Class 9 Science · Chapter 10
Short answer:
Sound Waves: Characteristics and Applications — Class 9 Science Exploration textbook ka Chapter 10 hai. Ye batata hai ki awaaz kaise banti hai (vibration se), kaise travel karti hai (longitudinal wave — compression & rarefaction), aur uske characteristics — wavelength (λ), frequency (f), amplitude, time period (T), speed (v = fλ) — kaise pitch aur loudness decide karte hain. Reflection of sound (echo, reverberation), SONAR, aur human ear ki working bhi isi chapter me aati hai — in short, ye class 9 science sound chapter important questions ka core hai.
Agar tumne recent class 9 science new syllabus 2026-27 chapter list dekhi hai, toh pata hoga ki naye Exploration textbook me total 13 chapters hain, aur Sound waala chapter ab renumber hoke Chapter 10: "Sound Waves: Characteristics and Applications" ban gaya hai — pehle sirf "Sound" naam se jaana jaata tha. Title me hi "characteristics" (theory ka part — wavelength, frequency, amplitude) aur "applications" (SONAR, stethoscope, ultrasound cleaning, sonography) dono explicitly add kiye gaye hain, taaki students ko clear rahe ki chapter sirf definitions tak limited nahi, real-world use tak jaata hai.
Baaki physics chapters (jaise Describing Motion, How Forces Affect Motion) me hum dekhte hain "kya move ho raha hai" — is chapter me hum dekhte hain kaise ek vibrating object apni energy medium ke particles tak transfer karta hai, aur wo disturbance longitudinal wave ki form me travel karke tumhare kaan tak pahunchti hai. Ye chapter numerical-heavy bhi hai — v = fλ, echo distance, aur SONAR wale sawaal exams me consistently aate hain, isliye formula aur unit dono clearly yaad rakhna zaroori hai. Chapter khatam hone tak tum samajh jaoge ki bats ultrasound se kaise "dekhte" hain, doctors stethoscope me multiple reflection kaise use karte hain, aur samundar ki depth SONAR se kaise naapi jaati hai.
Note: agar tumhe poora syllabus ek jagah chahiye — class 9 science all chapters notes pdf ya class 9 science exploration book pdf download jaisi searches se pehle apne school se ya ncert.nic.in se official copy confirm kar lena, kyunki third-party PDFs me kabhi-kabhi old "Science" book ka content mix ho jaata hai.
Chapter 10 Summary — 5 Minute Revision
Sound ek form of energy hai jo vibrating (kampan karne waale) object se paida hoti hai aur ek material medium (solid, liquid, gas) ke through longitudinal wave ki form me travel karti hai — vacuum me sound travel nahi kar sakti kyunki usme particles hi nahi hote. Medium ke particles apni mean position ke aage-peeche vibrate karte hain, jisse compressions (high pressure/density regions) aur rarefactions (low pressure/density regions) banti hain.
Sound wave ki 4 main characteristics hoti hain: wavelength (λ) — do consecutive compressions/rarefactions ke beech ki distance; frequency (f) — per second oscillations ki sankhya, jo pitch decide karti hai; time period (T = 1/f) — ek oscillation ka time; aur amplitude — maximum displacement, jo loudness decide karta hai (loudness amplitude ke square ke directly proportional hoti hai). Speed ka formula v = fλ hai, aur sound solids me sabse fast, liquids me medium, aur gases me sabse slow travel karti hai (density aur elasticity ke wajah se) — kyunki solids me particles ek-dusre ke zyada paas aur tightly bonded hote hain.
Sound reflect bhi hoti hai — light ki tarah hi laws of reflection follow karti hai (angle of incidence = angle of reflection), bas isko reflect karne ke liye badi, hard surface chahiye. Reflection ke do practical results: echo (original sound se alag, clearly repeat hui sound — minimum 17.2 m distance chahiye kyunki human ear ko 0.1 s tak sound "persist" hoti dikhti hai) aur reverberation (band room me baar-baar reflection se sound ka lagataar gunjna, jise sound-absorbing material se control kiya jaata hai). Multiple reflection ke useful applications hote hain — megaphone, stethoscope, aur concert hall ki curved ceilings/soundboards.
Human ear 20 Hz se 20,000 Hz (20 kHz) tak ki frequency sun sakta hai — ise audible range kehte hain. Isse kam frequency infrasound (elephants, whales use karte hain) aur isse zyada ultrasound (bats, dolphins use karte hain) kehlaati hai. Ultrasound ke practical applications hain — hard-to-reach parts cleaning, metal me cracks detect karna (non-destructive testing), medical sonography, kidney stones todna, aur sabse important — SONAR (Sound Navigation And Ranging), jisse pulse bhejke aur uska echo receive karke depth ya distance nikaali jaati hai: distance = (speed × time)/2. Chapter ke end me human ear ki structure bhi cover hoti hai — outer ear (pinna → auditory canal → eardrum), middle ear (hammer, anvil, stirrup — vibrations ko amplify karte hain), aur inner ear (cochlea — vibrations ko electrical signal me convert karke auditory nerve ke through brain tak bhejta hai).
In-Text Questions — Solutions
Ek vibrating object se paida hui sound, medium ke through travel karke tumhare kaan tak kaise pahunchti hai?
Vibrating object apne aas-paas ke medium ke particles ko push (compress) aur pull (rarefy) karta hai. Ye disturbance ek particle se agle particle tak transfer hoti hai — particles khud aage nahi jaate, sirf energy/disturbance aage badhti hai — is tarah compressions aur rarefactions ki series ek longitudinal wave banati hai jo travel karke kaan ke eardrum tak pahunchti hai aur use vibrate karti hai.
School bell ki awaaz kaise paida hoti hai, samjhao.
Jab bell ko hathoda ya stick se strike kiya jaata hai, bell ki metal surface tezi se vibrate karne lagti hai. Ye vibration aas-paas ki hawa ke particles ko compress aur rarefy karta hai, jisse sound wave banti hai jo hawa me travel karke humare kaan tak pahunchti hai.
Sound wave ko longitudinal wave kyun kehte hain?
Kyunki sound wave me medium ke particles wave ki propagation direction ke parallel (aage-peeche) vibrate karte hain, na ki perpendicular. Isi wajah se compression (particles paas-paas) aur rarefaction (particles door-door) ki alternate regions banti hain — ye pattern transverse wave se different hai jahan particles direction ke perpendicular oscillate karte hain.
Same amplitude ki do sounds, alag frequency ki hon, unhe differentiate karne me kaunsi characteristic help karti hai?
Frequency (jo pitch decide karti hai) help karti hai. Amplitude same hone ka matlab dono equally loud hain, lekin high frequency waali sound sharp/high-pitched sunayi degi aur low frequency waali sound dull/low-pitched — is difference se hum unhe alag pehchan sakte hain.
Bijli chamakti hai aur thoda time baad hi thunder sunayi deti hai, saath-saath produce hone ke bawajood — kyun?
Light ki speed (~3 × 108 m/s) sound ki speed (~344 m/s) se lakhon guna zyada hai. Isliye flash turant dikh jaata hai lekin sound ko wahi distance cover karne me kaafi zyada time lagta hai, isliye thunder kuch second late sunayi deti hai.
Ek stone ko 500 m oonchi tower se pond me drop kiya jaata hai. Splash ki awaaz tower ke top par kab sunayi degi? (g = 10 m/s², vsound = 340 m/s)
Stone ko fall hone me time: h = ½gt₁² ⟹ 500 = ½ × 10 × t₁² ⟹ t₁² = 100 ⟹ t₁ = 10 s
Sound ko wapas top tak aane me time: t₂ = distance/speed = 500/340 = 1.47 s
Total time = t₁ + t₂ = 10 + 1.47 = 11.47 s (≈ 11.5 s)
Ek sound wave ki frequency 2 kHz aur wavelength 35 cm hai. Ise 1.5 km travel karne me kitna time lagega?
f = 2 kHz = 2000 Hz, λ = 35 cm = 0.35 m
Speed: v = f × λ = 2000 × 0.35 = 700 m/s
Distance = 1.5 km = 1500 m
Time = Distance/Speed = 1500/700 = 2.14 s (approx)
Air, water aur iron — teeno me se sound sabse fast kis medium me travel karti hai, ek particular temperature par?
Iron (solid) me sound sabse fast travel karti hai, uske baad water (liquid), aur sabse slow air (gas) me — kyunki solids me particles ek-dusre ke sabse zyada paas aur tightly bonded hote hain, jisse disturbance jaldi transfer hoti hai (vsolid > vliquid > vgas).
Ek person hill se 350 m door khada hokar gun fire karta hai. Kitne time baad use echo sunayi degi? (v = 346 m/s)
Sound ko hill tak jaana aur wapas aana hai, total distance = 2 × 350 = 700 m
Time = Distance/Speed = 700/346 = 2.02 s (approx)
Concert halls ki ceilings curved kyun hoti hain?
Curved ceilings sound waves ko multiple reflection ke through hall ke har corner me evenly spread karti hain, taaki audience ke sabhi seats par sound clearly aur uniformly pahunche — bina curved shape ke sound sirf kuch directions me concentrate ho jaati aur baaki jagah kam sunayi deti.
Average human ear ka audible range kya hai?
Human ear normally 20 Hz se 20,000 Hz (20 kHz) tak ki frequencies sun sakta hai. Isse kam ya zyada frequency waali sound waves humein sunayi nahi deti.
Infrasound aur Ultrasound se associated frequency range kya hai?
Infrasound: 20 Hz se kam frequency (elephants, rhinos, whales use karte hain). Ultrasound: 20,000 Hz (20 kHz) se zyada frequency (bats, dolphins use karte hain, aur medical/industrial applications me bhi use hota hai).
Ek submarine SONAR pulse bhejta hai jo underwater cliff se 1.02 s me wapas aata hai. Agar salt water me speed of sound 1531 m/s hai, cliff kitni door hai?
d = (v × t)/2 = (1531 × 1.02)/2 = 1561.62/2
d = 780.81 m (approx 780.8 m)
Human ear kaise kaam karta hai, briefly samjhao.
Outer ear (pinna) sound collect karke auditory canal se eardrum (tympanic membrane) tak bhejta hai, jo vibrate karta hai. Middle ear ki 3 chhoti bones (hammer, anvil, stirrup) in vibrations ko amplify karke inner ear tak pahunchati hain, jahan cochlea vibrations ko electrical signals me convert karta hai — ye signal auditory nerve ke through brain tak jaata hai jahan hume sound 'sunayi' deti hai.
Exercise Questions — Solutions (Q1–Q19)
Sound kya hai aur ye kaise produce hoti hai?
Sound ek form of energy hai jo humein hearing sensation deti hai. Ye kisi bhi vibrating object se produce hoti hai — jab object vibrate karta hai, to wo aas-paas ke medium ke particles ko bhi vibrate karne par majboor karta hai, jisse ek disturbance (wave) banti hai jo medium ke through travel karti hai.
Diagram ki help se samjhao ki sound source ke paas hawa me compressions aur rarefactions kaise produce hoti hain.
Jab vibrating object (jaise tuning fork ka prong) aage move karta hai, wo apne saamne ki hawa ke particles ko compress kar deta hai — is region me particle density high ho jaati hai, ise compression kehte hain. Jab wahi object peeche move karta hai, particles spread ho jaate hain aur density kam ho jaati hai — ise rarefaction kehte hain. Object ke continuous back-and-forth motion se compressions aur rarefactions ki alternate series banti hai jo wave ki form me aage badhti hai (diagram me high-density bands aur low-density bands alternate dikhaye jaate hain).
Ek experiment cite karo jo dikhaye ki sound ke propagation ke liye material medium zaroori hai.
Ek electric bell ko ek airtight glass jar ke andar rakh kar bajaya jaata hai — shuru me air present hone par bell ki awaaz saaf sunayi deti hai. Ab jar se dheere-dheere vacuum pump ke through hawa nikaali jaati hai — jaise-jaise air kam hoti jaati hai, awaaz bhi dheemi hoti jaati hai, aur complete vacuum banne par awaaz bilkul sunayi nahi deti (bell ka hammer phir bhi vibrate karta dikhta hai). Ye prove karta hai ki sound ke travel karne ke liye material medium zaroori hai.
Sound wave ko longitudinal wave kyun kaha jaata hai?
Kyunki sound wave me medium ke particles wave ki propagation direction ke parallel (same line me aage-peeche) vibrate karte hain — jisse compression aur rarefaction ki alternate regions banti hain. Ye transverse wave se different hai, jahan particles perpendicular direction me oscillate karte hain.
Same amplitude ki do sounds, jo do alag musical instruments se produce hui hon, unhe kaunsi characteristic differentiate karti hai?
Quality ya Timbre — ye characteristic hume same pitch aur loudness ki sounds ko bhi alag instruments (jaise guitar vs violin) ki based par differentiate karne me help karti hai, kyunki har instrument alag harmonics/overtones produce karta hai.
High pitch aur low pitch sound ke liye displacement-time graph draw karo (word me describe karo).
High pitch: graph me waves close together (short time period, high frequency) dikhengi — ek fixed time interval me zyada oscillations honge. Low pitch: graph me waves door-door (long time period, low frequency) dikhengi — same time interval me kam oscillations honge. Dono graphs sine-wave shape ke honge, bas x-axis par unki 'spacing' alag hogi.
Reverberation kya hai? Ise kaise kam kiya ja sakta hai?
Reverberation band room/hall me sound ki multiple, overlapping reflections ke wajah se persist hote rehna hai, jisse original sound khatam hone ke baad bhi awaaz gunjti rehti hai — bahut zyada reverberation se sound unclear ho jaati hai.
Kam karne ke tareeke: (1) walls aur ceiling par sound-absorbent material (compressed fibreboard, rough plaster) lagana, (2) heavy curtains/draperies use karna, (3) floor par carpet bichhana — ye sab reflected sound ko absorb karke echo/reverberation reduce karte hain.
Loudness of sound kya hai? Ye kin factors par depend karta hai?
Loudness sound ki intensity ka measure hai jo batata hai ki sound kitni tez ya dheemi sunayi de rahi hai — isse humara ear kitna sensitive respond karta hai wo bhi depend karta hai. Ye mainly amplitude par depend karta hai (loudness ∝ amplitude²) — bada amplitude, zyada loud sound. Ye source se distance aur surrounding noise par bhi depend karta hai.
Air, water, aur iron — in teeno media me se sound kis medium me sabse fast travel karti hai?
Iron me sound sabse fast travel karti hai (viron ≈ 5130 m/s), uske baad water (vwater ≈ 1493 m/s), aur sabse slow air me (vair ≈ 344 m/s) — kyunki vsolid > vliquid > vgas (particles ki closeness aur elasticity ke wajah se).
Ek echo 3 s me return hoti hai. Reflecting surface source se kitni door hai? (Speed of sound = 342 m/s)
d = (v × t)/2 = (342 × 3)/2 = 1026/2
d = 513 m
Concert halls ki ceilings curved kyun hoti hain?
Taaki sound waves multiple reflection ke through poore hall me evenly distribute ho jaayein, aur har seat par audience ko sound clearly aur uniformly sunayi de — flat ceiling me sound sirf kuch specific directions me hi concentrate rehti.
Average human ear ka audible range kya hai?
20 Hz se 20,000 Hz (20 kHz) — ye range audible range kehlaati hai.
Infrasound aur Ultrasound se associated frequency range kya hai?
Infrasound: frequency 20 Hz se kam. Ultrasound: frequency 20,000 Hz (20 kHz) se zyada.
Ek submarine SONAR pulse bhejta hai jo underwater cliff se 1.02 s me wapas aata hai. Salt water me speed of sound 1531 m/s hai — cliff kitni door hai?
d = (v × t)/2 = (1531 × 1.02)/2 = 1561.62/2
d = 780.81 m (approx)
Metal block me defects (cracks) detect karne ke liye ultrasound ka use kaise hota hai, samjhao.
Ultrasonic waves ko metal block ke ek side se bheja jaata hai. Agar block bilkul flawless hai, to sound seedha doosre end tak pahunch jaati hai. Lekin agar block ke andar koi crack ya cavity (defect) hai, to us point par ultrasonic wave reflect ho jaati hai aur detector ko turant ek reflected signal receive hota hai — is reflection ki position/timing se defect ka exact location detect kiya jaata hai. Ye method non-destructive testing kehlaata hai kyunki block ko todna nahi padta.
Bats ultrasound se prey (shikaar) kaise pakadte hain, explain karo.
Bats udte waqt continuously high-frequency ultrasonic waves emit karte hain. Ye waves aas-paas ki cheezon (jaise insects) se takra kar reflect (echo) hoti hain, aur bat unhe apne sensitive kaano se receive karta hai. Echo ke return-time aur direction se bat prey ki exact position, distance, aur size ka andaza laga leta hai — is technique ko echolocation kehte hain, jisse andhere me bhi bat accurately shikaar pakad leta hai.
Ek sound wave 339 m/s ki speed se travel karti hai. Agar iska wavelength 1.5 cm hai, wave ki frequency kya hogi? Kya ye audible hogi?
v = 339 m/s, λ = 1.5 cm = 0.015 m
f = v/λ = 339/0.015 = 22,600 Hz (22.6 kHz)
Ye frequency 20,000 Hz (20 kHz) se zyada hai, isliye ye audible NAHI hogi — ye ultrasound range me aati hai jo human ear sun nahi sakta.
Reflection of sound kya hai? Reflection of sound ke laws state karo.
Jab sound wave ek badi, hard surface (jaise wall, hill) se takra kar wapas medium me lautti hai, use reflection of sound kehte hain. Iske laws light ki tarah hi hain: (1) Angle of incidence hamesha angle of reflection ke barabar hota hai (∠i = ∠r), (2) Incident sound wave, reflected sound wave, aur normal — teeno point of incidence par same plane me hote hain.
Human ear kaise kaam karta hai, explain karo.
Outer ear (pinna) sound waves collect karke auditory canal ke through eardrum tak bhejta hai, jahan wo vibrate hone lagta hai. Middle ear ki teen chhoti bones — hammer (malleus), anvil (incus), aur stirrup (stapes) — in vibrations ko kai guna amplify karke inner ear ke oval window tak transfer karti hain. Inner ear ka cochlea (spiral-shaped, fluid se bhara) in vibrations ko electrical (nerve) signals me convert karta hai, jo auditory nerve ke through brain tak pahunchte hain — wahan par unhe sound ke roop me interpret kiya jaata hai.
Important Equations — Ek Nazar Me
| Quantity / Rule | Formula | Notes |
|---|---|---|
| Speed of sound | v = f × λ (also v = λ/T) | f in Hz, λ in m, v in m/s |
| Time period | T = 1/f | T in seconds, f in Hz |
| Frequency | f = 1/T = v/λ | Pitch is directly proportional to f |
| Loudness | Loudness ∝ (Amplitude)² | Measured in decibel (dB) |
| Distance travelled by wave | Distance = Speed × Time | Basic motion-sound relation |
| Echo distance (reflecting surface) | d = (v × t)/2 | t = time for sound to go and return |
| Minimum distance to hear echo | dmin = (v × 0.1)/2 ≈ 17.2 m | Persistence of hearing = 0.1 s, v ≈ 344 m/s |
| SONAR distance | d = (v × t)/2 | v = speed of sound in water/medium used |
| Laws of reflection of sound | ∠i = ∠r | Incident wave, reflected wave & normal lie in same plane, at the point of incidence |
| Audible range (human) | 20 Hz – 20,000 Hz (20 kHz) | <20 Hz = infrasound, >20 kHz = ultrasound |
| Speed of sound (approx, at room temp.) | Air ≈ 344 m/s, Water ≈ 1493 m/s, Iron ≈ 5130 m/s | vsolid > vliquid > vgas |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
Frequency aur Amplitude confuse karna — frequency pitch decide karti hai (chhoti/moti awaaz) jabki amplitude loudness decide karta hai (dheemi/tez awaaz). Ye dono independent hain — high frequency ka matlab loud nahi hota.
Sound ko transverse wave samajhna — sound hamesha longitudinal wave hai (particles wave ki direction ke parallel vibrate karte hain, perpendicular nahi), light/water waves ki tarah transverse nahi.
Echo distance ki formula me ×2 bhoolna — students d = v×t likh dete hain, lekin sound object tak jaati hai AUR wapas aati hai, isliye d = (v×t)/2 hoga, na ki v×t.
Speed of sound ko medium-independent maan lena — v = fλ me v medium ke hisaab se change hota hai (solid > liquid > gas); students galti se ek hi 'v' value har numerical me use kar dete hain jab question me alag medium diya ho.
Ultrasound aur infrasound ulta likhna — ultrasound = 20 kHz se zyada frequency (bats), infrasound = 20 Hz se kam frequency (elephants). Exam me ye swap ho jaata hai.
Reverberation aur Echo ko same maan lena — echo ek single, clearly separate reflected sound hai jo minimum 17.2 m distance par sunayi deta hai; reverberation ek continuous, overlapping multiple reflections ka result hai jo band room me hota hai — dono ka cause same (reflection) hai lekin effect alag.
Board-Style Important Questions
- 1 mark: Human ear ka audible range likho.
- 2 marks: Longitudinal wave me compression aur rarefaction ko define karo.
- 3 marks: Ek submarine SONAR pulse bhejta hai jo 1.02 s me underwater cliff se wapas aata hai. Agar salt water me sound ki speed 1531 m/s hai, cliff kitni door hai — poora calculation dikhao.
- 3 marks: Reverberation kya hai aur bade halls me isko kaise kam kiya jaata hai — koi teen tareeke likho.
- 5 marks: Human ear ki structure diagram sahit samjhao ki sound wave kaise electrical signal me convert hoke brain tak pahunchti hai.
- 5 marks: Echo kya hota hai, echo sunayi dene ke liye minimum distance kyun chahiye — persistence of hearing ke concept se numerically explain karo (v = 344 m/s lekar).
Aksar Poochhe Jaane Wale Sawaal
Class 9 Science Exploration me Sound chapter ka number kya hai?
Naye Exploration textbook (2026-27 rationalised syllabus, 13 chapters) me Sound Chapter 10 hai, poora naam "Sound Waves: Characteristics and Applications". Isse pehle ye purane book me alag number ke saath sirf "Sound" naam se tha.
Kya sound vacuum (space) me travel kar sakti hai?
Nahi. Sound ek mechanical wave hai jise travel karne ke liye material medium (solid, liquid ya gas) chahiye kyunki wave particles ke vibration se propagate hoti hai. Vacuum me particles hote hi nahi, isliye space me sound nahi sunayi degi — yehi wajah hai ki astronauts radio waves se baat karte hain.
SONAR ka full form aur use kya hai?
SONAR = SOund NAvigation Ranging. Ye ultrasonic waves bhejkar aur unka echo receive karke samundar ki depth naapne, submarine navigation, aur underwater objects detect karne me use hota hai. Formula: distance = (speed × time)/2.
Class 9 Science all chapters notes PDF ya Exploration book PDF kahan se download karein?
Sabse reliable source official ncert.nic.in hai — wahan se hi chapter-wise PDF free download hoti hai. Third-party sites pe "class 9 science all chapters notes pdf" ya "class 9 science exploration book pdf download" search karte waqt confirm kar lo ki wo naye 13-chapter Exploration edition ka hai, na ki purani "Science" book ka — dono ka content aur chapter naming alag hai.
Sound chapter se pehle aur baad me kaunse chapters hain, agar main aage/peeche revise karna chahoon?
Sound se pehle Physics-Chemistry ke chapters hain — Work, Energy and Simple Machines (numericals ke liye), Journey Inside the Atom aur Atomic Foundations of Matter (jinke liye 'structure of atom class 9 notes' aur 'atoms and molecules important questions class 9' searches useful hain). Sound ke baad Biology start hota hai — Reproduction ('reproduction class 9 ncert solutions') aur Patterns in Life – Diversity and Classification ('diversity and classification class 9 important questions').
Loudness aur pitch me kya farak hai?
Pitch sound ki frequency par depend karta hai — high frequency = high pitch (patli/teekhi awaaz), low frequency = low pitch (moti/bhaari awaaz). Loudness amplitude par depend karta hai — bada amplitude = zyada loud sound, chhota amplitude = dheemi sound. Ek high-pitch sound bhi dheemi ho sakti hai aur ek low-pitch sound bhi loud ho sakti hai — dono independent properties hain.
Class 9 Science — Saare Chapters
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