Class 9 Science NCERT Notes – Chapter 11: Sound (PDF, MindMap, Q&A, Quizzes)

Chapter 11 (Physics): Sound – Class 9 NCERT Science Detailed Study Notes.

1. The Nature of Sound

Sound is a form of energy that creates a sensation of hearing in our ears. According to the principle of conservation of energy, this energy is not created or destroyed but is converted from other forms, such as the mechanical energy used when clapping. The study of sound involves understanding how it is produced, how it travels, and how it is perceived.

1.1. Production of Sound

Sound originates from vibrating objects. A vibration is a rapid to-and-fro motion of an object. Sound can be produced by various actions that cause objects to vibrate, including striking (tuning fork, bell), plucking (rubber band, guitar string), scratching, rubbing, blowing, or shaking.

• Human Voice: Produced by vibrations in the vocal cords.
• Musical Instruments: Different parts of instruments vibrate to produce sound (e.g., strings on a violin, air column in a flute).
• Animals: A bee’s buzz comes from the vibration of its wings.

An object must be vibrating to produce sound. This can be demonstrated by touching a vibrating tuning fork to a suspended ball, causing the ball to be pushed away, or by dipping it in water, causing ripples and splashes.

1.2. Propagation of Sound

Sound requires a material substance, or medium, to travel. The medium can be a solid, liquid, or gas. Sound cannot travel in a vacuum, which is why no sound can be heard on the moon.

The process of sound propagation involves:

1. A vibrating object displaces the particles of the medium immediately surrounding it from their equilibrium positions.
2. These displaced particles exert a force on adjacent particles, causing them to be displaced.
3. The initial particles then return to their original positions.
4. This process continues, transferring the disturbance through the medium.

It is the disturbance, or energy, that travels, not the particles of the medium themselves. This disturbance travelling through a medium is defined as a wave. Because sound waves are characterized by the motion of particles in a medium, they are classified as mechanical waves.

2. Sound Waves

2.1. Compressions and Rarefactions

As a vibrating object moves, it creates a series of pressure and density variations in the medium.

• Compression (C): When the object moves forward, it pushes and compresses the medium in front of it, creating a region of high pressure and high particle density.
• Rarefaction (R): When the object moves backward, it creates a region of low pressure and low particle density.

The rapid back-and-forth motion of the source creates a continuous series of these compressions and rarefactions, which propagate through the medium as a sound wave.

2.2. Longitudinal Waves

Sound waves are longitudinal waves. In a longitudinal wave, the individual particles of the medium oscillate back and forth in a direction that is parallel to the direction of the wave’s propagation. This can be visualized by pushing and pulling a slinky, where the coils bunch up (compress) and spread out (rarefy) along the length of the slinky.

This is distinct from a transverse wave, where particles of the medium oscillate up and down, perpendicular to the direction of wave propagation. Examples of transverse waves include ripples on a water surface. Light is also a transverse wave, but it is not a mechanical wave as it does not require a medium.

3. Characteristics of a Sound Wave

A sound wave can be described by its frequency, amplitude, and speed.

• Wavelength (λ): The distance between two consecutive compressions or two consecutive rarefactions. Its SI unit is the meter (m).
• Frequency (ν): The number of complete oscillations (or the number of compressions/rarefactions passing a point) per unit time. Its SI unit is the hertz (Hz), named after Heinrich Rudolph Hertz.
• Time Period (T): The time taken for one complete oscillation to occur. Its SI unit is the second (s). Frequency and time period are inversely related: ν = 1/T. The time interval between successive compressions is equal to the time period.
• Amplitude (A): The magnitude of the maximum disturbance in the medium from the mean value of density or pressure. It determines the energy of the wave.
• Speed (v): The distance a point on a wave (like a compression) travels per unit time. The speed of a wave is related to its wavelength and frequency by the equation: Speed (v) = Wavelength (λ) × Frequency (ν).

4. Perceived Qualities of Sound

The characteristics of the sound wave determine the properties we perceive.

• Pitch: This is the brain’s interpretation of a sound’s frequency. A higher frequency corresponds to a higher pitch (e.g., a violin), while a lower frequency results in a lower pitch (e.g., a drum).
• Loudness: This is determined primarily by the amplitude of the sound wave. A wave with a larger amplitude carries more energy and is perceived as a louder sound. A wave with a smaller amplitude is a softer sound.
• Intensity: A physical quantity defined as the amount of sound energy passing through a unit area each second. While related, loudness is the subjective response of the ear to a sound, whereas intensity is an objective measurement.
• Quality (or Timbre): This is the characteristic that allows us to distinguish between two sounds that have the same pitch and loudness. A sound of a single frequency is called a tone. A sound produced by a mixture of several frequencies is a note, which is generally pleasant to hear. An unpleasant sound is classified as noise.

5. Speed of Sound

The speed of sound is not constant; it depends on the properties of the medium through which it travels, especially its state (solid, liquid, gas) and temperature.

• Dependence on Medium: Sound travels fastest in solids, slower in liquids, and slowest in gases. For example, at 25°C, the speed of sound in aluminum is 6420 m/s, while in air it is 346 m/s.
• Dependence on Temperature: The speed of sound in a medium increases as the temperature increases. For example, the speed of sound in air is 331 m/s at 0°C and 344 m/s at 22°C.
• Speed of Light vs. Sound: Sound travels much more slowly than light. This is why thunder is heard several seconds after a lightning flash is seen.

State	Substance	Speed in m/s (at 25 ºC)
Solids	Aluminium	6420
	Nickel	6040
	Steel	5960
	Iron	5950
	Brass	4700
	Glass (Flint)	3980
Liquids	Water (Sea)	1531
	Water (distilled)	1498
	Ethanol	1207
	Methanol	1103
Gases	Hydrogen	1284
	Helium	965
	Air	346
	Oxygen	316
	Sulphur dioxide	213

6. Reflection of Sound

Sound waves bounce off the surface of a solid or liquid. This phenomenon is called reflection and follows the same laws as the reflection of light:

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

6.1. Echo

An echo is the sound heard after reflection from a distant object like a building or a mountain. For a human to hear a distinct echo, the time interval between the original sound and the reflected sound must be at least 0.1 seconds.

• At a temperature of 22°C (speed of sound ≈ 344 m/s), the total distance the sound must travel is 344 m/s × 0.1 s = 34.4 m.
• Therefore, the minimum distance to the reflecting surface must be half of this, or 17.2 m.

Multiple echoes can be heard from successive reflections. The rolling of thunder is caused by successive reflections of sound from clouds and the land.

6.2. Reverberation

Reverberation is the persistence of sound in a large hall due to repeated reflections from walls, ceiling, and floor. While some reverberation can be desirable, excessive reverberation is undesirable as it can make sound unclear. To reduce it, auditoriums are often lined with sound-absorbent materials like compressed fibreboard, rough plaster, and draperies.

6.3. Applications of Multiple Reflection

• Megaphones, Horns, Trumpets: These instruments are designed with a conical opening that guides sound waves in a particular direction through successive reflections, preventing the sound from spreading out.
• Stethoscope: This medical device uses multiple reflections within its tubes to channel the sound of a patient’s heartbeat or breathing to the doctor’s ears.
• Concert Halls: The ceilings of concert halls and auditoriums are often curved to reflect sound and distribute it evenly to all corners of the hall. A curved soundboard can also be placed behind the stage for the same purpose.

7. Range of Hearing and Ultrasound

7.1. Audible, Infrasonic, and Ultrasonic Frequencies

• Audible Range: The average human ear can hear sounds with frequencies between 20 Hz and 20,000 Hz (or 20 kHz). Children under five and some animals like dogs can hear up to 25 kHz.
• Infrasound: Sounds with frequencies below 20 Hz are called infrasonic. Humans cannot hear them, but they are produced by sources like vibrating pendulums, rhinoceroses (as low as 5 Hz), whales, elephants, and earthquakes.
• Ultrasound: Sounds with frequencies above 20 kHz are called ultrasonic. It is produced and used by animals like dolphins, bats, and porpoises for navigation and hunting. Moths can hear the ultrasound squeaks of bats to evade them.

7.2. Applications of Ultrasound

Ultrasound’s high frequency allows it to travel in well-defined paths and makes it useful in industrial and medical fields.

• Cleaning: Used to clean hard-to-reach parts (e.g., spiral tubes, electronic components) by placing them in a solution and using ultrasonic waves to detach dirt and grease particles.
• Detecting Flaws: Ultrasonic waves are passed through metal blocks to detect internal cracks or flaws. The waves are reflected back from any defect, indicating its presence.
• Medical Imaging (Ultrasonography): An ultrasound scanner sends ultrasonic waves into the body. These waves reflect off regions where tissue density changes (e.g., organs, tumors, stones). The reflected waves are converted into images of internal organs like the liver, kidney, uterus, and heart (echocardiography). This is also used to monitor fetal development.
• Breaking Kidney Stones: High-intensity ultrasound can be focused to break small stones in the kidneys into fine grains that can be flushed out with urine.

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Q&A Section

Short-Answer Questions

(Answer in 2-3 sentences)

1. What is sound, and how is it fundamentally produced?
2. Explain why sound is considered a mechanical wave.
3. What is the role of a medium in the propagation of sound?
4. Describe what happens to the particles of the air when a compression is formed.
5. Describe what happens to the particles of the air when a rarefaction is formed.
6. How does a longitudinal wave differ from a transverse wave?
7. Define the wavelength of a sound wave. What is its SI unit?
8. Define the frequency of a sound wave and state its relationship with the time period.
9. What characteristic of a sound wave determines its loudness, and how?
10. What characteristic of a sound wave determines its pitch, and how?
11. Explain the difference between a tone and a note in music.
12. Why is thunder heard a few seconds after the lightning flash is seen?
13. In general, how does the speed of sound change when moving from a solid to a liquid to a gas?
14. What are the two main laws governing the reflection of sound?
15. What is an echo, and what is the minimum time interval required to hear one distinctly?
16. Calculate the minimum distance required to hear an echo in air at 22 ºC, where the speed of sound is 344 m/s.
17. What is reverberation, and how is it typically reduced in large halls?
18. How does a stethoscope utilize the principle of sound reflection?
19. What is the audible frequency range for a typical human being?
20. What is infrasound? Name two animals that use it.
21. What is ultrasound? Name two animals that use it.
22. Describe one industrial application of ultrasound.
23. Explain the basic principle behind using ultrasound for medical imaging (ultrasonography).
24. How does a hearing aid help a person with hearing loss?
25. Can you hear a sound produced by your friend on the moon? Explain why or why not.

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Multiple-Choice Questions (MCQ)

1. Sound is produced by:
   • A) Non-vibrating objects B) Vibrating objects C) Objects in a vacuum D) Objects at high temperature
2. A sound wave consists of a series of:
   • A) Crests and troughs only B) Compressions and rarefactions C) Perpendicular oscillations D) Electromagnetic fields
3. Sound waves are classified as:
   • A) Transverse waves B) Electromagnetic waves C) Longitudinal waves D) Surface waves
4. The property of a sound wave that corresponds to the sensation of pitch is:
   • A) Amplitude B) Speed C) Wavelength D) Frequency
5. Loudness of a sound is primarily determined by its:
   • A) Frequency B) Amplitude C) Speed D) Time Period
6. The SI unit of frequency is:
   • A) Meter (m) B) Second (s) C) Hertz (Hz) D) Meter per second (m/s)
7. A sound wave has a frequency of 220 Hz and a speed of 440 m/s. What is its wavelength?
   • A) 0.5 m B) 2.0 m C) 96800 m D) 440 m
8. In which of the following media will sound travel the fastest at 25°C?
   • A) Air B) Water (distilled) C) Steel D) Ethanol
9. The minimum time interval between the original sound and its echo for it to be heard distinctly is:
   • A) 1.0 s B) 0.01 s C) 0.1 s D) 10 s
10. The persistence of sound in a large hall due to repeated reflections is called:
    • A) Echo B) Pitch C) Reverberation D) Noise
11. The audible range of frequency for the average human ear is:
    • A) 20 Hz to 2,000 Hz B) 2 Hz to 20 Hz C) 20,000 Hz to 25,000 Hz D) 20 Hz to 20,000 Hz
12. Frequencies below 20 Hz are known as:
    • A) Supersonic B) Ultrasound C) Audible sound D) Infrasound
13. Which of the following animals produces ultrasound?
    • A) Elephant B) Rhinoceros C) Bat D) Whale
14. The technique of using ultrasonic waves to form an image of the heart is called:
    • A) Ultrasonography B) Echocardiography C) Metal flaw detection D) Reverberation
15. Megaphones and horns work on the principle of:
    • A) Reverberation B) Single reflection of sound C) Multiple reflection of sound D) Sound absorption
16. What is the relationship between frequency (ν) and time period (T)?
    • A) ν = T B) ν = 1/T C) ν = T² D) ν = λT
17. The time interval between two successive compressions from a source of sound is 0.002 s. The frequency of the source is:
    • A) 50 Hz B) 500 Hz C) 200 Hz D) 0.002 Hz
18. In a sound wave, the particles of the medium:
    • A) Travel from the source to the listener B) Oscillate about their mean positions C) Remain stationary D) Vibrate perpendicular to the wave direction
19. A sound of a single frequency is called a:
    • A) Note B) Noise C) Tone D) Timbre
20. What happens to the speed of sound in air as the temperature increases?
    • A) It decreases B) It remains the same C) It increases D) It becomes zero

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Answer Keys

Short-Answer Questions Answer Key

1. Sound is a form of energy that creates a sensation of hearing. It is produced by the rapid to-and-fro motion, or vibration, of objects.
2. Sound is a mechanical wave because it requires a material medium (like a solid, liquid, or gas) to propagate. The wave is characterized by the motion of the particles of the medium.
3. A medium is the substance through which sound is transmitted from the source to the listener. The particles of the medium vibrate and transfer the sound energy, but the particles themselves do not travel the entire distance.
4. A compression is a region in the medium where particles are crowded together. This results in a temporary region of higher density and higher pressure than the surrounding medium.
5. A rarefaction is a region in the medium where particles are spread apart. This results in a temporary region of lower density and lower pressure.
6. In a longitudinal wave, the particles of the medium oscillate parallel to the direction of wave propagation. In a transverse wave, the particles oscillate perpendicular to the direction of wave propagation.
7. Wavelength is the distance between two consecutive compressions or two consecutive rarefactions in a sound wave. Its SI unit is the meter (m).
8. Frequency is the number of complete oscillations per unit time. It is inversely related to the time period (T), which is the time for one oscillation, by the formula ν = 1/T.
9. Loudness is determined by the amplitude of the sound wave. A wave with a larger amplitude carries more energy and is perceived as a louder sound.
10. Pitch is determined by the frequency of the sound wave. A source vibrating faster produces a higher frequency and, therefore, a higher pitch.
11. A tone is a sound composed of a single frequency. A note is a sound created from a mixture of several frequencies and is generally pleasant to listen to.
12. Sound travels significantly slower (approx. 344 m/s in air) than light (approx. 3 × 10⁸ m/s). Because of this vast difference in speed, the light from the flash reaches an observer almost instantaneously, while the sound of the thunder takes several seconds to travel the same distance.
13. The speed of sound is generally fastest in solids, slower in liquids, and slowest in gases. This is due to the closer packing of particles in solids, which allows the vibration to be transmitted more efficiently.
14. The two laws of reflection are: 1) The angle of incidence is equal to the angle of reflection. 2) The incident sound wave, the reflected sound wave, and the normal to the surface all lie in the same plane.
15. An echo is a reflected sound. To hear it distinctly, the time interval between the original and reflected sound must be at least 0.1 seconds.
16. The total distance travelled is speed × time = 344 m/s × 0.1 s = 34.4 m. Since this is the distance to the obstacle and back, the minimum distance to the obstacle is half of this, which is 17.2 m.
17. Reverberation is the persistence of sound in a large space due to repeated reflections. It is reduced by covering surfaces like walls and ceilings with sound-absorbent materials such as rough plaster, draperies, or compressed fibreboard.
18. A stethoscope uses a long tube to channel the sounds from a patient’s body (like a heartbeat) to the doctor’s ears. The sound waves undergo multiple reflections inside the tube, guiding them along its path with minimal loss of intensity.
19. The audible frequency range for a typical human is approximately 20 Hz to 20,000 Hz (or 20 kHz).
20. Infrasound consists of sound waves with frequencies below 20 Hz. Rhinoceroses and elephants are two examples of animals that communicate using infrasound.
21. Ultrasound consists of sound waves with frequencies above 20 kHz. Bats and dolphins are two examples of animals that use ultrasound for navigation and hunting.
22. An industrial application of ultrasound is cleaning hard-to-reach machine parts. The parts are placed in a cleaning solution, and ultrasonic waves are used to dislodge particles of dust and grease.
23. An ultrasound scanner transmits high-frequency sound waves into the body. These waves travel through tissues and are reflected back when they encounter a change in tissue density, such as the boundary of an organ. A computer converts these reflected waves into a visual image.
24. A hearing aid receives sound through a microphone, which converts sound waves into electrical signals. An amplifier strengthens these signals, and a speaker converts them back into sound waves, which are then directed into the ear for clearer hearing.
25. No, sound cannot be heard on the moon. The moon has no atmosphere, meaning it is a vacuum. Since sound is a mechanical wave, it requires a medium to travel and cannot propagate in a vacuum.

Multiple-Choice Questions Answer Key

1. B) Vibrating objects
2. B) Compressions and rarefactions
3. C) Longitudinal waves
4. D) Frequency
5. B) Amplitude
6. C) Hertz (Hz)
7. B) 2.0 m (λ = v/ν = 440/220 = 2)
8. C) Steel
9. C) 0.1 s
10. C) Reverberation
11. D) 20 Hz to 20,000 Hz
12. D) Infrasound
13. C) Bat
14. B) Echocardiography
15. C) Multiple reflection of sound
16. B) ν = 1/T
17. B) 500 Hz (ν = 1/T = 1/0.002 = 500)
18. B) Oscillate about their mean positions
19. C) Tone
20. C) It increases

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Essay Questions and Answers

1. Describe the complete process of how sound from a ringing school bell travels through the air to reach a student’s ear. In your answer, define compressions, rarefactions, and explain why sound is a longitudinal wave.

Answer: When a school bell is struck, it begins to vibrate rapidly back and forth. This vibration is the source of the sound. As the surface of the bell moves forward, it pushes the air particles in front of it, crowding them together to create a region of high pressure and high density called a compression. When the bell’s surface moves backward, it leaves a space with fewer air particles, creating a region of low pressure and low density called a rarefaction.

This rapid, continuous vibration of the bell produces a series of alternating compressions and rarefactions that travel outwards through the air. The air particles themselves do not travel from the bell to the student; instead, they oscillate back and forth around their equilibrium positions, transferring energy to their neighbors. This propagating disturbance is the sound wave.

Sound is called a longitudinal wave because the oscillations of the air particles are parallel to the direction in which the wave is traveling. The particles move back and forth along the same line that the wave is moving. When this wave of pressure variations reaches the student’s ear, it causes the eardrum to vibrate, which the brain then interprets as the sound of the bell.

2. Explain the three main characteristics of a sound wave: wavelength, frequency, and amplitude. How do frequency and amplitude relate to the perceived qualities of pitch and loudness?

Answer: The three main characteristics of a sound wave are wavelength, frequency, and amplitude.

• Wavelength (λ) is the spatial period of the wave, defined as the distance between two consecutive corresponding points, such as two adjacent compressions or two adjacent rarefactions. It is measured in meters (m).
• Frequency (ν) is the number of complete oscillations or cycles that pass a given point per unit of time. It is measured in hertz (Hz), where 1 Hz is one cycle per second. Frequency is the inverse of the time period (T), which is the time for one full oscillation.
• Amplitude (A) is the magnitude of the maximum displacement or disturbance of the particles of the medium from their mean position. It is related to the energy of the wave; a higher amplitude signifies more energy.

These physical characteristics directly relate to the qualities of sound we perceive.

• Pitch is the perceptual property of sound that allows for its ordering on a frequency-related scale. It is determined by the frequency of the sound wave. A high-frequency sound wave results in a high pitch, while a low-frequency wave results in a low pitch.
• Loudness is the subjective perception of sound pressure. It is primarily determined by the amplitude of the sound wave. A sound wave with a large amplitude has more energy and is perceived as a loud sound, whereas a wave with a small amplitude is perceived as a soft sound.

3. What is an echo? Detail the specific conditions required for a human to perceive a distinct echo, including a calculation to find the minimum distance from a reflecting surface at 22 ºC (speed of sound = 344 m/s).

Answer: An echo is a repetition of a sound caused by the reflection of sound waves from a surface back to the listener. When we shout or clap near a large, suitable reflecting object, such as a tall building or a mountain, we may hear the same sound again a little later; this repeated sound is the echo.

For a human to perceive an echo as distinct from the original sound, there must be a sufficient time delay between them. The sensation of sound persists in the human brain for about 0.1 seconds. Therefore, the time interval between the original sound and the reflected sound must be at least 0.1 seconds.

Using this time interval, we can calculate the minimum distance to the reflecting surface. At a temperature of 22 ºC, the speed of sound in air is approximately 344 m/s. The total distance the sound wave must travel (from the source to the reflector and back to the source) in this minimum time is: Distance = Speed × Time, Distance = 344 m/s × 0.1 s = 34.4 meters

This 34.4 meters is the total path length. The actual distance between the sound source and the reflecting surface is half of this total path length. Minimum distance = 34.4 m / 2 = 17.2 meters. Therefore, to hear a distinct echo, the listener must be at least 17.2 meters away from the reflecting surface under these conditions.

4. Compare and contrast infrasound, audible sound, and ultrasound. Provide the frequency ranges for each and give examples of animals or phenomena associated with infrasound and ultrasound.

Answer: Sound can be categorized into three ranges based on its frequency: infrasound, audible sound, and ultrasound.

• Audible sound refers to the range of frequencies that the average human ear can detect. This range extends from about 20 Hz to 20,000 Hz (or 20 kHz). Sounds within this range are what we perceive in our daily lives, from conversations to music.
• Infrasound refers to sound with frequencies below 20 Hz, which is too low for humans to hear. These low-frequency waves are produced by large-scale natural phenomena and certain large animals. For example, rhinoceroses communicate using infrasound as low as 5 Hz, and whales and elephants also produce infrasonic calls. Earthquakes generate infrasound waves before the main shock, which some animals can possibly detect.
• Ultrasound refers to sound with frequencies above 20,000 Hz (20 kHz), which is too high for humans to hear. These high-frequency waves are used by various animals for echolocation (navigation and hunting). For example, bats, dolphins, and porpoises emit ultrasonic waves and interpret the returning echoes to “see” their environment.

In summary, the key difference is frequency. Audible sound is the range perceptible to humans, while infrasound is below this range and ultrasound is above it.

5. What is reverberation? Explain why it is often undesirable in spaces like auditoriums and describe three specific methods used in architectural design to control or reduce it.

Answer: Reverberation is the persistence of sound within an enclosed space, such as a large hall, as a result of multiple, continuous reflections of the sound from surfaces like walls, the ceiling, and the floor. The sound continues to be heard even after the source has stopped producing it, until the energy is absorbed and the sound becomes inaudible.

In spaces designed for listening, such as auditoriums, concert halls, and conference halls, excessive reverberation is highly undesirable. It can cause sounds to overlap and become blurred, making speech unintelligible and music muddy and unclear.

Architects and acoustical engineers use several methods to control and reduce reverberation:

1. Using Sound-Absorbent Materials: The primary method is to cover large surfaces with materials that absorb sound energy rather than reflecting it. Common materials include compressed fibreboard, rough plaster, thick draperies, and carpets. These porous and soft materials trap sound waves, converting their energy into a small amount of heat.
2. Acoustically Designed Seating: The seats in an auditorium are often upholstered with sound-absorbing fabric and materials. This not only makes them comfortable but also ensures that the acoustic properties of the hall do not change significantly whether it is full of people (who are also good sound absorbers) or empty.
3. Geometric Shaping of Surfaces: The shape of the room itself is crucial. Non-parallel walls, angled ceilings, and specially designed diffusing panels can be used to scatter sound waves rather than letting them reflect directly back and forth. Curved ceilings, for instance, can be designed to reflect sound towards the audience to improve clarity, while also helping to prevent the build-up of reverberant sound.

6. Explain how the principle of multiple reflection of sound is applied in the design and function of a stethoscope and a megaphone.

Answer: The principle of multiple reflection of sound is cleverly used to guide and amplify sound in specific directions in devices like stethoscopes and megaphones.

• Stethoscope: A stethoscope is a medical instrument used to listen to internal sounds of the body, such as a patient’s heartbeat or lung sounds. It consists of a chest-piece, rubber tubes, and earpieces. When the chest-piece is placed on the patient, the faint sound waves travel into the tubes. The sound waves then undergo multiple reflections off the inner walls of the tubes. These successive reflections guide the sound energy along the path of the tube directly to the doctor’s ears with very little loss of intensity, making the faint internal sounds audible.
• Megaphone: A megaphone (or loudhailer) is a cone-shaped device designed to amplify and direct the voice. When a person speaks into the narrow end, the sound waves travel into the conical tube. As the waves propagate towards the wide opening, they reflect multiple times off the inner surfaces of the cone. This process prevents the sound waves from spreading out in all directions and instead confines and guides most of the sound energy in a forward direction, making the voice audible over a much larger distance. Musical instruments like trumpets and shehnais use a similar principle.

7. List and describe three distinct applications of ultrasound technology in the medical field. For each application, explain the underlying physical principle.

Answer: Ultrasound technology, which uses high-frequency sound waves above 20 kHz, has several vital applications in medicine.

1. Medical Imaging (Ultrasonography): This is one of the most common uses. An instrument called an ultrasound scanner is used to get images of internal body organs like the liver, gall bladder, kidneys, uterus, and heart (in a technique called echocardiography). The principle involves a transducer that sends pulses of ultrasound into the body. These waves travel through tissue and are reflected back whenever they hit a boundary between tissues of different densities. The transducer detects the reflected waves (echoes), and a computer analyzes their timing and intensity to construct a real-time image of the organ. It is widely used to monitor fetal development during pregnancy.
2. Breaking Kidney Stones (Lithotripsy): Ultrasound can be used to non-invasively treat kidney stones. In this technique, high-intensity focused ultrasound waves are directed from outside the body to the precise location of the stone inside the kidney. The powerful vibrations of the ultrasonic waves cause the stone to break apart into fine grains. These tiny particles are then small enough to be passed naturally out of the body through urine, eliminating the need for invasive surgery.
3. Detecting Flaws in Structures (Industrial/Medical): While also an industrial application, the same principle can apply to medical implants. Ultrasound is used to detect cracks and flaws in metal blocks used in construction and machinery. High-frequency ultrasonic waves are passed through the material, and detectors are placed to receive them. If there is an internal flaw like a crack, the ultrasound wave will be reflected back instead of passing through. The reflection indicates the presence and location of the defect, which is crucial for ensuring structural integrity.

8. A sound wave has a frequency of 2 kHz and a wavelength of 35 cm. How long will it take to travel 1.5 km? Show all calculations and explain each step.

Answer: This problem requires calculating the time taken for a sound wave to travel a certain distance, given its frequency and wavelength. Step 1: Identify the given information and convert to SI units.

• Frequency (ν) = 2 kHz = 2 × 1000 Hz = 2000 Hz
• Wavelength (λ) = 35 cm = 35 / 100 m = 0.35 m
• Distance (d) = 1.5 km = 1.5 × 1000 m = 1500 m

Step 2: Calculate the speed of the sound wave. The speed of a wave (v) is related to its frequency (ν) and wavelength (λ) by the formula: v = λ × ν Substitute the values: v = 0.35 m × 2000 Hz v = 700 m/s The speed of the sound wave is 700 meters per second.

Step 3: Calculate the time taken to travel the given distance. The relationship between speed, distance, and time is: Speed = Distance / Time. Rearranging the formula to solve for time (t): Time (t) = Distance (d) / Speed (v). Substitute the values: t = 1500 m / 700 m/s t ≈ 2.14 s

Therefore, the sound wave will take approximately 2.14 seconds to travel a distance of 1.5 km.

9. Explain why sound waves travel at different speeds in solids, liquids, and gases. Refer to the properties of the particles in each state of matter in your explanation.

Answer: The speed of sound is determined by the properties of the medium through which it travels, specifically its elasticity and density. The difference in speed between solids, liquids, and gases is due to how closely the particles are packed and how strongly they are bonded.

• Solids: In solids, particles are packed very tightly together and are held in fixed positions by strong intermolecular forces. When one particle vibrates, it quickly transfers this vibration to its immediate neighbors because of this rigidity and proximity. This efficient transfer of energy allows the sound disturbance to propagate very rapidly. Therefore, sound travels fastest in solids (e.g., ~5960 m/s in steel).
• Liquids: In liquids, particles are still close together but are not held in fixed positions. The intermolecular forces are weaker than in solids, allowing particles to slide past one another. Because the particles are less tightly bound, the transfer of vibrational energy from one particle to the next is less efficient and slower than in solids. Consequently, the speed of sound is lower in liquids than in solids (e.g., ~1498 m/s in distilled water).
• Gases: In gases, particles are far apart and move randomly, with very weak intermolecular forces between them. For a vibration to be transferred, a particle must travel a significant distance before colliding with another. This makes the transfer of energy much slower and less efficient than in liquids or solids. As a result, sound travels slowest in gases (e.g., ~346 m/s in air at 25°C).

In summary, the speed of sound is fastest in solids and slowest in gases because the efficiency of energy transfer via particle vibrations depends directly on the proximity and bonding of the particles in the medium.

10. You and a friend are on opposite ends of a long aluminum rod. Your friend strikes their end of the rod with a stone. Explain why you would perceive the sound twice and which sound you would hear first. Use data from the text to support your answer (Speed of sound in aluminum = 6420 m/s; speed in air = 346 m/s at 25°C).

Answer: When the friend strikes one end of the aluminum rod, the sound is generated and travels to the observer at the other end through two different media simultaneously: the solid aluminum rod and the surrounding air. Because sound travels at different speeds in different media, the observer will perceive the sound at two different times.

The two paths for the sound are:

1. Through the aluminum rod itself.
2. Through the air surrounding the rod.

According to the data provided, the speed of sound in aluminum is 6420 m/s, while the speed of sound in air is 346 m/s (at 25°C). Since the speed of sound in aluminum is significantly faster than the speed of sound in air (more than 18 times faster), the sound wave traveling through the solid rod will reach the observer’s ear much more quickly than the sound wave traveling through the air.

Therefore, the observer would hear the sound twice. The first sound they would hear would be the one that travelled through the aluminum rod. A short moment later, the second sound they would hear would be the one that travelled through the air. This phenomenon demonstrates that the speed of sound is dependent on the medium and is much faster in solids than in gases.

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Glossary of Key Terms

• Amplitude: The magnitude of the maximum disturbance in the medium on either side of the mean value. It determines the loudness of a sound.
• Audible Range: The range of sound frequencies that can be heard by the average human, typically from 20 Hz to 20,000 Hz.
• Compression: A region in a longitudinal wave where the particles of the medium are crowded together, corresponding to a region of high pressure.
• Crest: The point on a wave with the maximum value of upward displacement within a cycle. For a sound wave graph, this represents maximum compression.
• Echo: A sound heard after it has been reflected from a distant, hard surface.
• Frequency (ν): The number of complete oscillations or cycles of a wave that occur per unit of time. Its SI unit is the hertz (Hz).
• Hertz (Hz): The SI unit of frequency, equal to one cycle per second.
• Infrasound: Sound with frequencies below the lower limit of human audibility (below 20 Hz).
• Intensity: The amount of sound energy passing each second through a unit area.
• Longitudinal Wave: A wave in which the particles of the medium vibrate parallel to the direction of wave propagation. Sound waves are longitudinal waves.
• Loudness: The perceptual attribute of sound that corresponds to its physical strength (amplitude). It is the ear’s response to the intensity of a sound.
• Mechanical Wave: A wave that requires a medium to propagate its energy. Sound is a mechanical wave.
• Medium: The matter or substance through which a wave, such as sound, is transmitted. Can be solid, liquid, or gas.
• Note: A sound produced by a mixture of several frequencies, typically perceived as pleasant.
• Pitch: The quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone, as perceived by the ear. It is determined by frequency.
• Quality (or Timbre): The characteristic of a sound that distinguishes it from others of the same pitch and loudness.
• Rarefaction: A region in a longitudinal wave where the particles of the medium are spread apart, corresponding to a region of low pressure.
• Reflection of Sound: The bouncing back of sound waves from a surface. It follows the same laws as the reflection of light.
• Reverberation: The persistence of sound in an enclosed space as a result of multiple reflections.
• Sound: A form of energy that propagates as waves through a medium and produces a sensation of hearing.
• Speed of Sound: The distance traveled per unit of time by a sound wave as it propagates through a medium.
• Time Period (T): The time taken for one complete oscillation or cycle of a wave. It is the reciprocal of frequency (T = 1/ν).
• Tone: A sound consisting of only a single frequency.
• Transverse Wave: A wave in which the particles of the medium vibrate perpendicular to the direction of wave propagation.
• Trough: The point on a wave with the maximum value of downward displacement within a cycle. For a sound wave graph, this represents maximum rarefaction.
• Ultrasound: Sound with frequencies above the upper limit of human audibility (above 20,000 Hz).
• Vibration: A rapid to-and-fro motion of an object about a central position. Vibration is the source of all sound.
• Wavelength (λ): The distance between two consecutive compressions or two consecutive rarefactions in a sound wave.