The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 12 Question Answer – Sound are essential study tools for DAV public school students in Class 8.
DAV Class 8 Science Ch 12 Question Answer – Sound
DAV Class 8 Science Ch 12 Solutions – Sound
Something To Know
A. Fill in the blanks:
Question 1.
Birds have a ring of cartilage, called ________ in their wind pipes.
Answer:
Syrinx
Question 2.
The SI unit of loudness’ of sound, is ________ .
Answer:
Decibel
Question 3.
The pitch of sound is determined by its ________ .
Answer:
Frequency
Question 4.
Sounds, of frequency more than 20 kHz, are known as ________ .
Answer:
Ultrasound
Question 5.
An irregular, irritating sound is referred to as a ________ .
Answer:
Noise
B. State True or False for the following statements:
1. Human beings, as well as all animals, have a voice box for producing their characteristic sounds.
Answer:
False
2. Of the three mediums, solids, liquids and gases, sound propagates fastest in liquids.
Answer:
False
3. Sound cannot propagate through vacuum.
Answer:
True
4. Light propagates, through vacuum, with a speed that is nearly a million times more than the speed of sound in air.
Answer:
True
5. The frequency of a given sound determines its ‘shrillness’ or ‘flatness’.
Answer:
True
6. Irregular non-periodic vibrations generally produce musical sounds.
Answer:
False
C. Tick (✓) the correct option:
Question 1.
The maximum displacement of a vibrating body, on either side of its mean position, is known as its-
(a) Frequency
(b) Loudness
(c) Amplitude
(d) Pitch
Answer:
(c) Amplitude
Question 2.
The frequency of a given sound is 1.5 kHz. The vibrating body is then-
(a) Completing 1,500 vibrations in one second.
(b) Taking 1,500 seconds to complete one vibration.
(c) Taking 1.5 seconds to complete one vibration.
(d) Completing 1.5 vibrations in one second.
Answer:
(b) Completing 1,500 vibrations in one second.
Question 3.
Sound can propagate through-
(a) Vacuum as well as gases.
(b) Only through gases and liquids.
(c) Only through gases and solids.
(d) All the three states of matter.
Answer:
(d) All three states of matter.
Question 4.
When lightning and thunder take place, they occur-
(a) Together and are also observed together.
(b) One after the other but are observed together.
(c) Together but the thunder is observed a little after the lightning.
(d) Together but thunder is observed a little before the lightning.
Answer:
(d) Together but thunder is observed a little after the lightning.
Question 5.
A given sound is inaudible to the human ear. We can then say that this is so-
(a) Only because its amplitude is too small.
(b) Only because its frequency is below 20 Hz.
(c) Only because its frequency is above 20 Hz.
(d) Because it has any one, or more, of the three characteristics listed above.
Answer:
Only because its frequency is below 20 Hz.
Question 6.
The following are some statements about sound:
(i) Sound propagates faster in steel than in air
(ii) Sounds are produced through vibrations
(iii) Sound propagates faster than light
(iv) Sound does not require a medium ior its propagation Out of these, the correct statements are-
(a) (i) and (ii) only
(b) (i) and (iii) only
(c) (i) and (ii) only
(d) (ii) and (iv) only
Answer:
(c) (i) and (ii) only
D. Answer the following questions in brief:
Question 1.
Name the cause that is regarded as the basic cause of all sounds.
Answer:
Vibrations
Question 2.
Name two musical instruments which produce sound through vibrating strings.
Answer:
Sitar and guitar
Question 3.
Through which state of matter does sound propagate with the
(a) Slowest speed
(b) Fastest speed
Answer:
(a) Gas
(b) Solid
Question 4.
Define the following terms. Also write their SI units.
(a) Amplitude
(b) Time period
(c) Frequency
Answer:
(a) The maximum distance from the mean position to which a vibrating body moves is called amplitude. SI unit of amplitude is meter.
(b) Time taken to complete one oscillation is called time period. The SI unit of time period is second.
(c) Number of oscillations per second is called frequency. The SI unit of frequency is hertz.
Question 5.
Name the characteristic of a vibrating body which determines the- (a) Loudness (b) Pitch of the sound Produced by it.
Answer:
(a) Amplitude
(b) Frequency
Question 6.
The irequencies of the male and female voices in human beings are nearly 1 kHz and 1.5 kHz. Which of the two voices
(a) Is likely to be flatter? (b) Has a higher pitch?
Answer:
(a) Male
(b) Female
Question 7.
When is a given sound regarded as a musical sound or music?
Answer:
When the sound is pleasant it is called music. Musical sounds are produced by regular or periodic vibrations.
E. Answer the following questions:
Question 1.
Explain briefly the mechanism of production of sound in human beings.
Answer:
The lungs force air through the slit between the vocal chords. It causes vibrations in vocal chords and sound is produced.
Question 2.
How does the mechanism of production of sounds in birds differ from that in human beings?
Answer:
Birds have a cartilaginous ring; called syrinx; to produce sound. Humans have voice box (larynx) to produce sound.
Question 3.
Describe a simple experiment to show that sound cannot propagate through vacuum.
Answer:
Take a bell jar, an electric bell, a vacuum pump, battery and wires.
- Fix the electric bell inside the bell jar and connect the bell to the battery.
- Attach the bell jar with vacuum pump.
- Switch on the electric bell to hear its sound.
- With the help of vacuum pump, create vacuum inside the bell jar.
- Now, switch on the electric bell and try to hear its sound.
Observations: When air was present in the bell jar, you could hear the sound of the electric bell. But when vacuum was present in the bell jar, you could not hear the sound of electric bell.
Conclusion: Sound cannot propagate through vacuum.
Question 4.
On a hot summer day, a little mosquito produces a warning sound near your ear by vibrating its wings at the rate of 3,000 wing beats per five seconds. State the frequency, in hertz, of this sound.
Answer:
Given number of oscillations = 3,000, time = 5 s Frequency = Number of oscillations/time = 3,000/5 = 600 Hz
Question 5.
Explain why we always hear the thunder a little after we see the flash of lightning.
Answer:The speed of light is more than the speed of sound. Due to this, we always hear
the thunder a little after we see the flash of lightning.
Question 6.
(a) Define “loudness’ and ‘pitch’.
(b) State the effect on the
(i) Loudness of a sound when the amplitude of the vibrating body increases.
(ii) Pitch of a sound when the frequency of the vibrating body increases.
Answer:
The intensity of sound is called its loudness. Loudness increases with increase in amplitude.
Pitch: The shrillness or flatness of sound is called its pitch. Pitch increase with increase in frequency.
Question 7.
Why is noise pollution regarded as harmful for human beings?
Answer:
Noise pollution results in headache, hypertension and temporary loss of hearing. Hence, noise pollution is regarded as harmful for human beings.
Value Based Question
It was Shikha’s birthday. Her friends and cousins came to her house to wish her and to celebrate the day. They all wanted to dance and enjoy, with loud music in the background. Shikha, however, kept the music only at a moderate level. She was aware of the harms and dangers of noise pollution. She also did not want to cause any discomfort and irritation to the old lady, in the neighbourhood, who was recovering from her illness. She was of the firm view that our happiness and celebrations should not cause any inconvenience and discomfort to others.
Question 1.
State the values displayed by Shikha.
Answer:
Shikha shows concern for others.
Question 2.
Why is ‘noise pollution’ undesirable? Suggest some ways of minimizing it.
Answer:
Noise pollution causes headache, hypertension and temporary loss of hearing. Hence, it is undesirable. Some ways of minimizing noise pollution are as follows:
- Installing sound barriers along the roads and bridges.
- Banning loud music in residential areas.
- Avoiding unnecessary honking of horns.
Question 3.
Have a ‘group discussion’ in which different students think of situations in which they can have their ‘happiness calibrations’ without causing discomfort, or inconvenience to others.
Answer:
Do it yourself.
Something To Do
Question 1.
Take some steel or porcelain bowls of the same size. Put different amounts of water in all the bowls. Beat them slowly with a spoon one by one. You will hear different types of sound from different bowls. This is your Jaltarang.
Answer:
Do it yourself.
Question 2.
Identify the sources of noise pollution in your locality. Suggest ways and means to control the noise pollution caused by them. Discuss your report with your friends, parents and teachers.
Answer:
Do it yourself.
Question 3.
Musical instruments are categorized into three main types- Stringed (Tantu vadya) instruments, wind (Sushir vadya) instruments and membrane (Avnaddhu vadya) instruments. Make a list of some well known musical instruments of each type and try to understand the way they work.
Answer:
Do it yourself
Question 4.
Solve the crossword puzzle.
| Across → | Down ↓ |
| 1. Sound of frequencies above 20 kHz. | 1. Form of energy produced due to vibration. |
| 2. Amplitude of sound. | 2. Voice box |
| 3. Ring of cartilage. | 3. SI unit of frequency |
| 4. Unpleasant sound. | 4. Sound of frequencies below 20 Hz. |
| 5. Reciprocal of time period. | 5. Shrillness or flatness of sound. |
Answer:
DAV Class 8 Science Chapter 12 Solutions – Sound Important Questions and Answers
I. Fill in the blanks:
1. Sound is a form of ________ .
Answer:
Energy
2. Voice of females is of ________ pitch.
Answer:
High
3. Voice of males is of ________ pitch.
Answer:
Low
4. Sound of guitar is produced because of vibrations in ________ .
Answer:
String
5. Sound of table is produced because of vibrations in ________ .
Answer:
Membrane
II. Tick the correct answer:
Question 1.
Which of these is the SI unit of amplitude?
(a) Metre
(b) Metre per second
(c) Hertz
(d) Decibel
Answer:
(a) Metre
Question 2.
Which of these is the SI unit of frequency?
(a) Metre
(b) Metre per second
(c) Hertz
(d) Decibel
Answer:
(c) Hertz
Question 3.
Decibel is the unit of which of these?
(a) Loudness of sound
(b) Noise
(c) Amplitude
(d) Pitch
Answer:
(a) Loudness of sound
Question 4.
What is the frequency of infrasonic sound?
(a) 20 Hz
(b) 20 Hz to 20,000 Hz
(c) Less than 20 Hz
(d) More than 20,000 Hz
Answer:
(c) Less than 20 Hz
Question 5.
What is the frequency of ultrasonic sound?
(a) 20 Hz
(b) 20 Hz to 20,000 Hz
(c) Less than 20 Hz
(d) More than 20,000 Hz
Answer:
(d) More than 20,000 Hz
III. Answer the following questions:
Question 1.
What is sound?
Answer:
Sound is a form of energy which produces the sensation of hearing in our ears.
Question 2.
How does an insect produce sound?
Answer:
Insects; like mosquitoes and cockroaches produce sound by vibrating their wings.
Question 3.
What is the speed of sound in air?
Answer:
Speed of sound through air is about 330 m/s.
Question 4.
Explain the structure and function of the inner ear in humans.
Answer:
The inner ear is made up of cochlea and semi-circular canals. The sound waves reach the cochlea from where they are sent to the brain via auditory nerve. The brain interprets these signals and we are able to hear sound. The semi-circular canal plays a role in maintaining the balance of the body.
Question 5.
What do you understand by time period?
Answer:
The time taken to complete one oscillation is called time period. One cycle of to and fro motion along a mean position is counted as one oscillation. The SI unit of time period is second.
Question 6.
What do you understand by audible sound?
Answer:
Human beings can hear the sound between 20 Hz to 20,000 Hz. The sound within this range of frequencies is called audible sound.
Question 7.
What do you understand by inaudible sound?
Answer:
Human beings cannot hear sound with frequency below 20 Hz or more than 20,000 Hz. The sound below 20 Hz and more than 20,000 Hz is called inaudible sound.
IV. Projects:
Question 1.
Devise an experiment to show that vibrations produce sound.
Answer:
For this, take a rubber band and stretch it across your fingers. Produce vibrations in the rubber band by striking it. Try to hear the sound.
Question 2.
Device an experiment to show that sound cannot travel through vacuum.
Answer:
For this, conduct the Bell’s experiment in your school laboratory. Take help of your teacher.