Sound
Production, Propagation and Properties of Sound
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Overview
Sound is a fundamental topic in upper primary physics that bridges everyday experience with core scientific principles. For GTET-2, this topic tests your understanding of how sound is produced, how it travels through different media, and its measurable properties like frequency, amplitude, and speed.
Questions typically focus on distinguishing between sound and light propagation, calculating speed or frequency relationships, and understanding practical applications like echoes and the human auditory range. Mastery of this topic requires clear conceptual understanding rather than complex calculations—most problems involve direct formula application or reasoning about medium-dependent behaviour.
This topic connects naturally to wave motion concepts and has practical relevance in teaching scenarios involving musical instruments, noise pollution, and hearing. Expect 2–4 questions combining both conceptual understanding and numerical problems.
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Key Concepts
- **Sound is a mechanical wave**: Unlike light, sound requires a material medium (solid, liquid, or gas) to travel. It cannot propagate through vacuum.
- **Sound is a longitudinal wave**: Particles of the medium vibrate parallel to the direction of wave propagation, creating alternate compressions (high pressure) and rarefactions (low pressure).
- **Vibration is essential for sound production**: Every sound source—tuning fork, vocal cords, drum membrane, guitar string—must vibrate to produce sound.
- **Speed depends on medium**: Sound travels fastest in solids, slower in liquids, and slowest in gases. This is because particles are closest in solids, allowing faster energy transfer.
- **Frequency determines pitch**: Higher frequency produces higher pitch (shriller sound); lower frequency produces lower pitch (grave sound).
- **Amplitude determines loudness**: Greater amplitude means louder sound; smaller amplitude means softer sound.
- **Human audible range is 20 Hz to 20,000 Hz**: Sounds below 20 Hz are infrasonic; sounds above 20,000 Hz are ultrasonic.
- **Echo requires minimum distance**: Sound must travel a minimum distance (about 17 metres from the reflecting surface) for the human ear to perceive echo separately from the original sound.
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Formulas / Key Facts
| Formula/Fact | Context | |--------------|---------| | **v = f × λ** | Speed (v) equals frequency (f) times wavelength (λ) | | **Speed in air ≈ 340 m/s** | At room temperature (around 25°C) | | **Speed in water ≈ 1500 m/s** | Sound travels ~4 times faster in water than air | | **Speed in steel ≈ 5000 m/s** | Solids transmit sound fastest | | **Time period (T) = 1/f** | T is time for one complete vibration | | **Echo condition: d ≥ 17 m** | Minimum distance to reflecting surface for audible echo | | **Persistence of hearing = 0.1 s** | Minimum time gap for ear to distinguish two sounds | | **Loudness unit: decibel (dB)** | Normal conversation ~60 dB; painful threshold ~120 dB | | **Frequency unit: hertz (Hz)** | 1 Hz = 1 vibration per second |
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Worked Examples
### Example 1: Basic Wave Equation **Problem**: A sound wave has frequency 256 Hz and wavelength 1.3 m. Find the speed of sound.
**Solution**:
- Given: f = 256 Hz, λ = 1.3 m
- Using v = f × λ
- v = 256 × 1.3 = 332.8 m/s
- **Answer**: Speed of sound ≈ 333 m/s
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### Example 2: Echo Calculation **Problem**: A person claps near a cliff and hears the echo after 4 seconds. If speed of sound is 340 m/s, find the distance to the cliff.
**Solution**:
- Total distance travelled by sound = v × t = 340 × 4 = 1360 m
- Sound travels to cliff and back, so total distance = 2d
- Therefore, d = 1360 ÷ 2 = 680 m
- **Answer**: Distance to cliff = 680 m
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### Example 3: Frequency and Time Period **Problem**: A tuning fork completes 512 vibrations in 2 seconds. Find (a) frequency and (b) time period.
**Solution**:
- (a) Frequency = Number of vibrations ÷ Time
- f = 512 ÷ 2 = 256 Hz
- (b) Time period T = 1/f = 1/256 = 0.0039 s ≈ 0.004 s
- **Answer**: Frequency = 256 Hz; Time period ≈ 0.004 s
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Common Mistakes
- **Thinking sound travels in vacuum** → Sound is a mechanical wave and absolutely requires a medium. Electric bells in vacuum chambers demonstrate this—the bell vibrates but no sound is heard.
- **Confusing frequency with amplitude** → Frequency affects pitch (high/low), while amplitude affects loudness (soft/loud). A high-frequency sound can still be soft if amplitude is small.
- **Forgetting the "2d" in echo problems** → Sound travels to the reflecting surface AND back. Total distance = 2 × distance to reflector. Many students calculate only one-way distance.
- **Assuming sound speed is constant everywhere** → Speed varies with medium AND temperature. Sound travels faster in hot air than cold air because molecules move faster.
- **Mixing up infrasonic and ultrasonic** → Infrasonic is below 20 Hz (elephants, earthquakes); ultrasonic is above 20,000 Hz (bats, dolphins, medical imaging). The prefix "infra" means below, "ultra" means beyond.
- **Thinking higher speed means higher pitch** → Speed depends on medium; pitch depends on frequency of the source. A 256 Hz sound has the same pitch whether in air or water, even though speeds differ.
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Quick Reference
- Sound needs medium; no sound in vacuum.
- Speed order: Solid > Liquid > Gas.
- v = f × λ (memorise this fundamental relationship).
- Human hearing: 20 Hz – 20,000 Hz.
- Echo needs minimum 17 m distance; use 2d in calculations.
- Pitch ↔ Frequency; Loudness ↔ Amplitude.