Lakhmir Singh & Manjit Kaur: Sound- 3 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.71.
(A) What type of pollution is caused by the working of mixer and grinder in the kitchen?

Noise pollution is caused by the working of mixer and grinder in the kitchen.

(B) Why should we not put a pin or pencil in our ears?

No, we must never put a pin or pencil in our ears, because it can tear our ear diaphragm.

Q.72. Name any two common musical instruments and identify their vibrating parts.

Dholak - Diaphragm (stretched membrane)
Sitar- Strings of the Sitar
Flute - Air column inside the flute

Q.73. What is the difference between noise and music? Can music become noise sometimes?

The Unwanted sound that is unpleasant to ear is called noise. A pleasant sound is called music.
Music can become noise in many instances. Music is of many types and everybody may not like each type of music. When someone listens very loud music, it can be noise for someone else.

Q.74. Draw a labelled diagram of larynx and explain its functions.

In humans, the sound is produced by the box, which is called voice box or the larynx. Larynx is a part of the throat. It is responsible for production of sound. It has two vocal cord which are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the air. When the lungs force the air through the slit, the vocal cords vibrate, producing sound.

Q.75.
(A) Give two causes of noise pollution from the homes.

The causes of noise pollution are as follows:

• Televisions running at high volumes.
• Use of Loudspeakers in religious or other functions.
• Use of crackers.

(B) What are the usual causes of the partial hearing loss suffered by a person?

Loud noise and noise pollution are the usual causes of the partial hearing loss suffered by a person.

Q.76. How can you show that sound cannot travel through a vacuum? Draw a labelled diagram of the apparatus used.

The bell jar experiment is a common experiment to show that sound cannot travel through a vacuum
A bell jar’s shape is similar to that of a bell. A bell jar is placed on a base and connected to a vacuum pump as shown in the picture below. The air in the jar is varied by pumping the air out of it.

Now, we put an electrical bell in the bell jar. As the air is pumped, vacuum is being created inside the jar slowly and steadily and the sound from the bell jar fades. At a point when complete vacuum is maintained, no more sound is heard from the bell. Thus, the sound is not audible to our ears because of the vacuum inside the jar.

Q.77.
(A) What is meant by the ‘time-period’ of a vibrating object? State its unit.

Time Period given by the inverse of the frequency.

Time period = 1/frequency 

Its unit is seconds.

(B) Define ‘frequency’ of a vibrating object. Name the unit in which frequency is measured.

The frequency is described as the number of vibrations or oscillations per second. Its unit is Hertz (Hz)

(C) A pendulum oscillates 40 times in 4 seconds. Calculate its
(i) time-period, and
(ii) frequency.
Can we hear the sound produced by the oscillations of this pendulum? Give reason for your answer.

Frequency = 40 vibration/ 4 seconds = 10 Hz

Thus, time period = 1/f = 1/10 = 0.1 seconds.

No, we cannot hear because the audible human range is from 20Hz to 20,000 Hz.

Q.78. Draw a neat and labelled diagram of the human ear. Explain its working.

The ear has three main parts:

• External part – It is called the pinna and is made of ridged cartilage covered by skin.
• Middle – This is the part through which sound travels inside the ear to the eardrum, through the auditory canal. Sound causes the eardrum and its tiny attached bones in the middle portion of the ear to vibrate, and the vibrations are conducted to the nearby cochlea.
• Inner part – The cochlea is part of the inner ear. Its main function is to transform sound into nerve impulses.

Q.79.
(A) What is noise? Give two examples of sounds which are considered noise.

The Unwanted sound that is unpleasant to ear is called noise.
Example

• Loud music
• The noise of machines.

(B) What is a musical sound ? Give two examples of musical sounds.

A pleasant sound is called music. Its like a treat to the ears.

Example

• Beats
• ones

Q.80.
(A) What is meant by noise pollution? Mention some of the sources of noise pollution in your surroundings.

The harmful and annoying noise to the ears is called noise pollution. The sources of noise pollution are as follows:

• Televisions running at high volumes.
• Use of Loudspeakers in religious or other functions.
• Use of crackers.
• Horns of buses, cars, and trucks

(B) Explain how, noise pollution (or excessive loud noise) is harmful to human beings.

Noise pollution has adverse effects physical as well as mental behavior. The constant exposure to noise pollution can create many health related problems like hypertension, insomnia, mental disorder and may even lead to loss of hearing.

(C) State the various measures which can be taken to control (or reduce) noise pollution in our surroundings.

The following measures which can be taken to control (or reduce) noise pollution in our surroundings:

• Ban the use of loudspeakers after 9 pm.
• Ban the use of horns in the areas nearby hospital and school.

(D)  What can be done along the roads to reduce noise pollution caused by traffic from reaching the residents of the area?

The following measures which can be taken to reduce noise pollution caused by traffic from reaching the residents of the area:

• The residential area must be declared as no use of horns.
• The colonies must be established at a distance than the roads.

Q.81. Voice of which of the following is likely to have the minimum frequency?
(a) baby girl
(b) baby boy
(c) a man
(d) a woman

Correct Answer is Option (c)
The voice of man has the least frequency.

Q.82. Sound can travel through:
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids and gases

Correct Answer is Option (b)

Sound does not need any material medium to travel.

Q.83. Which of the following vibrates when a musical note is produced by the cymbals in an orchestra?
(a) stretched strings
(b) stretched membranes
(c) metal plates
(d) air columns

Correct Answer is Option (c)

Metal plates vibrates when a musical note is produced by the cymbals in an orchestra.

Q.84. A musical instrument is producing a continuous note. This note cannot be heard by a person having a normal hearing range. This note must then be passing through:
(a) water
(b) wax
(c) vacuum
(d) empty vessel

Correct Answer is Option (c)
This note must then be passing through vacuum.

Q.85. Which of the following sound frequencies can be heard by a woman having a normal hearing range?
(i) 25000 Hz
(ii) 15kHz
(iii) 40000 Hz
(iv) 25Hz
(a) i and ii
(b) ii and iii
(c) ii and iv
(d) only iv

Correct Answer is Option (d)

25Hz because the human audible range is 20Hz to 20,000 Hz.

Q.86. When we change a feeble sound to a loud sound, we increase its:
(a) frequency
(b) amplitude
(c) speed
(d) timbre

Correct Answer is Option (b)

Q.87. Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the strings suitably. By doing so he is adjusting:
(a)  intensity of sound only
(b) amplitude of sound only
(c) frequency of the sitar string, with the frequency of other musical instruments
(d)  loudness of sound

Correct Answer is Option (c)
He is adjusting the frequency of the sitar string, with the frequency of other musical instruments.

Q.88. A key of mechanical piano is first struck gently and then struck again but much harder this time. In the second case:
(a) sound will be louder but pitch will not be different
(b) sound will be louder and the pitch will also be higher
(c) sound will be louder but pitch will be lower
(d) both loudness and pitch will remain unaffected

Correct Answer is Option (a)

In the second case sound will be louder but pitch will not be different.

Q.89. One of the following can hear infrasound. This one is:
(a) dog
(b) bat
(c) rhinoceros
(d) humans

Correct Answer is Option (a)
dog can hear both the infrasound as well as ultrasound.

Q.90. The speed of highly penetrating ultrasonic waves is:
(a) lower than those of audible sound waves
(b) higher than those of audible sound waves
(c) much higher than those of audible sound waves
(d) same as those of audible sound waves

Correct Answer is Option (d)

The speed of sound waves is same in a particular medium.

Q.91. The ultrasound waves can penetrate into matter to a large extent because they have:
(a) very high speed
(b) very high frequency
(c) very high quality
(d) very high amplitude

Correct Answer is Option (b)
The ultrasound waves can penetrate into matter to a large extent because they have very high frequency.

Q.92. The frequencies of four sound waves are given below. Which of these sound waves can be used to measure the depth of sea?
(a) 15,000 Hz
(b) 10 kHz
(c) 50 kHz
(d) 10,000 Hz

Correct Answer is Option (c)
The ultrasound waves can be used to measure the depth of sea.

Q.93. Which of the following frequency of sound can be generated by a vibrating simple pendulum as well as by the vibrating vocal cords of a rhinoceros?
(a) 5 kHz
(b) 25 Hz
(c) 10 Hz
(d) 15,000 Hz

Correct Answer is Option (c)
10Hz can be generated by a vibrating simple pendulum as well as by the vibrating vocal cords of a rhinoceros.

Q.94. Which of the following are used to study the growth of fetus inside the mother’s womb?
(a) radio waves
(b) X-rays
(c) infrared waves
(d) sound waves

Correct Answer is Option (c)
Infra-red Waves are used to study the growth of fetus inside the mother’s womb

Q.95. We can distinguish between the musical sounds produced by different singers on the basis of the characteristic of sound called:
(a) frequency
(b) timbre
(c) pitch
(d) loudness

Correct Answer is Option (b)
Timbre can distinguish between the musical sounds produced by different singers on the basis of the characteristic of sound.

Q.96. The maximum speed of vibrations which produce audible sound will be in:
(a) dry air
(b) sea water
(c) ground glass
(d) human blood

Correct Answer is Option (c)
The sound travels faster in solids than liquids than gases.

Q.97. The sound waves travel fastest:
(a) in solids
(b) in liquids
(c) in gases
(d) in vacuum

Correct Answer is Option (a)
The sound travels faster in solids than liquids than gases.

Q.98. The speeds of sound in four different media are given below. Which of the following is the most likely speed in m/s with which the two under water whales in a sea talk to each other when separated by a large distance ?
(a) 340
(b) 5170
(c) 1280
(d) 1530

Correct Answer is Option (d)

Q.99. The velocities of sound waves in four media P, Q, R and S are 18,000 km/h, 900 km/h, 0 km/h, and 1200 km/h respectively. Which medium could be a liquid substance?
(a) P
(b) Q
(c) R
(d) S

Correct Answer is Option (d)
Because the speed of sound in liquids is more than gas but less than solids.

Q.100. Which of the following modes is utilized in the production of sound by humans?
(a) vibrating membranes
(b) vibrating plates
(c) vibrating strings
(d) vibrating air columns

Correct Answer is Option (c)
The vibrating strings modes is utilized in the production of sound by humans.

Q.101. Three different vibrating objects produce three types of sounds X, Y and Z. The sounds X and Y cannot be heard by a man having normal range of hearing but sound Z can be heard easily. The sound X can be heard by a bat whereas the sound Y can be heard by a rhinoceros. What type of sounds are X, Y and Z?

X : Ultrasonic sound waves (Greater than 20,000Hz)
Y : Infrasonic sound waves (0-20 Hz)
Z : Audible sounds (20Hz-20,000Hz)

Q.102. Your parents are going to buy a house. They have been offered one house on the roadside and another house three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

There will be more noise in the house which is along the roadside due to the noise produced by transportation vehicles. But, the house which is three lanes away from the roadside would have comparatively less noise. Therefore, it is better to take the house that is three lanes away from the roadside.

Q.103. The sound from an insect is produced when it vibrates its wings at an average rate of 500 vibrations per second:
(a) What is the time period of the vibrations ?
(b) What is the frequency of the vibrations in hertz?
(c) Can we hear this sound? Why or why not?

(a) The time period = 1/f = 1/500 = 0.002 secs.

(b) The frequency of the vibrations in hertz is 500 Hz. Because 1Hz = 1 vibration/second.

(c) We can hear this because it is in the range of 20Hz to 20,000Hz.

Q.104. There are three small bones in the middle ear.
(a) Name the three bones
(b) Which of these bones is in touch with oval window?
(c) Which of these bones is in touch with the eardrum?
(d) Which bone is in touch with the other two bones?

(a) Hammer, Anvil, Stirrup are the three ear bones.
(b) Strirrup is in touch with oval window.
(c) Hammer is in touch with the ear drum.
(d) Anvil is in touch with the other two bones.

Q.105. Explain why, if we strike a steel tumbler with a metal spoon lightly, we hear a feeble sound but if we hit the tumbler hard, a loud sound is heard.

This is because the amplitude of vibrations is small when we beat the spoon lightly and so feeble sound is produced but when it is beaten with force then, the amplitude of vibrations is large, so loud sound is produced.