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c. Steel (Explanation: Sound travels fastest in solids; steel has a speed of approx. 5100 m/s compared to 1450 m/s in water and 340 m/s in air).a. Two (Explanation: Humans actually have only two vocal cords, and the range of sounds is due to their vibrations).c. Hertz (Hz)b. 17 m (Explanation: 
).d. Thermocol sheet (Explanation: Soft, fluffy, and porous materials like thermocol are bad reflectors and absorb sound).
hearingfrequencylongitudinalultrasoniclarynx (or voice box)
F (Sound cannot travel through a vacuum as it requires a material medium to propagate).T (Two waves of the same frequency can indeed have different amplitudes).F (Sound travels slowest in gases, faster in liquids, and fastest in solids).F (Infrasonic sounds have frequencies less than 20 Hz and are completely inaudible to human ears).T (The larynx is smaller in females, making vocal cords tight and thin, resulting in a higher frequency/shrill voice).
Amplitude: The maximum displacement of a wave on either side of its mean (equilibrium) position. Its unit is the metre (m).Echo: The sound that is heard by an observer after it is reflected back from a rigid, distant surface.Wavelength: The length of a wave along the x-axis, represented by the distance between two successive crests or troughs. It is denoted by the Greek letter lambda ( 
).Infrasonic Sounds: Sounds of frequencies less than 20 Hz, which are completely inaudible to the human ear.
‘A’ represents Amplitude. Its unit is the metre (m).‘B’ represents Wavelength. It is represented by the Greek letter lambda ( ).
Given: Oscillations ( 
) = 50, Time ( 
) = 5 seconds.a. Frequency 
.b. Time Period 
.
Materials Required: An electric bell, a glass bell jar, a battery, connection wires, an airtight cork, and a vacuum pump.Procedure:Invert the glass bell jar over the electric bell setup.Connect the electric bell to the battery using wires passed through an airtight cork.Strike the gong of the bell to verify the sound is loudly audible when air is present.Connect the vacuum pump to the bell jar and gradually pump out/remove the air from inside the jar while the bell is continuously ringing.Observation: As the air inside the jar is gradually removed by the vacuum pump, the sound of the bell becomes fainter and fainter, and finally becomes completely inaudible, even though the hammer can still be seen actively striking the gong.Conclusion: Sound cannot travel through a vacuum. It requires a material medium (like air) to propagate and transmit vibrations to our ears.
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Chapter: 06. Sound
SECTION A: MULTIPLE CHOICE QUESTIONS (MCQs)
SECTION B: FILL IN THE BLANKS
SECTION C: TRUE OR FALSE
SECTION D: EXPLANATION OF TERMS
SECTION E: IDENTIFY AND DIFFERENTIATE
Q1. Stringed Instruments vs. Percussion Instruments
Feature
Stringed Instruments
Percussion Instruments
How Sound is Produced
Stretched strings are plucked or stroked with a bow to vibrate.
A tightly stretched skin or membrane is struck to produce vibrations.
Air Column
Hollow body enhances sound via an trapped air column.
Hollow frame supports the stretched membrane.
Examples
Guitar, violin, veena, santoor, harp.
Drum, tabla, snare drum.
Q2. Ultrasonic vs. Infrasonic sounds
Feature
Ultrasonic Sounds
Infrasonic Sounds
Frequency Range
Frequencies higher than 20,000 Hz.
Frequencies less than 20 Hz.
Human Audibility
Inaudible to humans (but audible to dogs, bats, etc.).
Inaudible to humans.
Common Sources/Users
Bats, dolphins, medical ultrasound scanners.
Earthquakes, heavy machinery, elephants.
SECTION F: DIAGRAM-BASED QUESTIONS
SECTION G: SHORT ANSWER QUESTIONS
Q1. Answer:
This occurs because the speed of light is vastly superior to the speed of sound. The speed of light in air is constant at 
(instantaneous for terrestrial distances), whereas the speed of sound in air is only about 
. Therefore, the light flash reaches our eyes almost instantly, while the sound wave takes a few seconds to travel the same distance to our ears.
Q2. Answer:
Q3. Answer:
Bats produce high-frequency ultrasonic sounds as they fly. These waves travel through the air, strike objects (like insects, walls, or obstacles) in their path, and bounce back as echoes. Bats detect these reflected ultrasonic echoes, allowing them to map out their surroundings, detect obstacles, and pinpoint the exact location of prey in the dark.
Q4. Answer:
Thick carpets and rough, porous wall coatings are highly effective sound-absorbing materials. When sound waves hit these soft, fluffy, or uneven surfaces, they are absorbed rather than reflected. This helps to prevent echoes and excessive reverberations (overlapping sounds) inside the auditorium, ensuring clear sound quality during the music recital.
SECTION H: LONG ANSWER QUESTIONS
Q1. Answer:
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