Chapter 5 — Acoustics: Long Answer Questions
Production of Sound
Sound is produced by the vibration of an object. When an object vibrates:
- It pushes the surrounding air molecules, creating regions of compression (high pressure)
- When it moves back, it creates regions of rarefaction (low pressure)
- These alternating compressions and rarefactions travel outward as a longitudinal wave
Examples:
- Vocal cords vibrate → voice
- Tuning fork prongs vibrate → sound
- Guitar string vibrates → musical note
- Loudspeaker cone vibrates → amplified sound
Propagation of Sound
Sound requires a medium (solid, liquid, or gas) to travel. It cannot travel through vacuum.
Speed of sound in different media:
| Medium | Speed of sound (approx.) |
|---|---|
| Air (20°C) | 343 m/s |
| Water | 1500 m/s |
| Steel | 5100 m/s |
Sound travels fastest in solids and slowest in gases.
Effect of temperature: Speed of sound in air increases by ~0.6 m/s for every 1°C rise in temperature.
Properties of Sound Waves
Reflection of Sound
Sound waves, like light waves, obey the laws of reflection: 1. The angle of incidence = angle of reflection (measured from the normal to the reflecting surface) 2. The incident sound, reflected sound, and normal all lie in the same plane
Hard, smooth surfaces (walls, cliffs, metal sheets) are good reflectors of sound. Soft, porous surfaces (cloth, foam) absorb sound.
Echo
An echo is the repetition of a sound caused by the reflection of sound waves from a distant surface.
Condition for hearing a distinct echo: The reflected sound must reach the ear at least 0.1 seconds after the original sound (because the human ear can distinguish two sounds separated by at least 0.1 s).
Minimum distance for echo: If speed of sound = v m/s, in 0.1 s sound travels = v × 0.1 m This distance is covered twice (to the wall and back): minimum distance = v × 0.1/2 = v/20
At 20°C (v = 340 m/s): Minimum distance = 340/20 = 17 m (approximately)
So the reflecting surface must be at least 17 m away from the source for a distinct echo to be heard.
Formula for distance calculation:
Where d = distance to reflecting surface, v = speed of sound, t = time between producing sound and hearing echo
Reverberation
Reverberation is the persistence of sound in an enclosed space due to multiple reflections from walls, ceiling, and floor — the reflected sounds arrive within 0.1 s of each other and are perceived as prolonged sound.
Optimum reverberation time: 1–2 seconds for music; shorter for speech.
Control: Carpets, cushioned seats, curtains, acoustic tiles, false ceilings absorb excess sound.
Principle
SONAR works on the principle of reflection of sound (echo): 1. A transducer (ultrasound transmitter-receiver) attached to the ship sends a pulse of ultrasound downward (or in any direction) 2. The ultrasound pulse travels through water, hits an object or the sea floor, and reflects back 3. The returning echo is detected by the receiver 4. The time interval between sending and receiving the echo is measured 5. Distance to the object is calculated:
Where v = speed of sound in water (~1500 m/s), t = time for echo to return
Worked Example
An ultrasound pulse is sent from a ship. The echo from the seabed returns in 4 seconds.
- d = 1500 × 4/2 = 3000 m (depth of ocean)
Applications of SONAR
| Application | Details |
|---|---|
| Depth sounding | Measuring depth of seas and oceans (echo sounding) |
| Detecting submarines | Naval warfare — locate enemy submarines |
| Detecting fish shoals | Fishing industry — locate large groups of fish |
| Seabed mapping | Creating detailed ocean floor maps |
| Detecting icebergs | Preventing ship collisions |
| Navigation | Guiding ships in shallow or hazardous waters |
Ultrasound
Ultrasound has frequency above 20,000 Hz (beyond human hearing range).
Properties that make it useful:
- Short wavelength → can detect very small objects
- High energy → can penetrate matter
- Can be directed as a narrow beam
- Reflected clearly from boundaries between different materials
Applications of Ultrasound
Echolocation
Echolocation is the biological use of sound echoes by animals to navigate, hunt, and detect objects.
Bats:
- Emit very high-frequency ultrasound pulses (20 kHz–200 kHz) through their mouth or nose
- Detect the echoes with their large ears
- Can determine direction, distance, size, and speed of insects and obstacles
- Fly in complete darkness without collision
- Their brains process echo information in real-time
Other animals using echolocation:
- Dolphins and whales — underwater echolocation
- Cave-dwelling birds (oilbirds, swiftlets) — navigate in dark caves
- Some blind humans — have learned to echolocation by clicking their tongue
Inspiration for technology: SONAR and ultrasound detection systems are inspired by bat echolocation.
Classification of Sound by Frequency
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