Sound Waves, Speed and Echo
~8 min read
- Nature: Longitudinal mechanical waves. Require a medium — cannot travel in vacuum.
- Speed: Air ~343 m/s at 20 °C. Water ~1480 m/s. Steel ~5960 m/s. Faster in denser elastic media.
- Echo: Reflected sound heard distinctly when the reflecting surface is at least 17 m away (so total delay ≥ 0.1 s).
Sound is a longitudinal pressure wave that needs a medium to travel. CDS/OTA tests its speed in different media, echo conditions, and applications like SONAR.
Nature of Sound
- Sound is a longitudinal wave — particles vibrate parallel to direction of propagation (compressions and rarefactions).
- It is a mechanical wave — requires a material medium. Cannot travel in vacuum.
- An electric bell in a sealed jar becomes inaudible as air is pumped out — classic demonstration.
- Wave equation: v = fλ.
Speed of Sound
| Medium | Speed (m/s) |
|---|---|
| Air (0 °C) | 331 |
| Air (20 °C) | 343 |
| Water | ~1480 |
| Sea water | ~1530 |
| Steel / Iron | ~5960 |
- In gases: speed ∝ √T (increases with temperature). About +0.6 m/s per °C.
- In a given gas, independent of pressure (Newton-Laplace formula).
- Solids > liquids > gases — because solids are denser AND more elastic.
Echo and Reverberation
- Echo: distinct repetition of sound after reflection. Persistence of hearing is 0.1 s, so total travel must take at least 0.1 s — minimum distance to reflector = (343 × 0.1)/2 ≈ 17.15 m.
- Reverberation: overlapping echoes that prolong the original sound — e.g. in large halls. Excess reverberation reduces clarity; reduced by curtains, carpets, perforated panels.
- SONAR (Sound Navigation and Ranging): ultrasonic pulse sent into water; time of echo measures distance to ocean bed, submarines, fish shoals.
CDS/OTA PYQ Examples
Q: Sound cannot travel through:
(a) Water (b) Steel (c) Air (d) Vacuum
Answer: (d) Vacuum — no material medium.
Q: Speed of sound is greatest in:
(a) Air (b) Water (c) Steel (d) Vacuum
Answer: (c) Steel — densest and most elastic.
Q: For an echo to be heard distinctly, the minimum distance from the reflecting surface is:
(a) 5 m (b) 10 m (c) 17 m (d) 50 m
Answer: (c) ~17 m, using v=343 m/s and persistence 0.1 s.
Q: SONAR uses:
(a) Infrasonic waves (b) Audible sound (c) Ultrasonic waves (d) Radio waves
Answer: (c) Ultrasonic waves — high frequency, directional.
Drill Sound Waves, Speed and Echo for CDS/OTA
CDS/OTA-pattern items on Sound Waves, Speed and Echo with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does sound travel faster in solids than in gases?
Solids are far more elastic; though they are also denser, the increase in elasticity outweighs the increase in density, raising v = √(E/ρ).
Why does sound travel faster on a hot day?
Speed in gases ∝ √(absolute temperature). Hot air molecules move faster, transmitting the wave more quickly.
How does SONAR estimate depth?
If echo returns after time t, depth = v×t/2, where v is sound speed in water (~1500 m/s).