Sound Waves: Characteristics and Applications — Important Questions

Answers

• Strike the tuning fork gently on a rubber pad.
• Bring the vibrating prongs of the tuning fork close to your ear; you will hear a sound.
• Gently touch the surface of water with one of the vibrating prongs. You will observe ripples or waves forming on the water surface, which indicates that the prong is vibrating.
  4. Experiment to demonstrate sound travel through solids:
• Place your ear against one end of a long wooden table or a metal rod.
• Ask a friend to gently tap or scratch the other end of the table/rod.
• You will be able to hear the sound clearly through the solid material, even if the sound produced in the air is faint. This demonstrates that sound can propagate through solids.
  5. Astronauts cannot hear each other speak directly on a spacewalk because sound requires a material medium (like air, water, or solids) to propagate. The space outside the Earth's atmosphere is a near vacuum, meaning there is no medium for sound waves to travel. Therefore, sound cannot travel from one astronaut to another. They use special radio devices for communication.
  6. When a piston in a tube moves forward, it pushes the air particles in front of it, compressing them into a small region of higher density. This region is called a compression (C). These compressed particles then collide with the particles further ahead, passing the compression forward. As the piston moves backward, it creates a region of lower density behind it, called a rarefaction (R). The oscillating piston produces alternate compressions and rarefactions that propagate through the air as a sound wave.
  7. Mechanical waves are waves that require a material medium for propagation. Sound waves are an example of mechanical waves because they need a medium (solid, liquid, or gas) to travel from the source to the listener. Other examples include waves on a string.
  8. Infrasonic waves: These are sound waves with frequencies below the human audible range (below 20 Hz). They can travel long distances.
• Application: Detecting natural events like earthquakes and volcanic eruptions.
  Ultrasonic waves: These are sound waves with frequencies above the human audible range (above 20 kHz).
• Application: Used in medical imaging (sonography) to visualize internal organs.
  9. Frequency (ν) = 512 Hz.
  Time period (T) = 1/ν
  T = 1 / 512 s ≈ 0.00195 s.
  If the frequency were doubled to 1024 Hz, the pitch of the sound would increase, making the sound shriller.
  10. Sound wave (a) has a greater wavelength. In graph (a), the distance between consecutive crests (or troughs) is larger compared to graph (b). Wavelength is defined as the distance between two consecutive crests or troughs.
  11. An echo is the reflection of sound from a hard surface that reaches the listener with a noticeable time delay after the original sound. For an echo to be heard distinctly, the time interval between the original sound and the reflected sound must be at least 0.1 seconds. This requires the reflecting surface to be at a minimum distance of about 17 meters from the source of the sound.
  12. Speed of sound in air (v_air) ≈ 340 m/s.
  Speed of sound in steel (v_steel) ≈ 15 × v_air
  v_steel ≈ 15 × 340 m/s = 5100 m/s.
  13. Reverberation is the phenomenon where sound persists in a large hall or auditorium due to multiple reflections from the walls, ceiling, and floor. This occurs when reflected sounds arrive at the listener with a time difference less than 0.05 seconds from the direct sound. To control reverberation, sound-absorbing materials like curtains, carpets, upholstered furniture, and special acoustic panels are used. These materials reduce the intensity of reflected sound.
  14. Time taken for the sonar signal to return = 5 s.
  Time taken to reach the sunken ship = 5 s / 2 = 2.5 s.
  Speed of sound in seawater = 1525 m/s.
  Depth of the ocean = Speed × Time = 1525 m/s × 2.5 s = 3812.5 meters.
  15. Bats navigate and hunt using echolocation. They emit high-frequency ultrasonic sound pulses. When these sound waves strike an object (like prey or an obstacle), they get reflected back as echoes. The bat listens to these echoes and uses the time taken for the echo to return, as well as the difference in intensity and frequency of the echoes received by its two ears, to determine the distance, direction, size, and texture of the object.
  16. Frequency (ν) = 40 kHz = 40,000 Hz.
  Speed of sound in air (v) = 345 m/s.
  Wavelength (λ) = v / ν = 345 m/s / 40,000 Hz = 0.008625 m = 8.625 mm.
  17. Time taken for echo = 2 s.
  Speed of sound in air (v) = 340 m/s.
  Total distance traveled by sound (to the cliff and back) = speed × time = 340 m/s × 2 s = 680 m.
  Distance from the cliff = Total distance / 2 = 680 m / 2 = 340 m.
  18. The flash of light from an explosion is due to light waves, which travel at an extremely high speed (approximately $3\times {10}^8$ m/s). The sound of the explosion is due to sound waves, which travel much slower in air (approximately 340 m/s). Therefore, the light reaches the observer much faster than the sound, causing us to see the flash before hearing the sound.
  19. Wavelength (λ) = 3.44 m.
  Speed of sound (v) = 344 m/s.
  Speed = Frequency × Wavelength
  Frequency (ν) = v / λ = 344 m/s / 3.44 m = 100 Hz.
  Time period (T) = 1 / Frequency = 1 / 100 Hz = 0.01 s.
  20. Frequency of vibration (ν) = 512 Hz.
  Time period (T) = 1 / ν = 1 / 512 Hz ≈ 0.00195 s.
  If the tuning fork is struck harder:
• Loudness: The amplitude of vibration will increase, leading to a louder sound.
• Pitch: The frequency of vibration (and hence the pitch) will remain essentially unchanged, assuming the tuning fork is not damaged.