DAV Class 8 Science Chapter 12: Question Answers: Sound
Every sound you hear, from a school bell to a mosquito buzzing near your ear, begins with something shaking back and forth. Chapter 12 of the DAV Class 8 Science book is all about this idea: how sound is made, how it travels, and what makes one sound loud, shrill or unpleasant. This post gives easy answers to every question in the chapter, with a short reason behind each one, so you can understand the answer and write it in your own words.
The chapter in one minute
All sounds are made by vibrations, which are quick to-and-fro movements of an object. A vibration needs something to travel through, which scientists call a medium. Sound can move through solids, liquids and gases, but it cannot move through a vacuum, which is empty space with no matter in it. Sound travels fastest in solids and slowest in gases.
Three terms describe a vibration. Amplitude is how far the object moves from its resting position, measured in metres. Time period is the time taken for one complete vibration, measured in seconds. Frequency is the number of vibrations in one second, measured in hertz (Hz). The two are linked: frequency = number of vibrations ÷ time taken, and time period = 1 ÷ frequency.
A bigger amplitude gives a louder sound, and loudness is measured in decibels. A higher frequency gives a shriller sound, which means a higher pitch. Humans can hear sounds only between about 20 Hz and 20,000 Hz (20 kHz). Sounds above 20 kHz are called ultrasonic.
A. Fill in the blanks
1. Birds have a ring of cartilage in their windpipe called the syrinx.
2. Loudness of sound is measured in decibels.
3. The pitch of a sound depends on its frequency.
4. Sounds with a frequency above 20 kHz are called ultrasonic sounds.
5. An unpleasant, irregular sound is called noise.
B. True or false
| Statement | Answer | Why |
|---|---|---|
| 1. Humans and all animals have a voice box to produce their sounds. | False | Birds use the syrinx instead of a voice box, and many animals make sound in other ways. |
| 2. Sound travels fastest through liquids. | False | It travels fastest through solids. |
| 3. Sound cannot travel through a vacuum. | True | There is no matter to carry the vibrations. |
| 4. Light in a vacuum is nearly a million times faster than sound in air. | True | Light moves at about 3 lakh km per second, sound at about 0.33 km per second. |
| 5. Frequency decides whether a sound is shrill or flat. | True | Higher frequency means a shriller sound. |
| 6. Irregular, non-periodic vibrations usually produce musical sounds. | False | Musical sounds come from regular, periodic vibrations. Irregular ones make noise. |
C. Choose the correct option
1. The greatest distance a vibrating object moves on either side of its resting position is called its amplitude.
2. If a sound has a frequency of 1.5 kHz, the vibrating object is completing 1,500 vibrations in one second. This is because 1 kHz is 1,000 Hz, so 1.5 kHz is 1,500 Hz.
3. Sound can travel through all three states of matter, which are solid, liquid and gas.
4. When lightning and thunder happen, they occur together, but we hear the thunder a little after seeing the lightning.
5. If a sound cannot be heard by the human ear, the reason is that it falls outside the range of hearing or is too faint. Choose the option in your book that says one or more of the characteristics of sound (loudness, pitch or quality) is not suitable for the ear.
6. The correct statements are (i) and (ii) only. Sound travels faster in steel than in air, and sounds are produced by vibrations. Sound does not travel faster than light, and it always needs a medium.
D. Short answer questions
1. What is the basic cause of all sounds?
Vibration.
2. Name two musical instruments that make sound through vibrating strings.
Guitar and sitar. Violin and veena are other examples.
3. In which state of matter does sound travel (a) slowest and (b) fastest?
(a) Slowest in gases. (b) Fastest in solids.
4. Define the following and give their SI units.
(a) Amplitude: It is the maximum distance a vibrating object moves away from its mean (resting) position on either side. The SI unit is the metre.
(b) Time period: It is the time an object takes to complete one full vibration. The SI unit is the second.
(c) Frequency: It is the number of vibrations an object completes in one second. The SI unit is the hertz (Hz).
5. Which feature of a vibrating object decides (a) loudness and (b) pitch?
(a) Loudness depends on amplitude. (b) Pitch depends on frequency.
6. Male and female voices have frequencies of about 1 kHz and 1.5 kHz. Which voice (a) is flatter and (b) has a higher pitch?
(a) The male voice is flatter, because its frequency is lower. (b) The female voice has the higher pitch, because its frequency is higher.
7. When is a sound called a musical sound?
A sound is called musical when it is produced by regular, repeating vibrations and feels pleasant to listen to.
E. Long answer questions
1. How do human beings produce sound?
Humans produce sound with the voice box, which is also called the larynx. Two thin bands called vocal cords are stretched across it, leaving a narrow slit in between. When we speak, the lungs push air through this slit. The moving air makes the vocal cords vibrate, and these vibrations produce sound.
2. How is sound production in birds different from that in humans?
Humans use the larynx, or voice box, while birds use a part called the syrinx. The syrinx is a ring of cartilage at the lower end of a bird’s windpipe. When air passes through it, it vibrates and gives each bird its own typical call.
3. Describe a simple experiment to show that sound cannot travel through a vacuum.
Here is how the experiment is done, step by step:
- Take a bell jar and fit a rubber cork firmly into its neck. The cork has a small hole in the centre.
- Hang a small bell inside the jar from a glass rod, with the top end of the rod coming out through the hole in the cork.
- Seal the gap around the rod so that the jar is airtight.
- Shake the jar. The bell rings, and you can hear it clearly.
- Now connect the jar to a vacuum pump and slowly pump the air out.
- Shake the jar again. You can still see the bell moving, but the sound becomes fainter and fainter, and finally you cannot hear it at all.
Conclusion: when the air is removed, there is nothing to carry the vibrations to your ear. This shows that sound needs a medium to travel.
4. A mosquito beats its wings 3,000 times in five seconds. Find the frequency of the sound.
Frequency = number of vibrations ÷ time taken
Frequency = 3,000 ÷ 5 = 600
Answer: 600 Hz.
5. Why do we hear thunder a little after seeing lightning?
Lightning and thunder actually happen at the same moment. But light travels much faster than sound. Light moves at about 3 lakh kilometres per second, while sound in air moves at only about 330 metres per second, so light is nearly a million times faster. The flash reaches our eyes almost instantly, but the sound of thunder takes a few seconds to arrive. That is why we hear it later.
6. Define loudness and pitch. What happens to each when amplitude or frequency increases?
Loudness is the feeling our ear gets that tells us how strong or weak a sound is. Pitch is the quality that tells us whether a sound is shrill (high) or flat (low).
(i) When the amplitude of the vibrating object increases, the sound becomes louder.
(ii) When the frequency of the vibrating object increases, the pitch of the sound becomes higher.
7. Why is noise pollution harmful to humans?
Noise pollution affects both the mind and the body. Constant loud noise makes people irritable and anxious, and it makes it hard to concentrate or remember things. It can cause sleeplessness, high blood pressure and heart problems. It can also damage hearing. In the short term it can cause temporary deafness, and if a person is exposed to loud noise for many years, the damage to the ears can become permanent.
Mistakes to avoid in the exam
Students often mix up loudness and pitch. Remember it this way: amplitude controls loudness, and frequency controls pitch.
Another common error is forgetting to convert kilohertz into hertz. Always remember 1 kHz = 1,000 Hz.
In the vacuum experiment, do not write that the bell stops vibrating. The bell keeps vibrating, but the sound does not reach you because there is no air.
Finally, write the units in numerical answers. In the mosquito question, the answer is 600 Hz, not just 600.
Frequently asked questions
Why can’t sound travel in space?
Space is a vacuum, so there is no air or other matter to carry the vibrations from one place to another.
What is the audible range of human hearing?
Humans can hear sounds from about 20 Hz to 20,000 Hz. Sounds above this range are called ultrasonic.
What is the difference between noise and music?
Music comes from regular, repeating vibrations and sounds pleasant. Noise comes from irregular vibrations and is unpleasant.
Which has a higher pitch, a drum or a whistle?
A whistle has the higher pitch, because it vibrates at a much higher frequency than a drum.
Written for students following the DAV Science book for Class 8. Please cross-check answers with your own textbook and teacher.