Chapter 11 Sound Exercise
Exercises
1. What is sound and how is it produced?
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Answer
Sound is produced due to vibrations. When a body vibrates, it forces the adjacent particles of the medium to vibrate. This results in a disturbance in the medium, which travels as waves and reaches the ear. Hence, the sound is produced.
2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.
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Answer
When the school bell is hit with a hammer, it moves forward and backwards, producing compression and rarefaction due to vibrations. When it moves forward, it creates high pressure in its surrounding area. This high-pressure region is known as compression. When it moves backwards, it creates a low-pressure region in its surrounding. This region is called rarefaction.
3. Why is sound wave called a longitudinal wave?
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Answer
The vibration of the medium that travels parallel to the direction of the wave or along in the direction of the wave is called a longitudinal wave. The direction of particles of the medium vibrates parallel to the direction of the propagation of disturbance. Therefore, a sound wave is called a longitudinal wave.
4. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?
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Answer
Quality of sound is a characteristic that helps us identify the voice of a particular person. Two people may have the same pitch and loudness, but their qualities will be different.
5. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?
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Answer
The speed of sound is $344 m / s$, whereas the speed of light is $3 \times 10^{8} m / s$. The speed of light is less when compared to that of light. Due to this reason, thunder takes more time to reach the Earth as compared to light speed, which is faster. Hence, lightning is seen before whenever we hear thunder.
6. A person has a hearing range from $20 Hz$ to $20 kHz$. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as $344 m s^{-1}$.
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Answer
For sound waves,
Speed $=$ Wavelength $\times$ frequency
$v=\lambda \times v$
Speed of sound wave in air $=344 m / s$
(a) For $v=20 Hz$
$\lambda_1=v / v_1=344 / 20=17.2 m$
(b) For $v_2=20,000 Hz$
$\lambda_2=v / v_2=344 / 20,000=0.0172 m$
Therefore, for human beings, the hearing wavelength is in the range of $0.0172 m$ to $17.2 m$.
7. Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.
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Answer
Consider the length of the aluminium rod $=d$
Speed of sound wave at $25^{\circ} C, V Al=6420 ms-1$
Time taken to reach the other end is,
$T Al=d /(V Al)=d / 6420$
Speed of sound in air, V air = $346 ms-1$
Time taken by sound to each other end is,
$T$ air $=d /(V$ air $)=d / 346$
Therefore, the ratio of time taken by sound in aluminium and air is,
$T$ air $/ t Al=6420 / 346=18.55$
8. The frequency of a source of sound is $100 Hz$. How many times does it vibrate in a minute?
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Answer
Frequency $=$ (Number of oscillations) $/$ Total time
Number of oscillations $=$ Frequency $\times$ Total time
Given,
Frequency of sound $=100 Hz$
Total time $=1 \min (1 min=60 s)$
Number of oscillations or vibrations $=100 \times 60=6000$
The source vibrates 6000 times in a minute and produces a frequency of $100 Hz$.
9. Does sound follow the same laws of reflection as light does? Explain.
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Answer
Yes. Sound follows the same laws of reflection as light. The reflected sound wave and the incident sound wave make an equal angle with the normal to the surface at the point of incidence. Also, the reflected sound wave, the normal to the point of incidence, and the incident sound wave all lie in the same plane.
10. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?
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Answer
An echo is heard when the time interval between the reflected sound and the original sound is at least 0.1 seconds. As the temperature increases, the speed of sound in a medium also increases. On a hotter day, the time interval between the
reflected and original sound will decrease, and an echo is audible only if the time interval between the reflected sound and the original sound is greater than $0.1 s$.
11. Give two practical applications of reflection of sound waves.
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Answer
(i) Reflection of sound is used to measure the speed and distance of underwater objects. This method is called SONAR.
(ii) Working of a stethoscope - The sound of a patient’s heartbeat reaches the doctor’s ear through multiple reflections of sound.
12. A stone is dropped from the top of a tower $500 m$ high into a pond of water at the base of the tower. When is the splash heard at the top? Given, $g=10 m s^{-2}$ and speed of sound $=$ $340 m s^{-1}$.
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Answer
Height (s) of tower $=500 m$
Velocity (v) of sound $=340 m s^{-1}$
Acceleration (g) due to gravity $=10 m s^{-1}$
Initial velocity (u) of the stone $=0$
Time $(t_1)$ taken by the stone to fall to the tower base:
As per the second equation of motion,
$s=ut_1+(1 / 2) g(t_1)^{2}$
$500=0 xt_1+(\frac{1}{2}) 10(t_1)^{2}$
$(t_1)^{2}=100$
$t_1=10 s$
Time $(t_2)$ taken by sound to reach the top from the tower base $=500 / 340=1.47 s$
$t=t_1+t_2$
$t=10+1.47$
$t=11.47 s$
13. A sound wave travels at a speed of $339 m s^{-1}$. If its wavelength is $1.5 cm$, what is the frequency of the wave? Will it be audible?
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Answer
Speed (v) of sound $=339 m s^{-1}$
Wavelength $(\lambda)$ of sound $=1.5 cm=0.015 m$
Speed of sound $=$ Wavelength $\times$ Frequency
$v=v=\lambda X v$ $v=v / \lambda=339 / 0.015=22600 Hz$
The frequency of audible sound for human beings lies between the ranges of $20 Hz$ to $20,000 Hz$. The frequency of the given sound is more than $20,000 Hz$; therefore, it is not audible.
14. What is reverberation? How can it be reduced?
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Answer
The continuous multiple reflections of sound in a big enclosed space are reverberation. It can be reduced by covering walls and ceilings of enclosed spaces with the help of sound-absorbing materials, such as loose woollens and fibre boards.
15. What is loudness of sound? What factors does it depend on?
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Answer
Loud sounds have high energy. Loudness directly depends on the amplitude of vibrations. It is proportional to the square of the amplitude of vibrations of sound.
16. How is ultrasound used for cleaning?
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Answer
Objects that need to be cleansed are put in a cleaning solution, and ultrasonic sound waves are passed through the solution. The high frequency of ultrasound waves helps in detaching the dirt from the objects. In this way, ultrasound is used for cleaning purposes.
17. Explain how defects in a metal block can be detected using ultrasound.
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Answer
Defective metal blocks will not allow ultrasound to pass through them and reflect it back. This technique is used in detecting defects in metal blocks. Make a set-up as shown in the figure, with ultrasound being passed through one end and detectors placed on the other end of a metal block. Since the defective part of the metal block does not allow ultrasound to pass through it, it will not be detected by the detector. In this way, defects in metal blocks can be detected with the help of ultrasound.
