📑 Topics on this page (Jump to Topic):
📌 1. Waves
| Type | Particle vibration | Examples |
|---|---|---|
| Transverse | Perpendicular to wave direction; crests and troughs | Light (all EM waves), wave on a string, water surface |
| Longitudinal | Parallel to wave direction; compressions and rarefactions | Sound in air, push on a spring, P-waves of earthquakes |
- v = f λ (speed = frequency × wavelength). Frequency unit hertz; period T = 1/f.
- Mechanical waves (sound) need a medium — sound cannot travel in a vacuum; astronauts on the Moon talk by radio.
📌 2. Speed & Range of Sound
- Speed of sound: solid > liquid > gas. In air ≈ 332 m/s (0 °C), ≈ 343 m/s (20 °C); in water ≈ 1480 m/s; in steel ≈ 5000+ m/s.
- The speed of sound in air increases with temperature and humidity; pressure (at constant temperature) has no effect.
- Lightning is seen before thunder is heard — light (3 × 10⁸ m/s) is far faster than sound.
- Mach number = object speed / speed of sound; Mach > 1 = supersonic, producing a sonic boom.
| Range | Frequency | Who hears / use |
|---|---|---|
| Infrasonic | < 20 Hz | Elephants, whales; waves before earthquakes |
| Audible | 20 Hz – 20 kHz | Humans |
| Ultrasonic | > 20 kHz | Bats, dolphins, dogs; SONAR, ultrasound scans, breaking kidney stones |
📌 3. Characteristics & Phenomena of Sound
| Property | Depends on | Meaning |
|---|---|---|
| Loudness | Amplitude | Measured in decibels (dB); above 80 dB is harmful |
| Pitch | Frequency | Women's and children's voices have higher pitch than men's |
| Quality (timbre) | Waveform / overtones | Sitar and flute are told apart even on the same note |
- For an echo, the reflecting surface must be at least ≈17 m away (sound persists in the ear for 0.1 s).
- Reverberation: persistence of sound by repeated reflections — halls use curtains and sound absorbers.
- Resonance: applied frequency = natural frequency → maximum amplitude; soldiers break step while crossing a bridge.
- Doppler effect: an approaching source sounds higher in pitch, a receding one lower — train whistle; police radar speed checks.
- SONAR = Sound Navigation And Ranging — uses ultrasound to find sea depth and submarines. A stethoscope works by multiple reflection of sound.
⚠️ Exam Traps:
- Loudness depends on amplitude, pitch on frequency — do not swap them.
- Sound travels fastest in solids and not at all in a vacuum.
- Decibel measures sound intensity level, not frequency.
📌 4. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. In which medium is the speed of sound highest? Practice
Correct answer: (A)
Solid > liquid > gas; sound cannot travel in a vacuum.
Solid > liquid > gas; sound cannot travel in a vacuum.
Q2. The audible range of human hearing is: Practice
Correct answer: (B)
Below 20 Hz is infrasonic, above 20 kHz is ultrasonic.
Below 20 Hz is infrasonic, above 20 kHz is ultrasonic.
Q3. Bats navigate at night using: Practice
Correct answer: (D)
Ultrasound reflects off obstacles (echolocation).
Ultrasound reflects off obstacles (echolocation).
Q4. The pitch of a sound depends on its: Practice
Correct answer: (A)
Higher frequency = higher pitch.
Higher frequency = higher pitch.
Q5. Loudness of sound depends on: Practice
Correct answer: (D)
Larger amplitude = louder sound.
Larger amplitude = louder sound.
Q6. Sound waves in air are: Practice
Correct answer: (A)
Particles form compressions and rarefactions along the wave direction.
Particles form compressions and rarefactions along the wave direction.
Q7. Two astronauts on the Moon cannot hear each other directly because: Practice
Correct answer: (C)
Sound is a mechanical wave and needs a medium.
Sound is a mechanical wave and needs a medium.
Q8. Sound intensity level is measured in: Practice
Correct answer: (C)
Long exposure above 80 dB is harmful.
Long exposure above 80 dB is harmful.
Q9. For a distinct echo, the reflecting surface must be at least about: Practice
Correct answer: (C)
Sound persists 0.1 s; 343 × 0.1 / 2 ≈ 17 m.
Sound persists 0.1 s; 343 × 0.1 / 2 ≈ 17 m.
Q10. The pitch of an approaching train whistle seems to rise. This is the: Practice
Correct answer: (C)
Relative motion of source and listener changes the apparent frequency.
Relative motion of source and listener changes the apparent frequency.
Q11. Soldiers are told to break step while crossing a bridge to avoid: Practice
Correct answer: (A)
If step frequency matches the bridge's natural frequency, amplitude can grow dangerously.
If step frequency matches the bridge's natural frequency, amplitude can grow dangerously.
Q12. SONAR is used to: Practice
Correct answer: (D)
Ultrasonic pulses are sent and the echo time is measured.
Ultrasonic pulses are sent and the echo time is measured.
Q13. As temperature rises, the speed of sound in air: Practice
Correct answer: (A)
It rises by about 0.6 m/s per °C.
It rises by about 0.6 m/s per °C.
Q14. A wave has frequency 500 Hz and wavelength 0.68 m. Its speed is: Practice
Correct answer: (D)
v = fλ = 500 × 0.68 = 340 m/s.
v = fλ = 500 × 0.68 = 340 m/s.
Q15. A supersonic aircraft moves at: Practice
Correct answer: (B)
Mach > 1; this creates a sonic boom.
Mach > 1; this creates a sonic boom.
Q16. A sitar and a flute can be told apart on the same note because of: Practice
Correct answer: (D)
Different overtones give different waveforms.
Different overtones give different waveforms.
Q17. Thunder is heard 3 s after lightning (speed of sound 340 m/s). The cloud is about: Practice
Correct answer: (B)
Distance = 340 × 3 = 1020 m (light's time negligible).
Distance = 340 × 3 = 1020 m (light's time negligible).
Q18. Elephants and whales can communicate using: Practice
Correct answer: (C)
Frequencies below 20 Hz travel long distances.
Frequencies below 20 Hz travel long distances.
Q19. Seismic P-waves are: Practice
Correct answer: (B)
P-waves pass through solids, liquids and gases; S-waves are transverse and cannot pass through liquids.
P-waves pass through solids, liquids and gases; S-waves are transverse and cannot pass through liquids.
Q20. Why are sound-absorbing materials fixed on walls of large halls? Practice
Correct answer: (B)
Repeated reflections cause booming; absorbers prevent it.
Repeated reflections cause booming; absorbers prevent it.