🧪 CBSE Class 9 Science Chapter 11 – Sound (ध्वनि) English & Hindi 📝 By Grandmaster Bikram Sutradhar (SirBikramSutradhar) | BASTRONAUTWAY

🧪 CBSE Class 9 Science Chapter 11 – Sound (ध्वनि) English & Hindi 📝 By Grandmaster Bikram Sutradhar (SirBikramSutradhar) | BASTRONAUTWAY
📘 Chapter Overview – English
Let’s explore the fascinating topic of “Sound” with this comprehensive bilingual guide crafted by Grandmaster Bikram Sutradhar (SirBikramSutradhar), exclusively for the BASTRONAUTWAY program.
This chapter explains how sound is created, how it travels through various substances, and how it reflects and behaves. Core concepts like frequency, amplitude, wavelength, echo, SONAR, and the ear’s mechanism are explained in simple bilingual points to help learners understand quickly and score better in exams.
📘 अध्याय सारांश – हिंदी
यह अध्याय Grandmaster Bikram Sutradhar (SirBikramSutradhar) द्वारा BASTRONAUTWAY पहल के अंतर्गत तैयार किया गया है, जिसमें ध्वनि से जुड़ी सभी आवश्यक अवधारणाओं को सरल दो-भाषीय प्रारूप में समझाया गया है।
इसमें आप जानेंगे कि ध्वनि कैसे उत्पन्न होती है, माध्यमों में कैसे यात्रा करती है, और इसकी परावर्तन, प्रतिध्वनि, सोनार एवं मानव कान की कार्यप्रणाली क्या होती है।








📒 Detailed Notes – English + Hindi
🔹 1. Definition of ध्वनि की परिभाषा
• Sound is a form of mechanical energy that enables hearing.
ध्वनि एक यांत्रिक ऊर्जा है जो हमें सुनने की क्षमता देती है।
🔹 2. Source of ध्वनि का स्रोत
• Whenever an object vibrates, it produces sound.
🎵 Examples / उदाहरण: Guitar strings, vocal cords, tuning fork
गिटार की तारें, स्वररज्जु, ट्यूनिंग फोर्क
🔹 3. How Sound Travels / ध्वनि कैसे फैलती है
• Sound travels in the form of longitudinal waves and cannot move through vacuum.
ध्वनि अनुदैर्ध्य तरंगों के रूप में फैलती है और निर्वात में नहीं चल सकती।
🌬 Mediums / माध्यम: Air (वायु), Water (जल), Solids (ठोस)





🔹 4. Features of Sound Waves / ध्वनि तरंगों की विशेषताएँ
Property | English Meaning | हिंदी अर्थ |
Frequency | Number of vibrations per second (Hz) | प्रति सेकंड कंपन की संख्या (Hz) |
Amplitude | Height of wave, shows loudness | तरंग की ऊँचाई, तीव्रता दर्शाती है |
Time Period | Time taken for one vibration cycle | एक पूर्ण कंपन में लगने वाला समय |
Wavelength | Distance between two compressions | दो संपीड़नों के बीच की दूरी |

📌 Sound speed in different mediums / विभिन्न माध्यमों में ध्वनि की गति:
- Air (वायु): 343 m/s
- Water (जल): 1482 m/s
- Iron (लोहा): 5960 m/s


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🔹 5. Sound Reflection / ध्वनि का परावर्तन
• When sound hits a surface and comes back, it is called reflection.
जब ध्वनि किसी सतह से टकराकर लौटती है, तो उसे परावर्तन कहते हैं।
🔁 Echo / प्रतिध्वनि: Sound heard after bouncing back with a gap of at least 0.1 second.
📏 Minimum distance for echo: 17.2 meters (at speed 343 m/s)

🔹 6. Hearing Range / श्रवण सीमा
• Human ears can perceive sound frequencies from 20 Hz to 20,000 Hz.
मनुष्य 20 हर्ट्ज़ से 20,000 हर्ट्ज़ तक की ध्वनि को सुन सकता है।
⚠️ < 20 Hz: Infrasonic (अवध्वनिक)
⚠️ > 20,000 Hz: Ultrasonic (पराश्रव्य)
🔹 7. Ultrasound & SONAR / पराश्रव्य तरंगें और सोनार
• Ultrasound is sound beyond the audible range (>20,000 Hz).
पराश्रव्य वे तरंगें हैं जिनकी आवृत्ति 20,000 हर्ट्ज़ से अधिक होती है।
• SONAR is a technique that sends out ultrasonic waves to detect underwater objects.
SONAR एक विधि है जो जल के नीचे वस्तुओं की जानकारी प्राप्त करने के लिए पराश्रव्य तरंगों का उपयोग करती है।
🛥 Applications / उपयोग: Submarines, sea exploration, fish finding
पनडुब्बियाँ, समुद्र विज्ञान, मछली ढूँढना

🔹 8. Structure of Human Ear / मानव कान की संरचना
👂 Process / प्रक्रिया:
- Outer ear collects sound →
- Sound moves via ear canal →
- Eardrum vibrates →
- Vibrations travel through middle ear →
- Inner ear converts them into electrical signals →
- Brain interprets them as sound
🧠 मस्तिष्क इन संकेतों को समझकर हमें ध्वनि का अनुभव कराता है।
✅ MCQs – Multiple Choice Questions (Paraphrased, Bilingual)
1️⃣ What is the unit for measuring frequency? / आवृत्ति मापने की इकाई क्या है?
🔘 (a) Joule 🔘 (b) Hertz 🔘 (c) Watt 🔘 (d) Decibel
✅ Answer: (b) Hertz / हर्ट्ज
2️⃣ Sound travels fastest in which of the following? / इनमें से किसमें ध्वनि सबसे तेज़ चलती है?
🔘 (a) Air 🔘 (b) Water 🔘 (c) Iron 🔘 (d) Space
✅ Answer: (c) Iron / लोहा
3️⃣ प्रतिध्वनि सुनने के लिए न्यूनतम दूरी क्या है?
🔘 (a) 10 m 🔘 (b) 17.2 m 🔘 (c) 5 m 🔘 (d) 2 m
✅ Answer: (b) 17.2 m / 17.2 मीटर
4️⃣ Which frequency represents ultrasonic sound? / कौन-सी आवृत्ति पराश्रव्य ध्वनि को दर्शाती है?
🔘 (a) 100 Hz 🔘 (b) 1000 Hz 🔘 (c) 20,000 Hz 🔘 (d) 40,000 Hz
✅ Answer: (d) 40,000 Hz / 40,000 हर्ट्ज

❓ FAQs – Frequently Asked Questions (Bilingual, Paraphrased)
🔹 Q1. Can sound exist in a vacuum? / क्या निर्वात में ध्वनि चल सकती है?
✅ No, it needs a medium to propagate.
✅ नहीं, ध्वनि को फैलने के लिए किसी माध्यम की आवश्यकता होती है।
🔹 Q2. What generates sound? / ध्वनि किससे उत्पन्न होती है?
✅ Sound is produced when objects vibrate.
✅ वस्तुओं के कंपन से ध्वनि उत्पन्न होती है।
🔹 Q3. What is the function of SONAR? / SONAR का कार्य क्या है?
✅ SONAR is used to find underwater objects using sound waves.
✅ सोनार का उपयोग जल के नीचे वस्तुओं का पता लगाने के लिए किया जाता है।


Written By
Full Stack Developer and 5-Time World Record Holder, Grandmaster Bikram Sutradhar
bAstronautWay
SirBikramSutradhar on YouTube
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CBSE Class 10 Mathematics NCERT Syllabus (2025) – Chapter-Wise List
chapter 4. Quadratic Equations
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CBSE Class 10 Science (2025) NCERT Syllabus:
CBSE Class 10 Science NCERT Syllabus (2025) – Chapter-Wise List
CBSE Class 10 Science NCERT Syllabus (2025) – Chapter-Wise List
Chapter 1 Chemical Reactions and Equations
Chapter 2 Acids, Bases and Salts
Chapter 3 Metals and Non-metals
Chapter 4 Carbon and its Compounds
Chapter 5 Life Processes
Chapter 6 Control and Coordination
Chapter 7 How do Organisms Reproduce?
Chapter 8 Heredity
Chapter 9 Light – Reflection and Refraction
Chapter 10 The Human Eye and the Colourful World
Chapter 11 Electricity
Chapter 12 Magnetic Effects of Electric Current
Chapter 13 Our Environment
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Table of Contents:
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📌 Exercise 12 MISCELLANEOUS – Focus Areas:
🧬 Consolidated practice from the entire Organic Chemistry chapter
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🧾 Writing structural formulas and chemical equations
💡 Distinguishing reactions, isomer identification, and conversions
📘 Full-syllabus revision to boost conceptual clarity and board exam readiness
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