Sound Waves: Characteristics and Applications — Class 9 Science
Exploration · Chapter 10
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- मुहावरे
Chapter summary
Key ideas & terms
- Longitudinal wave
- A wave in which the particles of the medium vibrate back and forth along the same direction in which the wave travels, as in sound.
- Compression and rarefaction
- Regions of a sound wave where particles are crowded (high pressure) and where they are spread apart (low pressure), respectively.
- Frequency
- The number of vibrations (or compressions) produced per second, measured in hertz (Hz); it determines the pitch of a sound.
- Amplitude
- The maximum displacement of particles from their mean position; it determines the loudness of the sound.
- Wave speed relation
- The speed of a wave equals its frequency times its wavelength,
. - Echo
- A repetition of sound heard after it is reflected from a distant surface, perceived when it returns after about 0.1 second.
Extra questions & answers
How is sound produced?
Sound is produced by vibrating objects; the vibrations disturb the surrounding medium and this disturbance spreads outward as a sound wave that carries energy.
Why is sound called a longitudinal wave?
Because the particles of the medium vibrate back and forth along the same line as the direction of travel, producing alternate compressions and rarefactions.
What are compressions and rarefactions?
Compressions are regions where the medium’s particles are pushed close together (high pressure) and rarefactions are regions where they are spread apart (low pressure).
Why can’t sound travel through a vacuum?
Sound needs particles of a material medium to pass on the vibrations; a vacuum has no particles, so there is nothing to carry the wave, which is why astronauts cannot hear each other directly in space.
How does frequency affect a sound?
Frequency, measured in hertz, decides the pitch: a high-frequency sound has a high (shrill) pitch and a low-frequency sound has a low pitch.
What decides the loudness of a sound?
The amplitude of the wave decides loudness — a larger amplitude carries more energy and produces a louder sound.
Write the relation between speed, frequency and wavelength.
The speed of sound
In which medium does sound travel fastest, and why?
Sound travels fastest in solids because their particles are closely packed and tightly bound, so vibrations pass quickly from one particle to the next; it is slowest in gases.
What is the audible range of human hearing, and what lie beyond it?
Humans hear roughly from 20 Hz to 20,000 Hz; sound above 20,000 Hz is ultrasound and below 20 Hz is infrasound.
How does SONAR use sound?
SONAR sends out ultrasound pulses into water and measures the time taken for echoes to return from objects or the sea bed, then uses
Think It Over
Two astronauts are repairing the arm of a space station together during a spacewalk. Can they talk to each other and hear the sounds of metal clanking as they do on the Earth?
No. Sound needs a material medium (solid, liquid or gas) to propagate. In a spacewalk there is near vacuum with no air between the astronauts. Sound waves are mechanical longitudinal waves where compressions and rarefactions travel by collisions of particles. Without a medium they cannot travel, so voice and clanking cannot be heard directly. Astronauts communicate through radio systems built into their spacesuits.
How do most bats use sound to locate their prey in the dark at night?
Most bats use echolocation. They emit short bursts of ultrasonic waves (frequency
Pause and Ponder
1. Explore various ways of producing sound.
Sound is produced by vibration of an object called the source. Ways include: vibrating stretched strings (rubber band, sitar, violin), vibrating membranes (drum, tabla, eardrum), vibrating air columns (flute, bansuri, whistle), striking or rubbing solid objects (bell, taal, metal plate) and vibration of vocal cords in humans and animals. In each case to-and-fro oscillations transfer energy to the surrounding medium.
2. Make a list of different types of musical instruments and identify their vibrating parts which produce sound.
Examples: (a) String instruments - sitar, guitar, tanpura, sarangi - vibrating stretched string; (b) Membrane instruments - tabla, mridangam, drum, dhol - vibrating stretched membrane with syaahi; (c) Wind instruments - bansuri, flute, shehnai - vibrating air column inside tube; (d) Solid percussion - taal, manjira, bell - vibrating metal body. In most instruments more than one part vibrates together to enrich the sound.
Pause and Ponder
3. Assertion (A): We cannot hear the sound of a bell ringing in a closed jar after most of the air is pumped out.
Reason (R): Sound requires a medium to travel.
Choose the correct statement:
(i) Both A and R are true, but R is not the correct explanation of A.
(ii) Both A and R are true, and R is the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
Correct choice is (ii). Both A and R are true, and R is the correct explanation of A. The bell jar experiment shows loudness decreases as air is removed and sound is almost inaudible near vacuum, even though the bell is still ringing. This proves sound waves are mechanical waves that need a material medium. Removing the medium removes propagation.
Pause and Ponder
4. Assertion (A): Compressions and rarefactions move through the medium.
Reason (R): Individual particles of the medium continuously move forward with the wave.
Choose the correct statement:
(i) Both A and R are true, but R is not the correct explanation of A.
(ii) Both A and R are true, and R is the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
Correct choice is (iii) A is true, but R is false. Compressions (higher density) and rarefactions (lower density) do propagate away from the source through successive collisions. Individual air particles do not travel with the wave; they only oscillate about their mean positions parallel to propagation and return after the wave passes. Energy is transferred, not mass.
Pause and Ponder
5. When sound travels from a tuning fork to your ear, which of the following actually reaches your ear?
(i) Air particles near the tuning fork
(ii) Energy carried by sound waves
(iii) The tuning fork material
(iv) A continuous stream of compressed air
Correct choice is (ii) Energy carried by sound waves. Particles vibrate locally and transfer disturbance through compressions and rarefactions. The particles near the fork stay near the fork; what propagates and reaches the eardrum is the density disturbance carrying energy, which makes the eardrum vibrate and is perceived as sound.
Pause and Ponder
6. The variation of density of the medium for two sound waves is shown in Fig. 10.17 (a) and (b). Label compression and rarefaction by C and R on it. In the graph given in Fig. 10.17 (c) and (d), label the axes and draw the curves corresponding to Fig. 10.17 (a) and (b).
In the dot patterns, dense dotted regions are compressions (C) and sparse regions are rarefactions (R): pattern (a) shows alternating dense-sparse every short distance (shorter wavelength) and (b) shows longer dense and sparse stretches (longer wavelength). On the graphs (c) and (d) the horizontal axis is Distance and vertical axis is Density (with Average density as dashed line). Graph (c) should show more crests/troughs in the same distance (higher frequency, shorter
Pause and Ponder
7. Conduct Activity 10.1 once again with a thick rubber band and then with a thin rubber band. Does the thin rubber band vibrate faster than the thick rubber band? If yes, how do the frequency and time period of the sound produced by the thin rubber band differ from that of the thick rubber band?
Yes, the thinner, lighter and more taut rubber band vibrates faster. Faster vibration means more oscillations per second, so frequency
8. If the frequency of a sound wave produced by an oscillating piston of a long tube filled with air is 20 Hz, then how many oscillations does the piston complete per minute?
Frequency
9. For the sound wave represented by the graph shown in Fig. 10.19, what is half of its wavelength?
From Fig. 10.19 the distance between two successive crests (or troughs) is
Pause and Ponder
10. Table 10.1 shows the speed of sound in a few media at atmospheric pressure. Table 10.1: Speed of sound in different media at 15 °C State Substance/Medium Approximate speed Solid Steel 5000 m s–1 Liquid Water 1500 m s–1 Gas Air 340 m s–1 Compare the speeds in different media by finding the ratio of (i) the speed of sound in water with respect to the speed in the air.
10. Table 10.1 shows the speed of sound in a few media at atmospheric pressure. Table 10.1: Speed of sound in different media at 15 °C State Substance/Medium Approximate speed Solid Steel 5000 m s–1 Liquid Water 1500 m s–1 Gas Air 340 m s–1 Compare the speeds in different media by finding the ratio of (ii) the speed of sound in steel with respect to the speed in the water.
11. Two friends are standing along a steel fence at a distance of 340 m from each other (Fig. 10.23). Gunjan places her ear over the fence and her friend knocks the fence with a metal object. Using the values of the speed of sound in steel and air given in Table 10.1, calculate the time difference between the sound that reached Gunjan through the air and the steel. Would it have been possible for her to distinguish between the two sounds? (The time interval between two sounds must be at least 0.1 s to be heard separately.)
Time = distance / speed. Through steel:
Pause and Ponder
12. An experiment is being set up that requires echoes to arrive at least 0.2 s after the emission of sound. What minimum distance should a reflecting surface be placed at? Assume the speed of sound to be 343 m s–1.
For an echo total path is go and return:
Pause and Ponder
13. Sound travels much farther in water than light, and thus, is used for various underwater applications. A sonar signal sent to find the depth of ocean takes 4 s to return. What is the depth of the ocean at that location if the speed of sound in seawater is 1500 m s–1?
Total time for go and return
Revise, Reflect, Refine
1. 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 is (ii) Sound needs a medium to propagate. Mechanical waves require a material medium because they propagate by particle collisions (compressions and rarefactions). Bell jar vacuum experiment and inability to hear in space support this, while light reflects and carries energy but is non-mechanical.
2. 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 is (iii) number of compressions per second. Frequency
3. 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 is (ii)
4. 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.
It will produce reverberation, not a distinct echo. A distinct echo needs at least
5. 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,
(i) The wave with fewer oscillations in the same distance has greater wavelength. If Fig. 10.30 (a) shows fewer crests/troughs than (b) in the same horizontal length, then (a) has greater
5. 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 (ii) smaller amplitude?
(ii) Amplitude is maximum deviation from average density (vertical height). The wave with smaller crest height and shallower trough depth has smaller amplitude. If (a) has lower peaks than (b), then (a) has smaller amplitude and carries less energy, sounding softer.
6. 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.
Since
7. Draw a graph to represent a sound wave for which the density amplitude is 3 units and wavelength is 4 cm.
Draw Distance on x-axis and Density on y-axis with horizontal dashed line for average density. Mark wavelength
8. 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?
Two errors: (a) Sound cannot be heard in space because space is vacuum and sound is a mechanical wave requiring a medium. No medium means no propagation, so explosion would be silent to a distant observer. (b) Even if both were produced, light travels at
9. 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.
Given
10. 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?
Total go-return time
11. 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.
Round-trip distance
12. The speed of sound in air is about 331 m s–1 at 0 ºC and nearly 344 m s–1 at 22 ºC. 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 ºC to 0 ºC? Assume that all other conditions remain unchanged.
At
13. 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.
In Fig. 10.32 the 8 cm span contains three compressions, i.e. two full wavelengths, so
14. 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.
In Fig. 10.33 wave A is the curve with the shorter wavelength (two full cycles across 5.0 cm), so
15. 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?
Let distance to cliff be