Sound Waves
Longitudinal mechanical waves and auditory physics.
Sound is a mechanical longitudinal wave that propagates through compression and rarefaction of molecules in a physical medium.
This unit covers acoustic wave speed, intensity levels measured in decibels, the Doppler Effect of moving sources, and air column resonance in pipes.
Key Takeaways
- •Sound waves require a physical medium to travel; they cannot propagate through a vacuum.
- •The speed of sound depends on the elasticity and density of the medium (travels fastest in solids).
- •The Doppler Effect describes frequency shifts when a source or observer is in motion.
Core Concepts & Definitions
1Longitudinal Propagation
Sound waves vibrate parallel to the direction of wave travel, creating pressure pulses.
•High pressure zones are compressions; low pressure zones are rarefactions.
•Audible human range is approximately 20 Hz to 20,000 Hz.
2The Doppler Effect
The apparent change in frequency of a wave caused by relative motion between the source and observer.
•Apparent frequency increases as source/observer approach each other.
•Apparent frequency decreases as they recede.
Quick Revision Notes
- •The speed of sound in air increases with temperature: v ≈ 331.3 + 0.606 * T (°C) m/s.
- •Decibels (dB) measure sound intensity logarithmically.