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Political Science NCERT Solutions Class 9 Science Chapter 11 – Sound
Class 9 Science
Last Updated: 15 April 2026
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हिंदी में (Hindi)
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Important Points
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Practice MCQ Questions (40 questions)
Q1. Sound is a form of ________ that produces a sensation of hearing.
✓ Correct Answer: B
Q2. Which action primarily produces sound in objects?
✓ Correct Answer: B
Q3. The human voice is produced due to vibrations in the ________.
✓ Correct Answer: B
Q4. The material through which sound travels is called a ________.
✓ Correct Answer: B
Q5. Which statement about sound propagation is TRUE?
✓ Correct Answer: B
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Frequently Asked Questions (FAQ)
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Tough Words (Glossary)
vibration
Meaning: rapid to-and-fro motion of an object
अर्थ: किसी वस्तु की तेज़ आगे‑पीछे गति (कंपन)
"Sound is produced when a body is in vibration."
propagation
Meaning: spreading or transmission through a medium
अर्थ: माध्यम के माध्यम से फैलना या संचरण
"The propagation of sound needs a material medium."
medium
Meaning: material (solid, liquid or gas) through which sound travels
अर्थ: वह पदार्थ (ठोस, द्रव या गैस) जिसमें ध्वनि चलती है
"Air is the most common medium for sound."
equilibrium
Meaning: state of rest or balance
अर्थ: विश्राम या संतुलन की अवस्था
"A particle is displaced from its equilibrium position by the vibrating source."
compression
Meaning: region of high pressure in a sound wave
अर्थ: ध्वनि तरंग में उच्च दाब का क्षेत्र (संकुचन)
"Compressions are formed when particles are pushed close together."
rarefaction
Meaning: region of low pressure in a sound wave
अर्थ: ध्वनि तरंग में निम्न दाब का क्षेत्र (प्र rarefaction / विरलन)
"Rarefactions occur when particles move apart."
longitudinal wave
Meaning: wave in which particles oscillate parallel to direction of travel
अर्थ: ऐसी तरंग जिसमें कण तरंग की दिशा के समानांतर दोलन करते हैं
"Sound in air is a longitudinal wave."
transverse wave
Meaning: wave in which particles oscillate perpendicular to direction of travel
अर्थ: ऐसी तरंग जिसमें कण तरंग की दिशा के लम्बवत दोलन करते हैं
"Water ripples are transverse waves."
oscillate
Meaning: to move back and forth repeatedly
अर्थ: बार‑बार आगे‑पीछे दोलन करना
"The particles of the medium oscillate about their mean positions."
wavelength (λ)
Meaning: distance between two consecutive compressions or rarefactions
अर्थ: दो लगातार संकुचनों या विरलनों के बीच की दूरी (तरंगदैर्घ्य)
"The wavelength of this sound is 0.8 m."
frequency (ν)
Meaning: number of oscillations per second
अर्थ: प्रति सेकंड दोलनों की संख्या (आवृत्ति)
"High frequency sound has a high pitch."
time period (T)
Meaning: time taken for one complete oscillation
अर्थ: एक पूर्ण दोलन में लगा समय (अवधि)
"The time period is the inverse of frequency."
amplitude
Meaning: maximum displacement from the mean position
अर्थ: औसत स्थिति से अधिकतम विस्थापन (आयाम)
"A louder sound has a greater amplitude."
pitch
Meaning: how high or low a sound seems; perception of frequency
अर्थ: ध्वनि की ऊँचाई‑निचाई; आवृत्ति की अनुभूति (पिच)
"A flute produces a higher pitch than a drum."
timbre (quality)
Meaning: character of sound that distinguishes sources with same pitch and loudness
अर्थ: ध्वनि का स्वरगुण जो समान पिच व तीव्रता में भी स्रोतों को अलग करता है
"We distinguish a violin from a flute by timbre."
intensity
Meaning: sound energy passing per second through unit area
अर्थ: एकक क्षेत्र से प्रति सेकंड गुजरने वाली ध्वनि ऊर्जा (तीव्रता)
"Two sounds of equal intensity may not be equally loud."
mechanical wave
Meaning: wave that needs a material medium to travel
अर्थ: ऐसी तरंग जिसे चलने के लिए माध्यम चाहिए (यांत्रिक तरंग)
"Sound is a mechanical wave."
crest
Meaning: point of maximum value on a wave graph
अर्थ: तरंग आरेख का अधिकतम बिंदु (शिखर)
"Compressions correspond to crests in the density graph."
trough
Meaning: point of minimum value on a wave graph
अर्थ: तरंग आरेख का न्यूनतम बिंदु (गर्त)
"Rarefactions appear as troughs in the graph."
speed of sound
Meaning: rate at which sound travels through a medium
अर्थ: ध्वनि के माध्यम में चलने की दर (ध्वनि का वेग)
"The speed of sound in air increases with temperature."
reflection
Meaning: bouncing back of sound from a surface
अर्थ: ध्वनि का सतह से लौटना (प्रतिबिंबन)
"Sound follows laws of reflection like light."
echo
Meaning: reflected sound heard separately after a time gap
अर्थ: समयांतराल के बाद अलग से सुनाई देने वाली परावर्तित ध्वनि (प्रतिध्वनि)
"We hear an echo near a tall building."
reverberation
Meaning: persistence of sound due to multiple reflections
अर्थ: बार‑बार प्रतिबिंबों से ध्वनि का बने रहना (प्रतिध्वनन)
"Reverberation is reduced by sound‑absorbing materials."
audible range
Meaning: frequency range heard by humans (≈20 Hz–20 kHz)
अर्थ: मनुष्य द्वारा सुनी जाने वाली आवृत्ति सीमा (लगभग 20 हर्ट्ज–20 किलोहर्ट्ज)
"Whistles often have frequencies in the audible range."
infrasound
Meaning: sound with frequency below 20 Hz
अर्थ: 20 हर्ट्ज से कम आवृत्ति वाली ध्वनि (अधःश्रव्य)
"Elephants communicate using infrasound."
ultrasound
Meaning: sound with frequency above 20 kHz
अर्थ: 20 किलोहर्ट्ज से अधिक आवृत्ति वाली ध्वनि (अल्ट्रासाउंड/अति‑श्रव्य)
"Bats use ultrasound for navigation."
microphone
Meaning: device that converts sound waves into electrical signals
अर्थ: ध्वनि तरंगों को विद्युत संकेतों में बदलने वाला यंत्र (माइक्रोफ़ोन)
"A hearing aid has a tiny microphone."
amplifier
Meaning: device that increases the strength of signals
अर्थ: संकेतों की शक्ति बढ़ाने वाला यंत्र (ऐम्प्लीफ़ायर)
"The amplifier boosts the weak audio signal."
stethoscope
Meaning: medical device to listen to internal body sounds
अर्थ: शरीर के आंतरिक ध्वनियाँ सुनने का यंत्र (स्टीथोस्कोप)
"A doctor uses a stethoscope to hear heartbeats."
megaphone
Meaning: cone‑shaped device that directs sound forward
अर्थ: ध्वनि को आगे केन्द्रित करने वाला शंक्वाकार यंत्र (मेगाफ़ोन)
"A megaphone sends sound in one direction."
draperies
Meaning: heavy cloth curtains used to absorb sound
अर्थ: मोटे परदे जो ध्वनि को सोखते हैं (ड्रेपरी)
"Auditoriums use draperies to reduce echoes."
absorbent
Meaning: able to take in and reduce sound energy
अर्थ: ध्वनि ऊर्जा को सोखने वाला (शोषक)
"Sound‑absorbent panels lower reverberation."
multiple reflection
Meaning: repeated bouncing of sound from many surfaces
अर्थ: कई सतहों से बार‑बार ध्वनि का लौटना (बहु‑प्रतिबिंब)
"Rolling thunder is due to multiple reflection of sound."
persistence of hearing
Meaning: sound sensation staying in brain for ~0.1 s
अर्थ: कान/मस्तिष्क में ध्वनि संवेदना का लगभग 0.1 सेकंड तक बने रहना
"Echo requires a gap exceeding the persistence of hearing."
density variation
Meaning: change in number of particles per volume
अर्थ: प्रति आयतन कणों की संख्या में परिवर्तन (घनत्व परिवर्तन)
"Sound travels as density variations in air."
pressure variation
Meaning: change in pressure above and below average
अर्थ: औसत से ऊपर‑नीचे दाब में परिवर्तन (दाब परिवर्तन)
"Compressions are pressure variations above normal."
lambda (λ)
Meaning: Greek symbol used for wavelength
अर्थ: तरंगदैर्घ्य के लिए प्रयुक्त यूनानी चिन्ह λ
"We denote wavelength by λ in formulas."
hertz (Hz)
Meaning: SI unit of frequency (one oscillation per second)
अर्थ: आवृत्ति की एसआई इकाई (प्रति सेकंड एक दोलन)
"Human hearing is about 20 Hz to 20 kHz."
slinky
Meaning: stretched spring toy used to model waves
अर्थ: खींचने‑सिकोड़ने वाला स्प्रिंग खिलौना (स्लिंकी), तरंगों का मॉडल
"A slinky shows longitudinal compressions and rarefactions."
auditorium
Meaning: large hall for audiences and performances
अर्थ: दर्शकों व प्रस्तुतियों के लिए बड़ा सभागार
"An auditorium needs acoustic treatment to reduce echoes."
ultrasonography
Meaning: imaging technique using ultrasound
अर्थ: अल्ट्रासाउंड से चित्र बनाने की विधि (अल्ट्रासोनोग्राफी)
"Ultrasonography helps detect kidney stones."
echocardiography
Meaning: ultrasound imaging of the heart
अर्थ: हृदय का अल्ट्रासाउंड चित्रांकन (इकोकार्डियोग्राफी)
"Echocardiography maps the motion of heart valves."
foetus
Meaning: developing baby inside the uterus
अर्थ: गर्भाशय में विकसित होता शिशु (भ्रूण)
"Ultrasound is used to examine the foetus."
congenital
Meaning: present from birth
अर्थ: जन्मजात
"Ultrasound can detect some congenital defects."
porpoise
Meaning: a small toothed whale related to dolphins
अर्थ: डॉल्फ़िन से मिलती‑जुलती छोटी तिमि (पॉरपॉइज़)
"Porpoises also use ultrasound clicks."
intestine (context: organ imaging)
Meaning: part of digestive system; internal organ
अर्थ: पाचन तंत्र का भाग; आंत (आंतरिक अंग)
"Ultrasound can image soft organs like the intestine."
reflection laws
Meaning: rules about equal angles and same plane
अर्थ: समान कोण व एक ही तल वाले प्रतिबिंबन के नियम
"Sound obeys the reflection laws like light."
heartbeat
Meaning: rhythmic sound produced by the heart
अर्थ: हृदय की धड़कन से उत्पन्न लयबद्ध ध्वनि
"A stethoscope carries the heartbeat by multiple reflections."
navigation (by bats)
Meaning: guiding movement using echoes
अर्थ: प्रतिध्वनियों से दिशा‑ज्ञान कर गति करना
"Bats use ultrasound for navigation at night."
intensity vs loudness
Meaning: physical power per area vs ear’s response
अर्थ: क्षेत्रफल पर शक्ति बनाम कान की अनुभूति (तीव्रता बनाम तेज़ी)
"Two equal intensities can differ in perceived loudness."
curved ceiling
Meaning: arched surface that spreads sound evenly
अर्थ: ध्वनि को समान फैलाने वाली घुमावदार छत
"Concert halls use curved ceilings for better acoustics."
soundboard
Meaning: reflecting panel placed to direct sound
अर्थ: ध्वनि को निर्देशित करने वाला परावर्तक फलक
"A soundboard spreads the singer’s voice across the hall."
cleaning bath (ultrasonic)
Meaning: liquid tank where ultrasound removes dirt
अर्थ: अल्ट्रासोनिक सफ़ाई के लिए द्रव टंकी (क्लीनिंग बाथ)
"Electronic parts are cleaned in an ultrasonic bath."
flaw detection
Meaning: finding internal cracks using ultrasound
अर्थ: अल्ट्रासाउंड से आंतरिक दरारें खोजना
"Ultrasound is used for flaw detection in metal blocks."