Chapter 10: Sound Waves: Characteristics and Applications
Verified step-by-step solutions for your class notes.
Which observation best supports the idea that sound is a mechanical wave?
(i) Sound shows reflection
(ii) Sound needs a medium to propagate
(iii) Sound has frequency
(iv) Sound carries energy
- Correct Option: (ii) Sound needs a medium to propagate
For a sound wave propagating in a medium, increasing its frequency will increase its
(i) wavelength
(ii) speed
(iii) number of compressions per second
(iv) time period
- Correct Option: (iii) number of compressions per second
If 20 compressions pass a point in 4 seconds, the frequency is
(i) 80 Hz
(ii) 5 Hz
(iii) 10 Hz
(iv) 0.2 Hz
- Correct Option: (ii) 5 Hz
In a room, the reflected sound reaches the ear 0.05 s after its production. Will it produce an echo or reverberation? Justify your answer.
- Result: Reverberation. The time gap of 0.05 s is less than the 0.1 s required by the human ear to distinguish a distinct echo.
Graphs representing two sound waves are given in Fig. 10.30. If the scales on the X and Y axes of the two graphs are the same, which of the two sound waves has (i) greater wavelength, and (ii) smaller amplitude?
- (i) Greater wavelength: Wave (a)
- (ii) Smaller amplitude: Wave (a)
The sound waves emitted by three sources A, B and C are represented in Fig. 10.31. If the frequency of A is maximum and C is minimum, identify the corresponding curves, and mark A, B and C on them.
- Curve A (Maximum Frequency): Red curve
- Curve B: Green curve
- Curve C (Minimum Frequency): Blue curve
Draw a graph to represent a sound wave for which the density amplitude is 3 units and wavelength is 4 cm.
- Graph details: Plot a standard wave curve where the peak height equals 3 units on the vertical density axis, and one full wave cycle spans 4 cm horizontally.
In a movie, while showing the explosion of a spacecraft in space, a flash of light is shown along with sound at the same time. What are the errors in this depiction?
- Error 1: Space is a vacuum, so sound cannot travel and no explosion audio should be heard.
- Error 2: Light travels much faster than sound, so the visual flash would arrive way before any sound.
A source produces a sound wave of wavelength 3.44 m. If the wave travels with a speed of 344 m s–1 find its time period.
- Frequency: 344 m s−1 / 3.44 m = 100 Hz
- Time Period: 1 / 100 Hz = 0.01 s
A ship searching for a sunken ship sent a sonar signal and detected an echo after 5 s. If ultrasonic wave travels at 1525 m s–1 in seawater, approximately how far down in the ocean is the wreckage of the sunken ship located?
- One-way time: 5 s / 2 = 2.5 s
- Distance (Depth): 1525 m s−1 × 2.5 s = 3812.5 m
A vehicle is fitted with an ultrasonic distance sensor as part of parking assistance system which provides echolocation, while the driver is reversing the vehicle. It emits ultrasonic wave (about 40 kHz) which is reflected by the obstacle. When the warning beep starts sounding at a distance of 1.2 m from the obstacle, how much time is taken by ultrasonic wave to travel to the obstacle and come back? Assume the speed of ultrasonic wave in air to be 345 m s–1.
- Total roundtrip distance: 2 × 1.2 m = 2.4 m
- Total roundtrip time: 2.4 m / 345 m s−1 = 0.00696 s
The speed of sound in air is about 331 m s–1 at 0 °C and nearly 344 m s–1 at 22 °C. Roughly how much extra time will the sound of thunder take to travel a distance of 1720 m, if the air temperature changes from 22 °C to 0 °C? Assume that all other conditions remain unchanged.
- Time at 22 °C: 1720 m / 344 m s−1 = 5 s
- Time at 0 °C: 1720 m / 331 m s−1 = 5.196 s
- Extra time: 5.196 s − 5 s = 0.196 s
The variation of density of medium for a sound wave propagating with a speed of 340 m s–1 is shown in Fig. 10.32. Calculate the wavelength and frequency of the sound wave.
- Wavelength: 8 cm / 2 = 4 cm = 0.04 m
- Frequency: 340 m s−1 / 0.04 m = 8500 Hz
The graphical representation of two sound waves A and B propagating at the same speed of 345 m s–1 is shown in Fig. 10.33. What is the wavelength of each of them? Also, calculate their frequencies.
- Wave A: Wavelength = 2.5 cm = 0.025 m; Frequency = 345 m s−1 / 0.025 m = 13800 Hz
- Wave B: Wavelength = 5.0 cm = 0.05 m; Frequency = 345 m s−1 / 0.05 m = 6900 Hz
Two identical sound sources are placed at A and B — one in air and one submerged in water (Fig. 10.34). Both produce sounds at the same time, which travel horizontally to the vertical side of the cliff and come back. If the time taken by the sound to return to A is 4.5 times than that of B, what is the ratio between the speeds of sound in air and water?
- Ratio of speed (Air : Water): 1 : 4.5 (or 2 : 9)