NCERT Science Notes - Class 8
Chapter 10 - Sound

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 10 - Sound. The chapter delves into the fascinating world of sound waves and their properties. This chapter explains how sound is produced by vibrating objects and how it travels through different mediums such as air, water, and solids. Students will explore concepts like frequency, amplitude, and pitch, and understand the human ear’s role in perceiving sound. Engaging activities and experiments, such as creating sound with musical instruments and observing vibrations, make learning interactive and fun. This chapter aims to provide a comprehensive understanding of sound, its characteristics, and its significance in our daily lives.

NCERT Science Notes - Class 8 Chapter 9 - Friction notes ajs, cbse notes class 10 ajslearning, cbse notes ajs, ajs notes class 10, ajslearning, ajs chalo seekhen

NCERT Science Notes - Class 8
Chapter 9 - Friction

10.1 Sound is Produced by a Vibrating Body

1. Touching the School Bell:
   • Observation: When the school bell is not in use, you feel nothing when touching it. However, when it produces sound, you can feel it vibrating.
   • Reason: Sound is produced by the vibrations of an object. In this case, the bell vibrates when struck, which produces sound.
   • 
     
   • Activity 10.1:
2. • Materials Needed: A metal plate (or pan), a stick.
   • Steps:
     1. Hang the metal plate or pan in such a way that it does not touch any wall.
     2. Strike the plate or pan with a stick.
   • Questions and Observations:
     1. Do you hear a sound? Yes, sound is heard when the plate is struck.
     2. Do you feel vibrations? When you touch the plate gently after striking it, you can feel the vibrations.
        • Reason: The vibrations of the plate cause it to produce sound, which you can both hear and feel.
     3. What happens if you hold the plate tightly after striking? The sound stops immediately, and you cannot feel the vibrations.
        • Reason: When the plate is held tightly, its vibrations stop, and thus no sound is produced.

• Vibrations: Rapid back-and-forth movements of an object.
• Sound: A form of energy that is produced by vibrating objects and propagates as a wave through a medium (like air).

• Sound is always associated with vibrations.
• When vibrations stop, the sound also stops.
• Touching a vibrating object, like a bell or a plate, helps to feel the vibrations directly.

Activity 10.2: Rubber Band Vibration and Sound Production

Materials Needed:

• A rubber band
• A pencil box
• Two pencils

1. Take a rubber band and stretch it around the longer side of a pencil box.
2. Insert two pencils between the box and the stretched rubber band to keep it taut.
3. Pluck the rubber band in the middle.

1. Do you hear any sound? Yes, when the rubber band is plucked, a sound is heard.
2. Does the band vibrate? Yes, the rubber band vibrates when plucked.

• Vibration and Sound Production:
  • When the rubber band is tightly stretched and plucked, it undergoes a back-and-forth or to-and-fro motion, which is called vibration.
  • As long as the rubber band is vibrating, it produces sound.
  • When the vibrations stop, the sound also stops.

• Vibration: The to-and-fro or back-and-forth motion of an object. Vibrations are essential for sound production.

• As explained in Class VII, any object that vibrates produces sound. For example, musical instruments like guitars, drums, and pianos produce sound due to the vibrations of their strings, membranes, or keys.

• A stretched object, like a rubber band, vibrates when plucked, and this vibration causes the sound.
• The sound continues as long as the object vibrates.
• No vibrations mean no sound.

Activity 10.3: Sound and Vibrations in a Metal Dish with Water

Materials Needed:

• A metal dish
• Water
• A spoon

1. Pour water into the metal dish.
2. Strike the edge of the dish with a spoon.

1. Do you hear a sound? Yes, a sound is heard when the dish is struck.
2. Can you feel the dish vibrating? After striking the dish, if you touch it, you can feel the vibrations.
3. Do you see any waves on the surface of the water? When you strike the dish and observe the water surface, you may see small waves.
4. What happens when you hold the dish? If you strike the dish and then hold it tightly, the sound stops, and the waves on the water disappear.

• Vibrations and Sound Production:
  • When you strike the dish, it vibrates and produces sound. These vibrations are transferred to the water, causing ripples or waves on the water's surface.
  • The waves on the water are a visual indication of the vibrations.
  • When you hold the dish tightly, the vibrations stop, and the waves in the water disappear, showing that vibrations are essential for both sound and wave formation.

• Amplitude: The magnitude or size of the vibrations. In some cases, like the ripples on water, the amplitude of the vibrations is large enough to be seen. In other cases, the amplitude may be too small to be visible, but the vibrations can still be felt or heard.

• Sound and Vibrations Connection: Vibrations produce sound, and these vibrations may be visible (like ripples in water) or invisible (too small to see but can be felt or heard).

• A vibrating object produces sound.
• Waves on the water are a visible sign of the dish's vibrations.
• When vibrations stop, both sound and waves disappear.
• Not all vibrations are visible; some can only be felt.

Activity 10.4: Making and Understanding Vibrations in Ektara

• Create a simple musical instrument using a hollow coconut shell or an earthen pot. This instrument is called an ektara.
• Play the instrument and identify the part that vibrates to produce sound.

Activity 10.5: Making a Jaltrang (Water Music Instrument)

Materials Needed:

• 6–8 bowls or tumblers
• Water
• A pencil (or any object to strike the bowls)

1. Fill the bowls with water at different levels, gradually increasing from one end to the other.
2. Strike the bowls gently with a pencil in succession.

1. What happens when you strike the bowls? You will hear pleasant, musical sounds from the bowls.
   • Reason: The different water levels cause each bowl to produce a different pitch, creating a musical scale.

• How Jaltrang Works:
  • When you strike each bowl, the water inside vibrates, producing sound. The amount of water affects the pitch of the sound; bowls with less water produce higher-pitched sounds, and those with more water produce lower-pitched sounds.

• Pitch: The quality of sound that makes it higher or lower in tone. It depends on the frequency of vibrations; faster vibrations produce higher pitches, while slower vibrations produce lower pitches.

• Sitar: When you pluck a string, the sound is not just from the string's vibration. The entire body of the sitar vibrates, producing the sound we hear.
• Mridangam: Striking the stretched membrane makes the entire body of the mridangam vibrate, and the combined vibration produces sound.

• Answer: Yes, when we speak, our vocal cords in the throat vibrate. These vibrations produce the sound of our voice.
  • Vocal cords are bands of tissue located in the larynx (voice box) that vibrate when air passes through them, creating sound.

Key Points:

• Jaltrang: Sound is produced by the vibration of water when bowls are struck.
• Vibration of Musical Instruments: In instruments like sitar and mridangam, not only the string or membrane vibrates, but the entire body of the instrument contributes to the sound.
• Vocal cords: Vibrations of the vocal cords produce sound when we speak.

10.2 - Sound Produced by Humans

Activity:

• Speak loudly, sing a song, or buzz like a bee.
• Place your hand on your throat as shown in Fig. 10.8.

• Do you feel any vibrations? Yes, you will feel vibrations in your throat when you speak, sing, or buzz.

• In humans, sound is produced by the voice box, also known as the larynx.
• The larynx is located at the upper end of the windpipe.
  • Vocal cords are two bands of tissue stretched across the larynx.
  • When you place your fingers on your throat, you can feel a hard bump that moves when you swallow. This is the voice box.

• Vocal cords have a narrow slit between them for the passage of air.
• When air from the lungs passes through this slit, the vocal cords vibrate, producing sound.
• The muscles attached to the vocal cords control their tightness or looseness:
  • When the vocal cords are tight and thin, the quality or type of voice changes (producing a higher-pitched sound).
  • When the vocal cords are loose, a lower-pitched sound is produced.

Key Definitions:

• Larynx (Voice Box): The organ in the throat that contains the vocal cords and produces sound when air passes through it.
• Vocal Cords: Two bands of tissue in the larynx that vibrate to produce sound when air passes through them.

Key Points:

1. Vibrations in the throat are felt when speaking or singing because the vocal cords vibrate.
2. Sound production in humans occurs when air from the lungs passes through the narrow slit between the vocal cords, causing them to vibrate.
3. The tightness or looseness of the vocal cords affects the pitch and quality of the voice.

Activity 10.7: Investigating Sound Propagation

1. Objective: To understand how sound travels and the necessity of a medium for sound propagation.
2. Materials:
   • A dry metal or glass tumbler.
   • A cell phone.
3. Steps:
   • Place the cell phone inside the dry tumbler (do not put it in water).
   • Have a friend call the cell phone from another phone and listen carefully to the ringing sound.
   • Surround the rim of the tumbler with your hands and place your mouth on the opening between your hands.
   • Ask your friend to ring the phone again while you suck air from the tumbler.
4. Observations:
   • Notice that the sound becomes fainter as you suck air from the tumbler.
   • When you remove the tumbler from your mouth, the sound becomes louder again.
5. Questions to Consider:
   • Why does the sound become fainter as you suck air from the tumbler?
   • What happens to the loudness of the sound when you remove the tumbler from your mouth?
   • Could the decreasing amount of air in the tumbler be related to the decreasing loudness of the ring?
6. Conclusion:
   • Sound needs a medium (like air) to travel.
   • When air is removed from the tumbler (creating a vacuum), the sound cannot travel, making it inaudible.
   • This experiment illustrates that sound cannot propagate in a vacuum and emphasizes the importance of a medium for sound transmission.

Activity 10.8: Testing Sound Travel in Liquids

1. Objective: To find out if sound can travel through liquids.
2. Materials:
   • A bucket or bathtub filled with clean water.
   • A small bell.
3. Steps:
   • Fill the bucket or bathtub with water.
   • Shake the bell inside the water to produce sound, making sure the bell does not touch the bucket or tub.
   • Place your ear gently on the water surface to listen for the sound.
4. Observations:
   • You can hear the sound of the bell through the water.
5. Conclusion:
   • Sound can travel through liquids.
6. Paheli’s Insight:
   • Whales and dolphins might communicate underwater using sound, since sound can travel through water.

Activity 10.9: Investigating Sound Propagation through Solids

1. Objective: To demonstrate that sound can travel through solids.
2. Materials:
   • A metre scale or a long metal rod.
   • A friend to help with the activity.
3. Steps:
   • Hold one end of the metre scale or metal rod to your ear.
   • Ask your friend to gently scratch or tap at the other end of the scale.
   • Observe if you can hear the sound of the scratching.
4. Alternative Method:
   • Place your ear at one end of a long wooden or metallic table.
   • Ask your friend to gently scratch the other end of the table.
5. Observations:
   • Confirm whether you can hear the sound of the scratching.
   • Ask your friends if they could hear the same sound from their respective positions.
6. Conclusion:
   • Sound can travel through solids such as wood or metal.
   • This activity illustrates that sound is transmitted through vibrations in solid materials.
   • Sound can also travel through strings, as demonstrated in toy telephones, showing that various solid mediums can carry sound effectively.
7. Key Concept:
   • Sound is produced by vibrating objects and travels in all directions through a medium, which can be a gas (air), liquid (water), or solid (wood, metal, strings).

10.3: Sound Needs a Medium for Propagation

1. Propagation of Sound:
   • Sound can travel from one place to another through a medium.
   • Example: When you call your friend from a distance, the sound travels through the air, allowing your friend to hear your voice.

Activity 10.7: Testing Sound Propagation

1. Place a dry cell phone inside a metal or glass tumbler.
2. Ask your friend to call this phone from another cell phone.
3. Listen to the ringtone.
4. Surround the rim of the tumbler with your hands and place your mouth over the opening.
5. Have your friend call again, and as the phone rings, suck the air from the tumbler.
6. Observe how the sound of the ringtone changes when you reduce the air in the tumbler.

• When you start sucking the air out of the tumbler, the sound becomes fainter.
• When you stop sucking the air, the sound becomes loud again.

• The reduction in sound loudness occurs because sound needs a medium (air) to travel.
• When air is removed, the sound becomes fainter because the medium for propagation is reduced.
• If all the air was removed, forming a vacuum, sound would not travel, and you wouldn't hear anything.

• Sound cannot travel through a vacuum because it requires a medium, such as air, liquids, or solids, to propagate.

Activity 10.8: Testing Sound Travel in Liquids

1. Objective: To find out if sound can travel through liquids.
2. Materials:
   • A bucket or bathtub filled with clean water.
   • A small bell.
3. Steps:
   • Fill the bucket or bathtub with water.
   • Shake the bell inside the water to produce sound, making sure the bell does not touch the bucket or tub.
   • Place your ear gently on the water surface to listen for the sound.
4. Observations:
   • You can hear the sound of the bell through the water.
5. Conclusion:
   • Sound can travel through liquids.
6. Paheli’s Insight:
   • Whales and dolphins might communicate underwater using sound, since sound can travel through water.

10.5: Amplitude, Time Period, and Frequency of a Vibration

1. Vibration and Oscillatory Motion:
   • The to and fro motion of an object is called vibration or oscillatory motion.
2. Oscillatory Motion:
   • The time period is the time taken for one complete oscillation.
   • Frequency refers to the number of oscillations in one second.
3. Frequency:
   • Definition: The number of oscillations per second is the frequency of oscillation.
   • Unit: Frequency is expressed in Hertz (Hz).
     • 1 Hz = 1 oscillation per second.
   • Example: If an object oscillates 20 times in one second, its frequency is 20 Hz.
4. Recognizing Sounds:
   • You can recognize familiar sounds without seeing the source. This is due to differences in sound properties such as amplitude and frequency.
5. Amplitude and Frequency:
   • Amplitude: Refers to the size or strength of the vibration, affecting the loudness of the sound.
   • Frequency: Determines the pitch of the sound (how high or low it is).

Loudness and Pitch

1. Loudness of Sound:
   • The loudness of a sound depends on the amplitude of vibration.
   • Larger amplitude produces louder sounds.
   • Smaller amplitude produces softer sounds.

Activity 10.11: Measuring Amplitude and Loudness:

Materials Needed:

• A metallic tumbler
• A tablespoon
• A small thermocol ball

1. Gently Strike the Tumbler:
   • Take the tablespoon and gently strike the brim of the metallic tumbler.
   • Observation: Listen to the sound produced.
2. Bang the Tumbler:
   • Now bang the spoon harder against the tumbler.
   • Observation: Hear the sound produced again and compare it to the previous sound.
   • Question: Is the sound louder when the tumbler is struck hard?
     • Answer: Yes, the sound is louder when struck harder due to increased amplitude of vibration.
3. Thermocol Ball Experiment:
   • Suspend a small thermocol ball so that it touches the rim of the tumbler (see Fig. 10.17).
   • Vibrate the tumbler by striking it.
   • Observation: Observe how far the ball is displaced when the tumbler vibrates.
   • Conclusion: The displacement of the ball indicates the amplitude of vibration of the tumbler.
4. Comparing Amplitudes:
   • Strike the tumbler gently and then strike it a little harder.
   • Observation: Compare the amplitudes of vibrations in both cases.
   • Question: In which case is the amplitude larger?
     • Answer: The amplitude is larger when the tumbler is struck harder.

1. Loudness:
   • Loudness is determined by the amplitude of the sound waves. A larger amplitude results in a louder sound.
2. Pitch:
   • Although not directly measured in this activity, pitch is related to the frequency of the sound waves. Higher frequencies produce higher pitches, and lower frequencies produce lower pitches.

Loudness of Sound

1. Loudness and Amplitude:
   • The loudness of sound is proportional to the square of the amplitude.
   • Example: If the amplitude doubles, the loudness increases by a factor of 4.
2. Unit of Loudness:
   • Loudness is measured in decibels (dB).
3. Examples of Sound Loudness:

Loudness and Pitch of Sound

1. Loudness and Amplitude:
   • The loudness of sound depends on its amplitude.
   • Large amplitude = Loud sound.
   • Small amplitude = Feeble sound.
2. Comparison of Sounds:
   • Baby's sound is usually feeble, while an adult's sound can be louder.
3. Boojho's Question:
   • He wonders why his voice is different from that of his teacher.
4. Pitch and Frequency:
   • The frequency of vibration determines the pitch or shrillness of a sound.
     • Higher frequency = Higher pitch (Shrill sound).
     • Lower frequency = Lower pitch (Deep sound).
5. Examples:
   • Drum: Vibrates with a low frequency → Produces a low-pitched sound.
   • Whistle: Vibrates with a high frequency → Produces a high-pitched sound.
6. Animal Sounds:
   • Bird: Makes a high-pitched sound.
   • Lion: Produces a low-pitched roar, but it is loud.
7. Human Voices:
   • Children’s voices generally have a higher frequency than adults'.
   • Women's voices have a higher frequency than men's.

10.6 - Audible and Inaudible Sounds

1. Vibrating Body and Sound:
   • Sound is produced by a vibrating body.
   • However, not all sounds produced by vibrating bodies can be heard by the human ear.
2. Audible Sounds:
   • Audible range for humans is between 20 Hz and 20,000 Hz (20 kHz).
3. Inaudible Sounds:
   • Frequencies below 20 Hz: Inaudible to humans (called infrasonic sounds).
   • Frequencies above 20,000 Hz: Inaudible to humans (called ultrasonic sounds).
4. Animals and High Frequencies:
   • Some animals, such as dogs, can hear sounds with frequencies above 20,000 Hz.
   • Police use high-frequency whistles that dogs can hear but humans cannot.
5. Ultrasound in Medicine:
   • Ultrasound equipment used in medical diagnostics operates at frequencies higher than 20,000 Hz.

10.7 - Noise and Music

1. Types of Sounds:
   • Sounds can be pleasant or unpleasant to the ear.
2. Noise:
   • Noise refers to unpleasant sounds that cause discomfort.
   • Examples of noise:
     • Construction work sounds.
     • Horns of buses and trucks.
     • In a classroom, when all students speak together, the resulting sound is noise.
3. Music:
   • Musical sound is pleasing to the ear.
   • Examples of musical sound:
     • Harmonium produces a musical sound.
     • The string of a sitar gives a musical sound.
4. Loudness and Music:
   • Musical sound can become unpleasant if it becomes too loud, even if it was initially melodious.

10.8 - Noise Pollution

1. Definition:
   • Noise pollution refers to the presence of excessive or unwanted sounds in the environment.
2. Sources of Noise Pollution:
   • Outside sources:
     • Vehicles.
     • Explosions, including bursting of crackers.
     • Machines.
     • Loudspeakers.
   • Home sources:
     • Television and transistor radios at high volumes.
     • Kitchen appliances.
     • Desert coolers and air conditioners.
3. Harms of Noise Pollution:
   • Causes health problems such as:
     • Lack of sleep.
     • Hypertension (high blood pressure).
     • Anxiety.
     • Hearing impairment (temporary or permanent) when exposed to loud sounds continuously.

Measures to Limit Noise Pollution

1. Control of Noise Sources:
   • Install silencing devices in:
     • Aircraft engines.
     • Transport vehicles.
     • Industrial machines.
     • Home appliances.
2. Regulation in Residential Areas:
   • Conduct all noisy operations away from residential areas.
   • Establish noise-producing industries away from residential neighborhoods.
3. Minimizing Noise:
   • Reduce use of automobile horns.
   • Keep TV and music systems at low volumes.
4. Environmental Solutions:
   • Plant trees along roads and around buildings to:
     • Cut down on sounds reaching residents.
     • Reduce harmful effects of noise pollution.

Hearing Impairment

1. Types of Hearing Impairment:
   • Total hearing impairment is rare and usually present from birth.
   • Partial hearing disability often occurs due to:
     • Disease.
     • Injury.
     • Age.
2. Care for Hearing-Impaired Children:
   • Children with hearing impairments need special care.
   • They can learn sign language to communicate effectively.
3. Impact on Speech:
   • Since speech develops through hearing, children with hearing loss may also have defective speech.
4. Technological Assistance:
   • Devices for the hearing-impaired help improve their quality of life.
5. Societal Support:
   • Society can help by creating a better living environment for the hearing-impaired, enabling them to lead normal lives.

NCERT Science Notes - Class 8 | Science | Chapter 10 - Sound

NCERT Science Notes - Class 8 | Science | Chapter 10 - Sound