## Explain how the human ear works.

[question] Question. Explain how the human ear works. [/question] [solution] Solution: Refer theory, page no.72. [solution]...

## Explain how defects in a metal block can be detected using ultrasound.

[question] Question. Explain how defects in a metal block can be detected using ultrasound. [/question] [solution] Solution: Refer theory, page no.70. [/solution]...

## A sonar device on a submarine sends out a signal and receives an echo 5 s later.

[question] Question. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m. [/question] [solution] Solution: Time taken between transmission and reception of signal = 5 sec. Distance of the object from the sub marine = 3625 m. Speed of sound in water $=\frac{\text { Distance travelled }}{\text { Time taken }}=\frac{7250}{5}=1450 \mathrm{~m} / \mathrm{sec}$ [/solution]...

## Explain the working and applications of a sonar.

[question] Question. Explain the working and applications of a sonar. [/question] [solution] Solution: Refer theory, page no.71. [/solution]...

## How is ultrasound used for cleaning ?

[question] Question. How is ultrasound used for cleaning ? [/question] [solution] Solution: The object to be cleaned is put in a tank fitted with ultrasonic vibrator. The tank is filled with water containing detergent. As the ultrasonic vibrator is switched on, the detergent rubs against the object at a very high speed and hence cleans it. [/solution]...

## Explain how bats use ultrasound to catch their prey

[question] Question. Explain how bats use ultrasound to catch their prey [/question] [solution] Solution: The bats produce high pitched ultrasonic waves which are not heard by human beings. The ultrasonic waves on striking the insect send back an echo, which is heard by the bat. As the echo is heard by the bat, it moves towards the insect and catches it. [/solution]...

## What is loudness of sound ? What factors does it depend on ?

[question] Question. What is loudness of sound ? What factors does it depend on ? [/question] [solution] Solution: The loudness of the sounds humans perceive relates to the intensity of the audible sound. Loudnessis a subjective property of the sound that depends on the human ear, the sensitivity of the ear to the frequency of the sound, and the distance from the source of the sound. Loudness of the sound depends inversely on the square of the distance of the observer from the source of sound, a...

## What is reverberation ? How can it be reduced ?

[question] Question. What is reverberation ? How can it be reduced ? [/question] [solution] Solution: Reverberation is the repeated/multiple reflections of sound in any big enclosed space. It can be reduced by covering the ceiling and walls of the enclosed space with some sound-absorbing materials like fibre board, loose woolens etc. [/solution]...

## A sound wave travels at a speed of 339 ms–1.

[question] Question. A sound wave travels at a speed of 339 ms–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible ? [/question] [solution] Solution: Speed of sound wave, $\mathrm{v}=339 \mathrm{~ms}^{-1}$; Wavelength, $\lambda=1.5 \mathrm{~cm}=0.015 \mathrm{~m}$; Frequency, $v=?$ Frequency, $v=\frac{v}{2}=\frac{339}{0}=22600 \mathrm{~Hz}$ The sound will not be audible, because human beings can hear only up to $20,000 \mathrm{~Hz}$. [/solution]...

## Flash and thunder are produced simultaneously.

[question] Question. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. Why ? [/question] [solution] Solution: The speed of light is $3 \times 10^{8} \mathrm{~ms}^{-1}$ and the speed of sound is $344 \mathrm{~ms}^{-1}$ in air. That is, the speed of light is very large as compared to the speed of sound. Thus, flash of lightning is seen at first, but sound takes a little bit more time to reach our ears. [/solution]...

## Which characteristic of the sound helps you to identify

[question] Question. Which characteristic of the sound helps you to identify your friend by his voice while sittingwith others in a dark room ? [/question] [solution] Solution: The characteristic of sound that helps you to identify your friend's voice is 'quality' or 'timbre'. [/solution]...

## Why is sound wave called a longitudinal wave ?

[question] Question. Why is sound wave called a longitudinal wave ? [/question] [solution] Solution: Sound wave is called longitudinal wave because the particles of the medium vibrate in the direction of the propagation of wave. [/solution]...

## Give an experiment to show that sound needs a material medium for its propagation.

[question] Question. Give an experiment to show that sound needs a material medium for its propagation. [/question] [solution] Solution: Take an electric circuit which consists of a power supply source, a switch and an electric bell arranged inside a bell jar, which stands on the platform of a vacuum pump. The switch of the bell is pressed to close the electric circuit. When there is air within the bell jar, sound is heard. Air is now pumped out of the bell jar. When the air is completely remove...

## Describe with the help of a diagram,

[question] Question. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound. [/question] [solution] Solution: When a vibrating body like a tuning fork moves forward, it creates a region of high pressure in its vicinity. This region of high pressure is known as compression. When it moves backward, it creates a region of low pressure in its vicinity. This region is known as a rarefaction. As the body continues to move forward and backward,...

## What is the sound and how it is produced?

[question] Question. What is the sound and how it is produced? [question] [solution] Solution: Sound is a form of energy which produces sensation of hearing. When an object is set into vibrations, sound is produced. [solution]...

## A submarine emits a sonar pulse,

[question] Question. A submarine emits a sonar pulse, which returns from an underwater cliff in $1.02 \mathrm{~s}$. If the speed of sound in salt water is $1531 \mathrm{~ms}^{-1}$, how far away is the cliff ? [question] [solution] Solution: Given, time taken by the sonar pulse, $\mathrm{t}=1.02 \mathrm{~s}$; speed of sound in salt water, $\mathrm{v}=1531 \mathrm{~m} \mathrm{~s}^{-}$ 1 ; distance of cliff, $\mathrm{s}=?$ Now, $s=\frac{v}{2}=\frac{1531 \quad 1.02}{2}=780.81 \mathrm{~m}$ [solution]...

## What is the range of frequencies associated with

[question] Question. What is the range of frequencies associated with (a) Infrasound ? (b) Ultrasound ? [/question] [solution] Solution: (a) Infrasound : Sound waves of frequencies between 1 to 20 Hz. (b) Ultrasound : Sound waves of frequencies above 20,000 Hz. [/solution]...

## What is the audible range of the average human ear ?

[question] Question. What is the audible range of the average human ear ? [/question] [solution] Solution: An average human ear can hear sound waves of frequencies between 20 Hz to 20,000 Hz. [/solution]...

## Why is the ceiling of concert halls curved ?

[question] Question. Why is the ceiling of concert halls curved ? [/question] [solution] Solution: The ceiling of concert halls is curved so that sound after reflection from it reaches all the corners of the hall and is audible to every person in the hall. [/solution]...

## An echo is returned in 3 s.

[question] Question. An echo is returned in $3 \mathrm{~s}$. What is the distance of the reflecting surface from the source, given that the speed of sound is $342 \mathrm{~ms}^{-1}$ ? [/question] [solution] Solution: Given, speed of sound, $\mathrm{v}=342 \mathrm{~m} \mathrm{~s}^{-1}$; time taken for hearing the echo, $\mathrm{t}=3 \mathrm{~s}$; distance of the reflecting surface, $\mathrm{s}=$ ? Now, $\mathrm{s}=\frac{\mathrm{v}}{2}=\frac{342}{2}=\mathbf{5 1 3} \mathbf{~ m}$ [/solution]...

## In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

[question] Question. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature? [/question] [solution] Solution: Sound travels the fastest in solids. Its speed decreases in liquids and it is the slowest in gases. Therefore, for a given temperature, sound travels fastest in iron. [/solution]...

## Distinguish between loudness and intensity of sound.

[question] Question. Distinguish between loudness and intensity of sound. [/question] [solution] Solution: Intensity is an objective property of the sound wave. In fact, it is related to the square of the wave amplitude, and does not depend on the particular characteristics of a person’s ears. The amount of sound energy passing each second through unit area is called the intensity of sound. Loudness, on the other hand, is a subjective property of the sound that depends on the human ear, the sens...

## A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound.

[question] Question. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source ? [/question] [solution] Solution: The time interval between two successive compressions is equal to the time period of the wave. It has nothing to do with the distance (450 m) of the person from the source. Time interval $=\frac{1}{\text { Frequency }}=\frac{1}{500}=2 \times 10^{-3} \mathrm{~s}$ [/so...

## Calculate the wavelength of a sound wave whose frequency is 220 Hz

[question] Question. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 ms–1 in a given medium. [/question] [solution] Solution: Given, frequency, $v=220 \mathrm{~Hz}$; speed of sound, $\mathrm{v}=440 \mathrm{~ms}^{-1}$ Speed, $v=v \lambda \quad$ or $\quad \lambda=\frac{v}{v}=\frac{440}{220}=2 \mathrm{~m}$ [/solution]...

## How are the wavelength and frequency of a sound wave related to its speed ?

[question] Question. How are the wavelength and frequency of a sound wave related to its speed ? [/question] [solution] Solution: Speed of sound = frequency × wavelength [/solution]...

## What are wavelength, frequency,

[question] Question. What are wavelength, frequency, time period and amplitude of a sound wave ? [/question] [solution] Solution: Wavelength : It is the distance between two consecutive compressions or two consecutiv rarefactions. Frequency : The number of compressions or rarefactions taken together passing through a point in one second is called frequency. Time Period : It is the time taken by two consecutive compressions or rarefactions to cross a point. Amplitude : It is the magnitude of maxi...

## Guess which sound has a higher pitch: guitar or car horn ?

[question] Question. Guess which sound has a higher pitch: guitar or car horn ? [/question] [solution] Solution: The frequency of vibration of a sound produced by a guitar is greater than that produced by a car horn. Since the pitch of a sound is proportional to its frequency, the guitar has a higher pitch than a car horn. [/solution]...

## Which wave property determines

[question] Question. Which wave property determines (a) loudness (b) pitch ? [/question] [solution] Solution: (a) The amplitude of the wave determines the loudness; more the amplitude of a wave, more is the loudness produced. (b) The pitch is determined by the frequency of the wave. Higher the frequency of a wave, more is its pitch and shriller is the sound. [/solution]...

## Suppose you and your friend are on the moon.

[question] Question. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend? [/question] [solution] Solution: No, I will not be able to hear sound, because Moon has no atmosphere. Therefore, no sound waves can travel to my ears and therefore, no sound is heard. [/solution]...

## Why are sound waves called mechanical waves ?

[question] Question. Why are sound waves called mechanical waves ? [/question] [solution] Solution: The waves which require a medium for their propagation are called mechanical waves. Sound waves also propagate through a medium because of the interaction of the particles present in that medium. Sound waves force the medium particles to vibrate. Hence, these waves are known as mechanical waves. [/solution]...

## Explain how sound is produced by your school bell.

[question] Question. Explain how sound is produced by your school bell. [/question] [solution] Solution: When we hit a school bell by a hard gong, it moves back and forth about its equilibrium position. Thus, the edge of the bell strikes the particles in the air. When the edge moves forward, air particles are driven forward. This forward motion of the bell produces a region where the air pressure is slightly higher than average. This region is called compression. When the edge moves backward, ai...

## How does the sound produced by a vibrating object in a medium reach your ear ?

[question] Question. How does the sound produced by a vibrating object in a medium reach your ear ? [/question] [solution] Solution: Air is the most common material through which sound propagates. When a vibrating object like prongs of tuning fork move forward, they push the molecules of the air in front of them. This is turn compresses the air, thus creating a region of high pressure and high density called compression. This compression in the air travels forward. When the prongs of the tuning ...