1. Production of Sound
- Definition: Sound is a form of energy causing hearing sensation, produced by vibrations (rapid to-and-fro motion).
- Examples: Vocal cords (human voice), bird wings (buzzing), plucked rubber band.
- Activities:
- 11.1: Strike tuning fork (Fig. 11.1); hear sound, touch prong (vibration stops), touch ball (ball moves due to vibration).
- 11.2: Tuning fork prong touches water surface (Fig. 11.2) or dips in water (Fig. 11.3); creates ripples due to vibrations.
- 11.3: List musical instruments (e.g., guitar: strings; flute: air column); identify vibrating parts.
- Conclusion: Sound requires a vibrating object; no vibration, no sound.
Activity: Pluck a rubber band to hear sound!
2. Propagation of Sound
- Medium: Sound travels through solids, liquids, or gases; particles vibrate but don’t travel.
- Mechanism: Vibrating object displaces medium particles, creating compressions (C, high pressure) and rarefactions (R, low pressure) (Fig. 11.4).
- Sound Waves: Mechanical waves; energy travels as a disturbance, not particles.
- Questions:
- 1. Sound reaches ear via medium particle vibrations creating compressions/rarefactions.
- 2. School bell: Striking causes bell to vibrate, producing sound waves.
- 3. Mechanical waves: Require a medium (unlike light).
- 4. On Moon: No sound (no air medium).
Activity: Tap a table to feel vibrations!
3. Characteristics of Sound
- Longitudinal Waves:
- Activity 11.4: Push slinky (Fig. 11.5); particles oscillate parallel to wave direction, forming compressions/rarefactions.
- Sound waves: Particles move parallel to propagation (unlike transverse waves, e.g., water ripples).
- Properties (Fig. 11.6):
- Wavelength (\( \lambda \)): Distance between consecutive compressions/rarefactions (m).
- Frequency (\( v \)): Oscillations per second (Hz); determines pitch (Fig. 11.7).
- Time Period (\( T \)): Time for one oscillation (s); \( v = \frac{1}{T} \).
- Amplitude (A): Maximum disturbance; determines loudness (Fig. 11.8).
- Speed (\( v \)): \( v = \lambda v \); constant for all frequencies in a medium.
- Other Properties:
- Pitch: Brain’s interpretation of frequency; higher frequency = higher pitch.
- Loudness: Depends on amplitude; higher amplitude = louder sound.
- Quality/Timbre: Distinguishes sounds of same pitch/loudness; tone (single frequency) vs. note (multiple frequencies).
- Intensity: Sound energy per unit area per second; loudness is ear’s response to intensity.
- Questions:
- 1. (a) Loudness: Amplitude; (b) Pitch: Frequency.
- 2. Guitar: Higher pitch (higher frequency than car horn).
- 3. Wavelength, frequency, time period, amplitude: Defined above.
- 4. Speed: \( v = \lambda v \); \( \lambda = \frac{v}{v} = \frac{440}{220} = 2 \, \text{m} \).
- 5. Time interval: \( T = \frac{1}{v} = \frac{1}{500} = 0.002 \, \text{s} \).
- 6. Loudness: Ear’s response to intensity; Intensity: Energy/area/time.
- Example 11.1: \( v = 2000 \, \text{Hz}, \lambda = 0.35 \, \text{m}, d = 1500 \, \text{m} \); \( v = \lambda v = 700 \, \text{m/s} \); time = \( \frac{1500}{700} = 2.1 \, \text{s} \).
Activity: Compare high and low pitch sounds!
4. Reflection of Sound
- Law of Reflection: Incident and reflected angles equal to normal, in same plane (Activity 11.5, Fig. 11.9).
- Echo: Reflected sound heard after 0.1 s; minimum distance = \( \frac{344 \times 0.1}{2} = 17.2 \, \text{m} \).
- Reverberation: Repeated reflections causing sound persistence; reduced by sound-absorbent materials (e.g., fibreboard, draperies).
- Uses of Multiple Reflections:
- Megaphones/horns: Direct sound forward (Fig. 11.10).
- Stethoscope: Heart/lung sounds via multiple reflections (Fig. 11.11).
- Curved ceilings/soundboards: Spread sound evenly in halls (Figs. 11.12, 11.13).
- Example 11.2: Echo after 2 s, \( v = 346 \, \text{m/s} \); distance = \( \frac{346 \times 2}{2} = 346 \, \text{m} \).
Activity: Shout near a wall to hear an echo!
5. Range of Hearing
- Audible Range: 20 Hz to 20 kHz for humans.
- Infrasound: < 20 Hz; produced by pendulums, rhinos, elephants; may warn animals of earthquakes.
- Ultrasound: > 20 kHz; used by bats, dolphins; moths detect bat ultrasound to escape.
- Hearing Aid: Microphone → electrical signals → amplifier → speaker for clear hearing.
- Questions:
- 1. Audible range: 20 Hz to 20 kHz.
- 2. (a) Infrasound: < 20 Hz; (b) Ultrasound: > 20 kHz.
Activity: Test your hearing range!
6. Applications of Ultrasound
- Properties: High-frequency waves travel in straight paths, even around obstacles.
- Uses:
- Cleaning: Ultrasonic waves detach dirt in solution (e.g., spiral tubes, electronics).
- Defect Detection: Ultrasound reflects from cracks in metal blocks (Fig. 11.14).
- Echocardiography: Images heart via reflected ultrasound.
- Ultrasonography: Images organs (e.g., liver, kidney) or foetus for abnormalities.
- Kidney Stones: Breaks stones into grains for urinary flushing.
Activity: Explore ultrasound in medicine!
7. Exercises
- Exercise Questions:
- 1. Sound: Energy from vibrations causing hearing; produced by vibrating objects (e.g., vocal cords).
- 2. Compressions/rarefactions: Vibrating object pushes air (compression), pulls back (rarefaction) (Fig. 11.4).
- 3. Longitudinal: Particles oscillate parallel to wave direction.
- 4. Quality/timbre: Distinguishes voice by unique frequency mix.
- 5. Thunder delay: Sound (344 m/s) slower than light (3 × 10^8 m/s).
- 6. Wavelengths: \( \lambda = \frac{v}{v} \); 20 Hz: \( \frac{344}{20} = 17.2 \, \text{m} \); 20 kHz: \( \frac{344}{20000} = 0.0172 \, \text{m} \).
- 7. Aluminium (6420 m/s), air (346 m/s); ratio: \( \frac{6420}{346} \approx 18.55 \).
- 8. 100 Hz: \( 100 \times 60 = 6000 \) vibrations/min.
- 9. Yes, sound follows reflection laws like light (equal angles, same plane).
- 10. Hotter day: Faster sound (e.g., 350 m/s); distance same, so echo still heard (time < 0.1 s).
- 11. Applications: Stethoscope (heart sounds), megaphones (directed sound).
- 12. Stone fall: \( t = \sqrt{\frac{2h}{g}} = \sqrt{\frac{2 \times 500}{10}} = 10 \, \text{s} \); sound return: \( \frac{500}{340} \approx 1.47 \, \text{s} \); total: \( 10 + 1.47 = 11.47 \, \text{s} \).
- 13. \( v = 339 \, \text{m/s}, \lambda = 0.015 \, \text{m} \); \( v = \frac{v}{\lambda} = \frac{339}{0.015} = 22600 \, \text{Hz} \); ultrasound, inaudible.
- 14. Reverberation: Sound persistence from reflections; reduced by absorbent materials (e.g., fibreboard).
- 15. Loudness: Ear’s response to sound; depends on amplitude.
- 16. Ultrasound cleaning: High-frequency waves detach dirt in solution.
- 17. Defect detection: Ultrasound reflects from cracks, detected by sensors (Fig. 11.14).
Activity: Solve a sound wave problem!
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