Class IX Solutions of Sound

I. Very Short Answer Type Questions: 

1. If 20 waves are produced per second, what is the frequency in Hertz?Ans. Frequency = 20 Hz

2. The frequency of a source of sound is 10 Hz. How many times does it vibrate in a minute?

Ans. Vibrations per second = 10 Vibrations per minute = 10 × 60 = 600 times

3. What type of wave is a sound?

Ans. Longitudinal wave

4. What is the audible range of the human ear?

Ans. 20 Hz to 20 kHz (20,000 Hz)

5. Define mechanical wave.

Ans. A mechanical wave is a wave that requires a medium (solid, liquid, or gas) to travel.

6. Define wave motion.

Ans. Wave motion is the transfer of energy and momentum from one point to another without the actual transport of matter.

7. What is supersonic speed?

Ans. Supersonic speed is the speed greater than the speed of sound in a given medium (more than 343 m/s in air at room temperature).

8. Write a difference between longitudinal and transverse 

waves.

Ans. Longitudinal Wave: Particles of the medium vibrate parallel to the direction of wave propagation (e.g., sound waves).

Transverse Wave: Particles of the medium vibrate perpendicular to the direction of wave propagation (e.g., light waves, water waves).

9. What are the three quantities that play an important role in describing the nature of a wave?

Ans. Wavelength (λ), Frequency (f), and Wave Speed (v)

10. Define frequency.
Ans. Frequency is the number of waves produced per second. It is measured in Hertz (Hz).

11. What is the time period?
Ans. Time period (T) is the time taken to complete one wave cycle. It is the inverse of frequency: T = 1/f

12. What is the unit of frequency?
Ans. Hertz (Hz)

13. Write a relation between wavelength, frequency, and wave speed.
Ans. Wave speed (v) = Frequency (f) × Wavelength (λ)

14. What are the factors that make the sound seem different to us?
Ans. Pitch, loudness, and quality (or timbre)

15. Define pitch.
Ans. Pitch is the perception of how high or low a sound is, determined by its frequency.

16. What is the relation between amplitude and loudness?
Ans. Loudness is directly proportional to the square of the amplitude:

$$\text{<span style="font-family: verdana;">Loudness</span>}<span\; style="font-family:\; verdana;">\propto </span>{\mathrm{<span\; style="font-family:\; verdana;">A</span>}}^{\mathrm{<span\; style="font-family:\; verdana;">2</span>}}$$

17. What do you mean by the quality of musical sound?
Ans. Quality (Timbre) of sound is the characteristic that helps us distinguish between different sources of sound, even if they have the same pitch and loudness.

18. What is the frequency range of hearing of the human ear?
Ans. 20 Hz to 20 kHz

19. Give one example of each of longitudinal and transverse waves.
Ans. Longitudinal wave: Sound wave
Transverse wave: Light wave

20. What is the frequency of a wave with a time period of 0.025 s?
Ans. Frequency = 1/T = 1/0.025=$40$ Hz

21. Why do we hear the sound of an approaching car before the car reaches us?

Ans. Because sound travels faster than the car itself in the air.

22. A baby recognizes her mother by her voice. Name the characteristic of sound involved.
Ans. Quality (Timbre)

23. What is the unit of wavelength of sound?
Ans. Meter (m)

24. Infrasound is produced by:
Ans. (c) Rhinoceros

25. Which of the following is used in echocardiography?
Ans.(a) Ultrasound waves

26. Speed of sound is maximum in:
Ans. (a) Solids

27. Light is a:
Ans. (b) Transverse wave

28. In compression, pressure density is:
Ans. (a) High

29. Frequency of an ultrasonic sound wave is:
Ans. (b) Greater than 20,000 Hz

30. Stethoscope works on the principle of:
Ans. (a) Multiple reflections of sound

31. Audible range of the human ear is:

Ans. (d) Both (a) and (c) → 20 Hz – 20 kHz or 20 Hz – 20,000 Hz

32. The order of bones in the human ear from outside to inside:
Ans. (b) Hammer, Anvil, and Stirrup.

II. Short Answer Type Questions:

1. Differentiate between longitudinal and transverse waves?

Ans.

• Longitudinal waves: Particles of the medium vibrate parallel to the direction of wave propagation (e.g., sound waves).
• Transverse waves: Particles of the medium vibrate perpendicular to the direction of wave propagation (e.g., light waves, water waves).

2. Define the terms 'crest' and 'trough' of a wave?

Ans.

• Crest: The highest point of a transverse wave.
• Trough: The lowest point of a transverse wave.

3. Define wave motion.
Ans. Wave motion is the transfer of energy from one point to another without the actual transfer of matter, through vibrations in a medium or field.

4. Draw figures to show the wave shapes of two sounds: guitar and car horn. Which sound has a higher pitch?
Ans. (You need to draw waveforms: a smooth sinusoidal wave for a guitar and an irregular wave for a car horn.)
A guitar produces a higher-pitched sound than a car horn because it has a higher frequency.

5. Why do we say that sound waves are longitudinal waves?
Ans. Sound waves are longitudinal because the air particles vibrate back and forth in the same direction as the wave propagation, forming compressions and rarefactions.

6. Define supersonic speed and sonic boom.

Ans. 

• Supersonic speed: Speed greater than the speed of sound (~340 m/s in air).
• Sonic boom: A loud explosive noise caused by an object moving faster than sound, producing shock waves.

7. Define longitudinal and transverse waves

Ans.

• Longitudinal waves: Waves in which particle vibrations are parallel to wave direction.
• Transverse waves: Waves in which particle vibrations are perpendicular to wave direction.

8. What do you mean by reverberation?
Ans. Reverberation is the persistence of sound due to multiple reflections from surfaces, creating a prolonged effect.

9. Define the terms: Wavelength and Frequency.

Ans. 

1. Wavelength (λ): The distance between two consecutive crests or troughs (in transverse waves) or compressions and rarefactions (in longitudinal waves).
2. Frequency (f): The number of wave cycles passing a point per second, measured in Hertz (Hz).

10. An underwater device detects ultrasounds of frequency 75 kHz towards the water surface. What is the wavelength of sound in the air above the water surface and what is its frequency?

Ans. 

• Given:
  • Frequency $f = 75$ kHz = $75 \times 10^3$ Hz
  • Speed of sound in air $v = 340$ m/s
• Wavelength $\lambda = \frac{v}{f} = \frac{340}{75 \times 10^3} = 0.00453$ m or 4.53 mm
• Frequency remains the same: 75 kHz

11. What is an echo? Name two areas of its application.

Ans. An echo is the reflection of sound when it bounces off a surface and is heard again.
Applications:

• SONAR (Sound Navigation and Ranging)
• Medical ultrasonography (ultrasound scanning)

12. Establish the relation for a wave: velocity = frequency × wavelength.

Ans. 

• Wave velocity $v$ is the distance covered by a wave per second.
• Since distance covered in one cycle is the wavelength $\lambda$, and frequency $f$ is the number of cycles per second, v=λ/T

  $$v=f\times \mathrm{\lambda \; (f=1/T)}$$

13. When a wave travels from one medium to another, the wavelength changes but not the frequency. The wavelength of sound disturbance is 30 cm in air, and velocity is 340 m/s. What will be the wavelength in Helium & water?

Ans. 

• Given: $\lambda_{\text{air}} = 30$ cm = 0.30 m, $v_{\text{air}} = 340$ m/s
• Speed in Helium: $v_{\text{helium}} = 970$ m/s $$\lambda_{\text{helium}} = \frac{v_{\text{helium}}}{v_{\text{air}}} \times \lambda_{\text{air}} = \frac{970}{340} \times 0.30 = 0.855 \text{ m}$$
  
• Speed in Water: $v_{\text{water}} = 1450$ m/s $$\lambda_{\text{water}} = \frac{1450}{340} \times 0.30 = 1.28 \text{ m}$$

14. Sound waves of wavelength λ travel from a medium in which velocity is v m/s into another medium in which velocity is 3v m/s. What is the new wavelength?

Ans. Let new wavelength=λ′ and new velocity = v′

According to question v′ =3v

$${\lambda }^{\mathrm{\prime }}=\frac{v^{\mathrm{\prime }}}{v}\times \lambda =\frac{3v}{v}\times \lambda =3\lambda$$

15. Why are sound waves called mechanical waves?

Ans. Sound waves require a medium (solid, liquid, or gas) to propagate, meaning they rely on particle interaction, making them mechanical waves.

16. Define:(a) Time Period (b) Amplitude.

Ans. (a) Time Period (T): The time taken for one complete wave cycle, $T = \frac{1}{f}$.
(b) Amplitude (A): The maximum displacement of particles from their mean position in a wave.

17. Write the differences between noise and music.

Ans. 

Noise	Music
(a)Irregular, unpleasant sound	Regular, pleasant sound
(b) No fixed frequency or waveform	Fixed frequency and harmonious waveform
Example: Traffic noise	Example: Guitar melody

18. What do you understand by loud and soft sound?

Ans. 

• Loud sound: High amplitude wave, more energy.
• Soft sound: Low amplitude wave, less energy.

19. A sound wave travels at 340 m/s. If the wavelength is 1.4 m, what is the frequency?

$$f = \frac{v}{\lambda} = \frac{340}{1.4} = 242.86 \text{ Hz}$$

20. Sound requires a medium to travel. Justify experimentally.

Ans. Experiment: Place an electric bell inside a vacuum jar. As air is pumped out, sound decreases and stops, proving sound cannot travel in a vacuum.

21. Explain how echoes are used by bats to locate obstacles and prey.

Ans. Bats emit high-frequency ultrasonic waves. These waves reflect off obstacles or prey and return as echoes, helping bats determine distance and shape of objects using echolocation.

III. Long Answer Type Questions:

1. Sound cannot travel in a vacuum. Describe an experiment to demonstrate this

Ans. Experiment: Bell Jar Experiment

• A bell is placed inside a glass jar connected to a vacuum pump.
• The bell is rung, and the sound is clearly heard.
• As air is gradually pumped out, the sound becomes fainter.
• When air is completely removed, the sound is no longer heard.
  Conclusion: Since sound needs a medium (air) to propagate, it cannot travel in a vacuum.

2. What are crest and trough of a wave?

Ans. In a wave, the crest and trough are the highest and lowest points, respectively, in the wave pattern.

1. Crest:

   • The crest of a wave is the highest point reached by the wave above its resting position (also called the equilibrium position).
   • It represents the maximum positive displacement of the medium from its mean position.
   • In a water wave, the crest is the peak of the wave.
   • In a sound wave, the crest represents a region of high pressure (compression).

2. Trough:

   • The trough of a wave is the lowest point reached by the wave below its resting position.
   • It represents the maximum negative displacement of the medium from its mean position.
   • In a water wave, the trough is the lowest dip between two crests.
   • In a sound wave, the trough represents a region of low pressure (rarefaction).

(Diagram of a transverse wave showing crests and troughs can be included.)

3. Explain the working and application of SONAR.

Ans. Working of SONAR:

• SONAR (Sound Navigation and Ranging) is used to detect underwater objects using ultrasonic waves.
• A transmitter emits ultrasonic waves, which travel through water.
• These waves reflect upon hitting an object and return as echoes.
• The time taken for the echo to return helps determine the object's distance using: $$\text{Distance}=\frac{\text{Velocity of sound}\times \text{Time}}{\mathrm{2<span\; style="font-family:\; verdana;"></span>}}$$

Applications:

• Locating submarines and icebergs.
• Mapping the seabed.
• Detecting defects in metallic structures.

4. Explain the structure of the human ear with the help of a diagram.

Ans. (A labeled diagram of the human ear should be drawn.)

Parts of the human ear:

1. Outer Ear: Captures sound waves.
2. Middle Ear: Contains the eardrum and three bones (malleus, incus, stapes) that amplify sound.
3. Inner Ear: Converts vibrations into electrical signals via the cochlea, which are sent to the brain.

5. A cork moves up and down in water completing 5 vibrations in 4 s. The waves travel 20 m to the shore in 10 s. Calculate: Speed, Frequency, Wavelength.

Ans. Given: Distance = 20m, Time = 10 s and Frequency = 5/4 Hz

• Speed: $v = \frac{\text{Distance}}{\text{Time}} = \frac{20}{10} = 2$ m/s
• Frequency: $f = \frac{\text{Vibrations}}{\text{Time}} = \frac{5}{4} = 1.25$Hz
• Wavelength: $\lambda = \frac{v}{f} = \frac{2}{1.25} = 1.6$ m

6. Train sound arrives from steel rails and air with a time difference of 3.5 s. Distance to railway station?

Ans. Given:

• Speed of sound in air = 340 m/s
• Speed of sound in steel = 5130 m/s
• Time difference = 3.5 s

Let distance = $d$,

$$\frac{d}{340} - \frac{d}{5130} = 3.5$$

Solving for $d$,

d = 1274m

Distance to the railway station: 1274 m.

7. How can ultrasound be used to detect defects in metal blocks?

Ans. 

• Ultrasound waves are sent through the metal.
• If the wave passes through without reflection, the metal is defect-free.
• If an internal crack is present, waves reflect early.
• The reflected signals are analyzed to detect defects.

8. What do you understand by low-pitched and high-pitched sound?

Ans. 

• Low-pitched sound: Low frequency, deep tone (e.g., drum sound).
• High-pitched sound: High frequency, sharp tone (e.g., whistle).

9. Why do we see light first and hear the sound later during a thunderstorm?

Ans. 

• Light travels faster (~$3 \times 10^8$ m/s) than sound (~340 m/s).
• We see the lightning first, but sound reaches later due to its slower speed.

10. What are the laws of reflection of sound?

Ans. 

1. Angle of incidence = Angle of reflection.
2. Incident wave, reflected wave, and normal lie in the same plane.

11. Why are ceilings of concert halls curved?

Ans. Curved ceilings help reflect and spread sound evenly, avoiding dead spots and enhancing acoustics.

12. What is reverberation? How is it reduced?

Ans. 

• Reverberation is the persistence of sound due to multiple reflections.
• Reducing reverberation:
  • Using carpets, curtains, foam panels.
  • Constructing halls with sound-absorbing materials.

13. Man hears echoes at intervals of 2 s and 3.5 s in a valley. Find: Distance between mountains and Man's position.

Ans. 

• Distance between mountains: $$d = \frac{340 \times 3.5}{2} = 595 \text{ m}$$
  
• Man's position: $$d = \frac{340 \times 2}{2} = 340 \text{ m}$$
  

Position of the man: 340 m from one mountain, 255 m from the other.

14. What is SONAR? Write its working.

(Already answered in Q3.)

15. A stone thrown in a pond produces 12 ripples per second. Distance between crest and trough = 10 cm. Calculate wavelength and velocity.

Ans. 

• Wavelength:
  • Given: Distance between crest and trough = 10 cm → Full wave = 20 cm = 0.2 m.
  • $\lambda = 0.2$ m.
• Velocity: $$v = f \times \lambda = 12 \times 0.2 = 2.4 \text{ m/s}$$
  

16. Why is lightning seen before the sound of thunder heard? What causes reverberation of thunder?

Ans. (Answered in Q9.)

• Reverberation occurs as sound waves reflect off clouds, mountains, and buildings, making thunder rumble.

17. On what factor does pitch of a sound depend?

Ans. Pitch depends on frequency—higher frequency = higher pitch.

18. Draw a diagram to represent the sound of:(a) High pitch (b) Low pitch

Ans. 

• (a) High pitch: More waves in a given time (high frequency).
• (b) Low pitch: Fewer waves in a given time (low frequency).

(Diagrams should show differences in wave frequency.