Sound
Production, Propagation and Properties of Sound
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Overview
Sound is a fundamental physics topic in the TS TET Paper II Mathematics and Science section. It connects everyday experiences—speech, music, echoes—to scientific principles, making it both conceptually rich and practically relevant for upper primary teaching.
For the exam, you must understand how sound is produced by vibrating objects, how it travels as a mechanical wave through different media, and what properties (frequency, amplitude, wavelength, speed) define its characteristics. Expect questions on distinguishing loudness from pitch, calculating echo distances, explaining why sound cannot travel in vacuum, and identifying applications like SONAR and stethoscopes.
Mastering this topic also helps in pedagogy questions where you may be asked how to demonstrate sound concepts through simple classroom activities.
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Key Concepts
- **Sound is produced by vibrations**: Any vibrating body (tuning fork, vocal cords, drum membrane, guitar string) produces sound. When vibration stops, sound stops.
- **Sound is a mechanical wave**: It requires a material medium (solid, liquid or gas) to travel. Sound cannot travel through vacuum—this is why astronauts use radios in space.
- **Sound is a longitudinal wave**: Particles of the medium vibrate parallel to the direction of wave propagation, creating compressions (high-pressure regions) and rarefactions (low-pressure regions).
- **Speed of sound depends on the medium**: Sound travels fastest in solids, slower in liquids, and slowest in gases. In air at 20°C, speed ≈ 344 m/s.
- **Frequency determines pitch**: Higher frequency → higher pitch (shrill sound like a whistle). Lower frequency → lower pitch (deep sound like a drum).
- **Amplitude determines loudness**: Greater amplitude → louder sound. Loudness is measured in decibels (dB).
- **Audible range for humans**: 20 Hz to 20,000 Hz. Below 20 Hz = infrasound. Above 20,000 Hz = ultrasound.
- **Echo and reverberation**: Echo is distinct repetition of sound due to reflection from a surface at least 17 metres away. Reverberation is repeated reflection causing prolonged sound.
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Formulas / Key Facts
| Quantity | Formula / Fact | |----------|----------------| | Wave equation | Speed (v) = Frequency (f) × Wavelength (λ) | | Speed in air (20°C) | ≈ 344 m/s (often rounded to 340 m/s in problems) | | Minimum distance for echo | d = 17 m (sound travels 34 m round trip in 0.1 s, the persistence of hearing) | | Echo distance calculation | 2d = v × t, so d = (v × t) / 2 | | Loudness unit | Decibel (dB) | | Frequency unit | Hertz (Hz) = cycles per second | | Time period | T = 1 / f | | Audible range | 20 Hz – 20,000 Hz | | Infrasound | < 20 Hz (elephants, earthquakes) | | Ultrasound | > 20,000 Hz (bats, dolphins, medical imaging) |
**Speed comparison**: Solids > Liquids > Gases Example: Steel ≈ 5000 m/s, Water ≈ 1500 m/s, Air ≈ 340 m/s
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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**: v = f × λ v = 256 × 1.3 v = 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 an echo after 3 seconds. If speed of sound is 340 m/s, find the distance to the cliff.
**Solution**: Total distance travelled by sound = v × t = 340 × 3 = 1020 m This is the round trip (to cliff and back). Distance to cliff = 1020 / 2 = 510 m **Answer**: The cliff is 510 metres away.
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### Example 3: Frequency and Time Period **Problem**: A tuning fork vibrates 512 times per second. Find its time period.
**Solution**: Frequency (f) = 512 Hz Time period (T) = 1 / f = 1 / 512 T = 0.00195 s ≈ 0.002 s (or about 2 milliseconds) **Answer**: Time period ≈ 0.002 seconds
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Common Mistakes
| Wrong Thinking | Correct Understanding | |----------------|----------------------| | "Sound can travel through vacuum because we hear sounds everywhere." | Sound needs a medium. The bell-jar experiment proves sound cannot travel through vacuum. | | "Louder sound means higher pitch." | Loudness (amplitude) and pitch (frequency) are independent. A loud drum is not high-pitched; a soft whistle can be high-pitched. | | "Echo occurs from any reflecting surface." | For a distinct echo, the reflecting surface must be at least 17 m away. Closer surfaces cause overlap (reverberation). | | "Speed of sound is same in all media." | Speed varies: fastest in solids, slowest in gases. Temperature also affects speed in air. | | "Ultrasound is harmful radiation." | Ultrasound is just high-frequency sound—safe enough for medical scans (sonography). It is mechanical, not electromagnetic radiation. |
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Quick Reference
- **Vibration → Sound**: No vibration = no sound.
- **Medium required**: Solid > Liquid > Gas for speed; vacuum = no transmission.
- **v = f × λ**: The universal wave equation.
- **Echo minimum**: 17 m distance (0.1 s persistence).
- **Pitch = Frequency; Loudness = Amplitude**: Don't confuse these.
- **Human hearing**: 20 Hz to 20,000 Hz; below = infrasound, above = ultrasound.
- **Applications**: SONAR (navigation), ultrasound (medical imaging), stethoscope (amplifies body sounds).