. The human ear has three parts — outer ear (pinna and ear canal that collect sound and direct it to the eardrum), middle ear (eardrum/tympanum and three tiny bones — hammer, anvil and stirrup — that amplify vibrations) and inner ear (cochlea and auditory nerve that convert vibrations into electrical signals carried to the brain).
Textbook Question Answers
1-Mark Questions
Q1. How is sound produced?
Answer: Sound is produced by the vibration of an object. When a body vibrates, it sets the surrounding air particles into vibration, and this disturbance travels as a sound wave to our ears.
Q2. Name the organ that produces voice in human beings.
Answer: The voice box or larynx produces voice in human beings.
Q3. What is the SI unit of frequency?
Answer: The SI unit of frequency is hertz (Hz).
Q4. What is the audible range of frequency for human beings?
Answer: The audible range of frequency for human beings is from 20 Hz to 20,000 Hz.
Q5. What is the unit of loudness?
Answer: The loudness of sound is measured in decibel (dB).
Q6. Can sound travel through a vacuum?
Answer: No, sound cannot travel through a vacuum because it requires a material medium for propagation.
Q7. What is ultrasound?
Answer: Sound of frequency above 20,000 Hz, which cannot be heard by humans, is called ultrasound.
Q8. Define amplitude of a wave.
Answer: Amplitude is the maximum displacement of a vibrating particle from its mean (rest) position.
Q9. What is meant by noise pollution?
Answer: The presence of excessive or unwanted sound in the environment that causes discomfort and harms human health is called noise pollution.
Q10. Name two animals that can hear ultrasound.
Answer: Bats and dolphins can hear ultrasound. (Dogs also hear ultrasound up to about 45,000 Hz.)
2-3 Mark Questions
Q1. Distinguish between music and noise.
Answer: Music is a sound produced by regular and periodic vibrations; it is pleasant to the ear (e.g., notes from a flute, violin or piano). Noise, on the other hand, is produced by irregular and non-periodic vibrations and is unpleasant to hear (e.g., the sound of a bursting cracker or a moving truck). Music has a definite pitch and rhythm, whereas noise does not.
Q2. How does sound travel through air? Explain.
Answer: Sound travels through air in the form of longitudinal waves. When an object vibrates, it pushes the air particles next to it forward, producing a region of higher pressure called a compression. When it moves backward, it creates a region of lower pressure called a rarefaction. These compressions and rarefactions travel outward from the source and reach our ears, where they cause the eardrum to vibrate and produce the sensation of sound.
Q3. Why is sound called a longitudinal wave?
Answer: Sound is called a longitudinal wave because the particles of the medium vibrate back and forth in the same direction as the direction of propagation of the wave. This produces alternate compressions and rarefactions through which sound energy is transmitted from one place to another.
Q4. Define frequency, time period and wavelength of a sound wave. Write the relation between frequency and time period.
Answer: Frequency (f) is the number of complete vibrations made by a particle in one second; its SI unit is hertz (Hz). Time period (T) is the time taken to complete one vibration, measured in seconds. Wavelength (λ) is the distance between two consecutive compressions or rarefactions in a sound wave. The relation between frequency and time period is: f = 1/T.
Q5. Why are we able to hear the voices of our friends without seeing them?
Answer: Each person’s vocal cords have a slightly different length, thickness and tension, so the sound they produce has a unique quality (timbre). Even though pitch and loudness may be similar, this characteristic quality lets us recognise familiar voices without having to see the speaker.
Q6. List four sources of noise pollution in your locality.
Answer: (i) Loud horns of vehicles and continuous traffic; (ii) Loudspeakers used during festivals and meetings; (iii) Bursting of crackers; (iv) Heavy machinery in factories and construction sites. Domestic appliances such as mixer-grinders and television at high volume also add to noise pollution.
5-6 Mark Questions
Q1. Describe an experiment to show that sound cannot travel through a vacuum.
Answer: Take a thick-walled glass jar (called a bell jar) connected to a vacuum pump. Suspend an electric bell inside the jar so that it does not touch the walls and connect it to a battery and switch outside. Switch on the bell — its sound is heard clearly because air is present inside the jar. Now start the vacuum pump to remove air gradually. As the air inside the jar reduces, the loudness of the bell decreases. When almost all the air is removed, the sound becomes inaudible, although the hammer of the bell is still seen striking the gong. When air is allowed back into the jar, the sound is heard again. This experiment proves that sound requires a material medium for propagation and cannot travel through a vacuum.
Q2. Describe the structure of the human ear with the help of a labelled sketch (in words). Explain how we hear sound.
Answer: The human ear has three parts:
- Outer ear: Consists of the pinna (visible flap) and the auditory canal. The pinna collects sound waves from the surroundings and directs them through the canal to the eardrum.
- Middle ear: Contains the eardrum (tympanum), a thin membrane that vibrates when sound waves strike it, and three small bones called the hammer (malleus), anvil (incus) and stirrup (stapes). These bones amplify the vibrations and pass them to the inner ear.
- Inner ear: Contains a coiled, fluid-filled tube called the cochlea. Vibrations in the cochlea generate electrical signals which travel through the auditory nerve to the brain. The brain interprets these signals as sound.
Thus, sound waves → pinna → auditory canal → eardrum → hammer-anvil-stirrup → cochlea → auditory nerve → brain.
Q3. Explain the harmful effects of noise pollution and suggest measures to control it.
Answer: Harmful effects: (i) Continuous exposure to loud noise causes hearing impairment and may lead to permanent deafness. (ii) It produces headache, irritability, anxiety and lack of concentration. (iii) It raises blood pressure and may cause hypertension and heart problems. (iv) It disturbs sleep and rest, causing fatigue. (v) It badly affects patients in hospitals and the mental development of small children.
Control measures: (i) Plant more trees along roadsides as trees absorb noise. (ii) Use silencers in vehicles and machines. (iii) Avoid blowing horns unnecessarily, especially in silence zones like hospitals and schools. (iv) Restrict the use of loudspeakers and crackers. (v) Use soundproof curtains, doors and walls in factories and recording studios. (vi) Switch off TV, radio and appliances when not in use. (vii) Locate noisy industries away from residential areas.
Q4. Explain the three characteristics of sound — loudness, pitch and quality — with examples.
Answer: (a) Loudness: Loudness depends on the amplitude of the vibration. Greater the amplitude, louder the sound. It is measured in decibels (dB). For example, a whisper is about 30 dB while a jet engine is about 140 dB.
(b) Pitch: Pitch depends on the frequency of vibration. Higher frequency means higher pitch (shrill sound), while lower frequency gives a low-pitched (deep) sound. The voice of a child or woman is high-pitched while that of a man is low-pitched. A mosquito’s buzz is high-pitched; a lion’s roar is low-pitched.
(c) Quality (Timbre): Quality is the characteristic of sound that distinguishes two sounds of the same pitch and loudness produced by different sources. It is due to the presence of additional frequencies (overtones). The note from a flute and the same note from a violin sound different because of differing quality.
Q5. Explain how human beings produce sound.
Answer: Human beings produce sound by means of the voice box or larynx, situated at the upper end of the windpipe (trachea). Inside the larynx are two thin, elastic membranes called vocal cords, stretched across the air passage in such a way that a narrow slit is left between them. When we speak or sing, air from the lungs is forced through this slit. The air sets the vocal cords into vibration, which produces sound. Muscles attached to the cords can stretch them tight (producing a thin, high-pitched voice) or make them loose and thick (producing a deep, low-pitched voice). The vocal cords are about 20 mm long in men, 15 mm in women and shorter in children — that is why men’s voices are usually deeper than those of women and children.
Additional MCQs
Q1. Sound is produced by —
(a) Heating a body (b) Vibrating a body (c) Compressing a body (d) Cooling a body
Answer: (b) Vibrating a body.
Q2. Sound waves are —
(a) Transverse (b) Longitudinal (c) Electromagnetic (d) Surface
Answer: (b) Longitudinal.
Q3. The pitch of sound depends on its —
(a) Amplitude (b) Frequency (c) Speed (d) Wavelength
Answer: (b) Frequency.
Q4. The loudness of sound depends on its —
(a) Frequency (b) Pitch (c) Amplitude (d) Wavelength
Answer: (c) Amplitude.
Q5. The unit of frequency is —
(a) Decibel (b) Hertz (c) Newton (d) Watt
Answer: (b) Hertz.
Q6. Sound travels fastest in —
(a) Air (b) Water (c) Steel (d) Vacuum
Answer: (c) Steel.
Q7. The audible range of frequencies for humans is —
(a) 2–200 Hz (b) 20–20,000 Hz (c) 200–2000 Hz (d) 20,000–2,00,000 Hz
Answer: (b) 20–20,000 Hz.
Q8. Sound produced by humans is generated by —
(a) Tongue (b) Lungs (c) Vocal cords (d) Lips
Answer: (c) Vocal cords.
Q9. The part of the ear that converts sound vibrations into nerve impulses is —
(a) Pinna (b) Eardrum (c) Cochlea (d) Hammer
Answer: (c) Cochlea.
Q10. Which of the following is a source of noise pollution?
(a) Singing of birds (b) Soft music (c) Bursting of crackers (d) Whisper
Answer: (c) Bursting of crackers.
Fill in the Blanks
Q1. Sound is produced by a __________ body.
Answer: vibrating.
Q2. Sound cannot travel through a __________.
Answer: vacuum.
Q3. The number of vibrations per second is called __________.
Answer: frequency.
Q4. Sound of frequency below 20 Hz is called __________.
Answer: infrasound.
Q5. The loudness of sound is measured in __________.
Answer: decibel (dB).
True or False
Q1. Sound can travel through solids, liquids and gases.
Answer: True.
Q2. The pitch of a sound depends on its amplitude.
Answer: False. (Pitch depends on frequency.)
Q3. Bats can hear ultrasound.
Answer: True.
Q4. Noise pollution causes only mild discomfort and no serious health problems.
Answer: False. (It can cause hearing loss, hypertension and other serious problems.)
Q5. The eardrum is part of the inner ear.
Answer: False. (The eardrum is part of the middle ear, at the boundary of the outer ear.)
Glossary
| Term | Meaning |
|---|---|
| Sound | A form of energy produced by vibrating bodies that gives the sensation of hearing. |
| Vibration | The rapid back-and-forth motion of a body about its mean position. |
| Larynx | The voice box in humans, located at the upper end of the windpipe. |
| Vocal cords | Two thin elastic membranes in the larynx that vibrate to produce voice. |
| Medium | The substance (solid, liquid or gas) through which sound travels. |
| Vacuum | A region completely empty of matter; sound cannot pass through it. |
| Compression | A region of high pressure in a longitudinal sound wave. |
| Rarefaction | A region of low pressure in a longitudinal sound wave. |
| Amplitude | The maximum displacement of a vibrating particle from its mean position. |
| Frequency | The number of vibrations per second; SI unit hertz (Hz). |
| Time period | The time taken to complete one full vibration; T = 1/f. |
| Wavelength | The distance between two successive compressions or rarefactions. |
| Pitch | The shrillness of sound; depends on frequency. |
| Loudness | How loud a sound is; depends on amplitude; measured in decibel. |
| Quality (Timbre) | Property that distinguishes sounds of the same pitch and loudness from different sources. |
| Audible range | Frequency range that humans can hear: 20 Hz to 20,000 Hz. |
| Ultrasound | Sound with frequency above 20,000 Hz. |
| Infrasound | Sound with frequency below 20 Hz. |
| Music | Pleasant sound produced by regular and periodic vibrations. |
| Noise | Unpleasant sound produced by irregular vibrations. |
| Noise pollution | Excessive or unwanted sound in the environment that harms human health. |
| Decibel (dB) | The unit used to measure the loudness of sound. |
| Pinna | The visible outer flap of the ear that collects sound waves. |
| Eardrum | The thin membrane (tympanum) that vibrates when sound waves strike it. |
| Cochlea | The coiled, fluid-filled organ in the inner ear that converts vibrations to nerve impulses. |
| Auditory nerve | The nerve that carries sound signals from the cochlea to the brain. |