Skip to content
CNMS ICSE - 7 Std
Share
Explore

Answers to textbook exercises

Prepared by: learnloophq@gmail.com

Chapter: 06. Sound

⁠

CONNECT (Page 115)

1. Silhouette of a crow (cawing):
Harsh
2. Red plastic whistle:
Shrill
3. Violin:
Melodious
4. Lion (roaring):
Loud
5. Black ceiling fan:
Soft
6. Red megaphone:
Loud
7. Tube of Pringles (shaking/opening):
Soft
8. Pigeon (cooing):
Melodious / Soft
⁠

CHECK YOUR PROGRESS (Page 121)

1. The to and fro motion when one full wave is constituted is known as:
b. oscillation.
2. Time period of a vibration is the reciprocal of its:
a. frequency.
⁠

REASON CORNER (Page 121)

If a wave oscillates 50 times in 5 seconds, what is its time period and frequency?
Frequency =
math
⁠
Time period =
math
⁠
⁠

REASON CORNER (Page 123)

1. You can recognise your friend by hearing her voice on the phone. You can also recognise any musical instrument by hearing the sound produced from it. Which characteristic of sound helps you do it?
Quality or Timbre.
2. How can you change the pitch of a sound produced by a flute?
By covering or uncovering the finger holes on the flute. This changes the length of the vibrating air column inside the flute, which alters the frequency (pitch) of the sound.
⁠

CHECK YOUR PROGRESS (Page 125)

1. The characteristic of sound that depends on its frequency is:
b. pitch.
2. As the distance between the listener and the source increases, loudness of sound:
b. decreases.
3. Sound cannot travel through:
c. vacuum.
⁠

SCIENCE TALK (Page 127)

You and your friend are given a loud bell, a stop watch, a measuring tape. Design an activity to measure the approximate speed of sound using these. Explain the steps.
Step 1: Use the measuring tape to measure a large straight-line distance (such as 150 meters) in an open field.
Step 2: Stand at one end of the measured distance with the stop watch, while your friend stands at the other end with the loud bell and a visual flag.
Step 3: Instruct your friend to strike the bell forcefully while simultaneously waving the flag down in a sharp motion.
Step 4: The moment you see the flag wave down, start the stop watch (since the speed of light is nearly instantaneous, you see the movement instantly).
Step 5: Stop the stop watch the exact moment you hear the sound of the bell.
Step 6: Calculate the speed of sound using the formula:
math
⁠
⁠

REASON CORNER (Page 130)

Anushka visited a big indoor auditorium to hear a music recital. She noticed that the floor was covered with a thick carpet and the walls were coated with rough materials. Why do you think this had been done?
Thick carpets and rough wall coatings are soft, porous, and fluffy materials. They act as excellent sound absorbers and bad reflectors of sound. Covering the floors and coating the walls with these materials prevents multiple reflections of sound (reverberation) and echo formation, making the music recital clear and pleasant to hear.
⁠

LIFE SKILLS (Page 130)

1. Find out the materials used to reduce the effect of sound in buildings such as theatres and hospitals.
Materials used include acoustic foam panels, glass wool, mineral wood, heavy carpets, thick curtains, perforated fiberboards, and double-glazed soundproof glass windows.
2. Why are some areas called silent zones?
Areas near hospitals, educational institutions, and courts are designated as silent zones to prevent high-decibel disturbances. This is done to help patients rest and heal peacefully, and to allow students and legal professionals to concentrate.
3. How does sound affect patients?
Loud and unwanted sounds increase stress, elevate blood pressure, disrupt critical sleep cycles, cause anxiety, and delay the overall recovery process of patients.
4. If you have a sick person at home, how should you behave around him/her? How can you help an adult take care of the patient?
Talk in soft, low voices, keep the volume of televisions or music systems very low, and avoid making sharp noises like slamming doors. You can help an adult by keeping the sick person’s room calm, bringing medicines/water on time, and carrying out quiet activities near them.
⁠

CHECK YOUR PROGRESS (Page 132)

1. Which of these is a good reflector of sound?
a. aluminium door
2. In which of the following mediums does sound travel the fastest?
c. solids
3. Galton’s whistle is a whistle that can produce frequencies greater than:
b. 20,000 Hz.
4. What is the minimum distance between a reflective surface and a source for an echo to be heard?
b. 17 m
5. Sounds of frequencies greater than 20,000 Hz are called:
c. ultrasonic sounds
6. Bats can produce and detect:
a. ultrasonic sounds
⁠

PREPPING FOR PISA (Page 134)

1. Which of the following conditions can a Doppler ultrasound diagnose?
d. all of these
2. Assertion (A): Ultrasound waves are used to detect cracks and defects in metal blocks. Reason ®: Ultrasound waves have low frequency due to which they can penetrate into matter to a great extent without causing damage.
c. A is true but R is false. (Reason is false because ultrasound waves have extremely high frequencies, not low frequencies).
⁠

EVALUATION (Pages 135-136)

A. Choose the correct option.

1. Speed of sound in air is around:
c. 340 m/s.
2. Sounds of frequency less than 20 Hz are called:
d. infrasonic sounds.
3. Audible range of frequency is:
a. 20 Hz–20,000 Hz.
4. Sound is produced in a flute by:
d. blowing.
5. Shrillness of a sound is determined by:
a. pitch.
6. In which of the following cases will echo be heard the most?
b. i and iv (an empty room and a room with walls made of non-porous material maximize sound reflection).

B. Name the following.

1. number of oscillations made by a wave in a unit time:
Frequency
2. sounds of frequencies less than 20 Hz:
Infrasonic sounds (or subsonic sounds)
3. an application of the reflected sound:
Echo (or Ultrasonography / Megaphone / SONAR)
4. humans produce sound with the help of this organ:
Larynx (or Voice box)

C. Match the following.

1. speed of light — d. 3 × 10^{8} m/s
2. reflected sound — e. echo
3. speed of sound in air — b. 340 m/s
4. curtains and rugs — c. sound absorbing materials
5. ultrasonics — a. frequency more than 20,000 Hz

D. Write true (T) or false (F) against the following statements.

1. All materials reflect sound waves equally.
False (F)
2. Bats can produce and detect infrasonics.
False (F) (They produce and detect ultrasonics)
3. Two waves of the same frequency can have different amplitudes.
True (T)
4. The unit of amplitude is second.
False (F) (The unit of amplitude is metre)
5. Waves of sound produced by a body move in all directions.
True (T)

E. Fill in the blanks.

1. Loudness, pitch, and quality (or timbre) are three characteristics of sound.
2. An echo can be clearly heard by us only if the reflecting surface is at least 17 m away from us.
3. Sound requires a medium to travel.
4. In a longitudinal wave, the particles vibrate in the same direction as that of the sound.
5. The SI unit of frequency is hertz (Hz).

F. Short-answer-type questions.

1. What is a vibration?
A vibration (or vibratory motion) is the rapid to-and-fro or back-and-forth movement of an object about its central mean (equilibrium) position.
2. What is frequency? Give the mathematical relation between frequency and time period.
Frequency is defined as the number of oscillations completed by a wave in one unit of time. Its mathematical relation to the time period (
math
⁠
) is:
math
⁠
3. What do you mean by reflection of sound?
The process in which sound waves hit a hard surface and bounce back into the same medium is called the reflection of sound.
4. What are wind instruments and stringed instruments? Give two examples of each.
Wind instruments: Musical instruments in which sound is produced by the vibration of air columns when air is blown into them. Examples: Flute, Bugle.
Stringed instruments: Musical instruments consisting of stretched strings that produce sound when they are plucked, struck, or bowed. Examples: Guitar, Violin.
5. Give two examples each of:
a. good reflectors of sound: Metals (like steel or copper) and concrete/brick walls.
b. bad reflectors of sound: Curtains (cloth) and sponge (or thermocol).
6. What is an echo? Where does echo generally occur?
An echo is the repetition of the original sound heard by an observer after it reflects off a hard, distant surface. Echoes generally occur in empty spacious spaces, high-ceilinged halls, mountains, and deep valleys.
7. Find the frequency of the wave with a time period 0.50 s.
math
⁠

G. Long-answer-type questions.

1. Prove with the help of an activity that sound travels better through liquids.
Aim: To demonstrate that sound can travel through liquids.
Materials Required: A squeaking toy, a plastic polythene bag, and a bucket half-filled with water.
Procedure:
Place the squeaking toy inside the polythene bag and tie it tightly to prevent water from entering.
Immerse the wrapped toy deep into the bucket of water.
Place your ear flat against the outer wall of the bucket.
Squeeze the toy with your hand inside the water to make it squeak.
Observation: You will clearly and loudly hear the squeaking sound of the toy traveling through the water and the bucket walls to your ear.
Conclusion: This activity proves that sound travels effectively through liquids.
2. How do the following produce sound?
a. Human beings: Humans produce sound through the voice box or larynx, which is located in the throat. The larynx contains a pair of stretchable vocal cords with a narrow slit between them. When the lungs force air through this slit, the vocal cords vibrate rapidly and produce sound. Muscles attached to these cords stretch or loosen them to change the pitch and frequency of the voice.
b. Animals: Animals with lungs, like frogs and mammals, blow air through their vocal cords. Birds produce sound via a specialized ring of cartilage called the syrinx at the base of their windpipe. Fish vibrate their air bladders, grasshoppers rub stiff hairs on their legs to produce chirps, and insects like bees and mosquitoes buzz by rapidly vibrating their wings.
3. How is sound produced by different types of musical instruments?
Stringed instruments: Sound is produced by plucking, striking, or bowing stretched strings, causing them to vibrate. A hollow wooden body amplifies this vibration.
Percussion instruments: Striking a tightly stretched skin or membrane across an open hollow frame causes the membrane to vibrate and produce sound.
Wind instruments: A musician blows air into the instrument, which sets the air column inside the hollow tubes into a state of vibration to produce sound.
Reed instruments: Sound is created when air is blown over thin metal reeds, causing the metal strips to vibrate rapidly.
4. Define the following: oscillation, wavelength, frequency, time period, amplitude
Oscillation: One complete to-and-fro movement of a particle of a medium about its mean position.
Wavelength (λ): The linear distance between two consecutive crests or two consecutive troughs of a wave.
Frequency: The total number of complete oscillations made by a wave in one second. Its unit is hertz (Hz).
Time period (T): The total time taken by a wave to complete one single oscillation. Its unit is second (s).
Amplitude: The maximum displacement of a vibrating particle or wave from its central mean position. Its unit is metre (m).
5. Explain the factors on which the loudness of sound depends.
Amplitude of vibration: Loudness is directly proportional to the square of the amplitude. A larger amplitude of vibration creates a louder sound.
Distance from the source: Loudness decreases as the distance between the source of sound and the listener increases.
Area of the vibrating body: A larger surface area of the vibrating object sets a larger volume of air into motion, producing a louder sound.
Sensitivity of the ear: The perceived loudness of a sound is subjective to how sensitive the listener’s ears are.
6. What are ultrasonic sounds? State five uses of such sounds.
Ultrasonic sounds are sound waves with frequencies higher than 20,000 Hz, which cannot be heard by human ears.
Five uses of ultrasonic sounds:
Ultrasonography: Used in hospitals for scanning and diagnostic imaging of internal organs.
Crack detection: Used in industrial manufacturing to detect fine cracks and hidden flaws in heavy metal sheets or machinery blocks.
Physiotherapy: Used to relieve pain in joints and muscles.
Cleaning: Used in ultrasonic cleaners to easily remove grease, dirt, and dental tartar from delicate instruments and teeth.
Fishing: Used by commercial fishermen and marine vessels to locate deep shoals of fish in the sea.
⁠

Higher Order Thinking Skills (HOTS) (Page 137)

1. Which of the following materials would be the most appropriate for covering the walls of a cinema hall?
a. a material that can absorb more sound waves than it reflects (to minimize echo and prevent sound leakage).
2. Assertion (A): It creates louder sound in the case of strings of musical instruments plucked with greater force than with force of smaller magnitude. Reason ®: When the string is plucked with greater force, the amplitude of the string will be larger.
a. Both A and R are true and R is the correct explanation of A.
⁠

Integrate: Patterns in music (Page 137)

1. SMGR SRGM matches with a. SPDN SNDP
2. RPMG RGMP matches with b. NMPD NDPM
3. GDPM GMPD matches with c. DGMP DPMG
Gallery
 
Want to print your doc?
This is not the way.
Try clicking the ··· in the right corner or using a keyboard shortcut (
CtrlP
) instead.