Sound
Physics · ध्वनि
📋Quick Overview
Sound is a form of energy produced by vibrating objects. It travels as longitudinal waves through a medium (solid, liquid, or gas). Sound CANNOT travel through vacuum — that's why there is no sound in space! Sound needs a material medium to travel. Speed of sound depends on the medium: fastest in solids, slower in liquids, slowest in gases.
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Speed of sound: In air ≈ 340 m/s (at 20°C), In water ≈ 1500 m/s, In steel ≈ 5000 m/s. Order: Solid > Liquid > Gas.
📖Properties of Sound
| Property | Depends On | Explanation | Unit |
|---|---|---|---|
| Pitch | Frequency | High frequency = high pitch (women/children voice), Low frequency = low pitch (men/lion) | Hertz (Hz) |
| Loudness | Amplitude | Larger amplitude = louder sound; measured in decibels (dB) | Decibel (dB) |
| Quality/Timbre | Waveform (harmonics) | Why sitar sounds different from guitar at same note — overtones differ | — |
📝Echo, SONAR & Ultrasound
- •ECHO: Reflected sound heard again. Minimum distance for echo = 17.2 meters (so sound travels 34.4m total in 0.1 second). Persistence of hearing = 0.1 second.
- •Human hearing range: 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz = Infrasonic. Sounds above 20 kHz = Ultrasonic.
- •SONAR (Sound Navigation And Ranging): Uses ultrasonic waves to find depth of ocean and detect underwater objects (submarines, fish). Used by ships and navy.
- •Ultrasound uses: Medical scanning (sonography/pregnancy check), cleaning teeth/jewelry, detecting cracks in metals, SONAR, kidney stone treatment
- •Doppler Effect: When a source of sound moves towards you, frequency appears to increase (higher pitch). When it moves away, frequency decreases (lower pitch). Example: Ambulance siren sounds different when coming vs going.
- •Decibel scale: Whisper ≈ 20 dB, Normal talk ≈ 60 dB, Traffic ≈ 80 dB, Rock concert ≈ 120 dB. Above 80 dB = noise pollution. Above 120 dB = painful.
📝Sonic Boom & Speed Categories
- •Subsonic: Speed < speed of sound (most vehicles, commercial planes)
- •Sonic: Speed = speed of sound (Mach 1 = 340 m/s in air). Creates SONIC BOOM (loud explosion-like sound)
- •Supersonic: Speed > speed of sound (fighter jets, Concorde). Creates shock waves.
- •Mach number = Speed of object / Speed of sound. Mach 2 = twice the speed of sound.