Class 8 Science (CTET Paper 2)Critical (2–3 MCQs guaranteed)8 min read
Chapter 13: Sound and Acoustic Frequency
ध्वनि (कंपन, आयाम, आवृत्ति, तारत्व, प्रबलता एवं श्रव्य परास)
CTET Theme: Acoustics, Mechanical Waves & Auditory Biology
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OVERVIEW1 of 9
Introduction: Sound and Acoustic Frequency
Welcome to Mentora CTET Audio Masterclass for Class 8 Science (CTET Paper 2), Chapter 13: Sound and Acoustic Frequency. Here are the high-yield facts tested in CBSE examinations.
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High-Weightage NCERT Facts & Key Terms (द्विभाषी)
5 Core Points- Sound Requires a Material Medium: Sound waves are mechanical pressure waves; they propagate through solids, liquids, and gases, but CANNOT travel in a vacuum (निर्वात). Sound travels fastest in Solids > Liquids > Gases.
- Vibration (कंपन): The rapid to-and-fro motion of an object produces sound (vocal cords vibrating in the human larynx/voice box).
- Amplitude and Loudness: Loudness is directly proportional to the SQUARE of the amplitude (Loudness ∝ Amplitude²). Measured in Decibels (dB). Above 80 dB, sound becomes physically painful noise.
- Frequency and Pitch (तारत्व): Frequency is the number of oscillations per second, measured in Hertz (Hz). Higher frequency = Higher Pitch / Shrill sound (bird chirping, female voice). Lower frequency = Deeper grave sound (drum beats, male voice).
- Human Audible Range: Frequencies between **20 Hz and 20,000 Hz (20 kHz)**. Infrasonic sound < 20 Hz (whales, elephants, earthquakes); Ultrasonic sound > 20,000 Hz (bats, dolphins, medical ultrasound scanners).
CBSE Distractor Traps (Avoid Negative Marking)
- Trap: What happens to loudness if amplitude is doubled? Correct: Loudness becomes **4 times greater** ($2^2 = 4$).
- Trap: What determines the shrillness (pitch) of a voice? Correct: **Frequency (आवृत्ति)**, NOT amplitude.
- Trap: Can sound travel in a vacuum? NO. Sound requires a material medium.
NCERT Core Facts: Wave Mechanics, Pitch vs. Loudness & Ultrasound
Class 8 Chapter 13 Sound provides classic analytical questions in CTET Paper 2 Science:
1. Differentiating Loudness from Pitch (The Critical Distinction)
| Acoustic Parameter | Physical Determinant | Unit | Perceptual Result on Human Ear |
|---|---|---|---|
| Loudness (प्रबलता) | Amplitude (आयाम) of vibration | Decibels (dB) | Large amplitude = Loud roaring sound; Small amplitude = Faint feeble sound. (\text{Loudness} \propto \text{Amplitude}^2) |
| Pitch / Shrillness (तारत्व) | Frequency (आवृत्ति) of vibration | Hertz (Hz) | High frequency = Shrill sharp voice (Women, children, whistles); Low frequency = Grave bass sound (Drums, men). |
2. Speed of Sound Across Physical Media
\text{Speed in Solids (Steel/Iron)} > \text{Speed in Liquids (Water)} > \text{Speed in Gases (Air)} > \text{Vacuum} = 0
- In air at room temperature: ≈ 340\text{ m/s}.
- In water: ≈ 1,500\text{ m/s}.
- In steel: ≈ 5,000\text{ m/s}.
- In a vacuum: Sound cannot travel at all because there are no material particles to oscillate.
3. Auditory Range Spectrum
- Infrasound (< 20 Hz): Produced by elephants, rhinos, and earthquake seismic waves.
- Audible Human Hearing (20 Hz – 20,000 Hz): Normal speech range is typically 500–3,000 Hz.
- Ultrasound (> 20,000 Hz): Utilized by Bats (चमगादड़) for echolocation navigation and by marine SONAR.
Official CTET Benchmark Test
Test Your Mastery: Sound and Acoustic Frequency
Authentic CBSE exam questions tested directly from this NCERT chapter.
Question 1 of 2(CTET 2021 & 2023)
If the amplitude of vibration of an acoustic instrument is doubled, how does its loudness change?
यदि किसी ध्वनि उत्पन्न करने वाली वस्तु के दोलन का आयाम दोगुना कर दिया जाए, तो उसकी प्रबलता में क्या परिवर्तन होगा?
Question 2 of 2(CTET Jan 2022)
The pitch or shrillness of a sound is determined exclusively by which wave characteristic?
किसी ध्वनि का तारत्व (Pitch) या तीखापन विशिष्ट रूप से किस तरंग विशेषता द्वारा निर्धारित होता है?
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Open Full OMR SimulatorFrequently Asked Questions (CTET Pedagogy)
If the amplitude of a vibrating body is tripled, by what factor does its loudness increase?
By 9 times, because loudness is proportional to the square of amplitude (3² = 9).
What is the normal audible frequency range of human hearing?
20 Hz to 20,000 Hz (20 kHz).