Sound Waves Summary

Properties of Sound

  • Sound waves are longitudinal waves.

  • Tuning fork vibrations create compressions (high pressure) and rarefactions (low pressure).

Speed of Sound

  • Sound requires a medium (solid, liquid, gas) to travel.

  • Speed of sound in air depends on temperature: v_{sound} = 331 + 0.6T

  • Generally, use 343 m/s (sea level, 20°C).

  • Sound travels faster in solids and liquids than in gases.

Sound Loudness

  • Related to the amplitude of the sound wave.

  • Amplitude measures pressure variation.

  • Larger pressure variations are perceived as louder sounds.

Sound Intensity

  • Intensity is the power of sound per unit area (sound level).

  • Measured in decibels (dB).

  • Compared to the softest audible sound (0 dB).

  • A 10 dB increase is perceived as "twice as loud."

  • Changes in air pressure:

    • 2 times: +6 dB

    • 10 times: +20 dB

    • 100 times: +40 dB

Pitch

  • Pitch is associated with frequency.

  • Higher frequency = higher pitch.

Range of Hearing

  • Human range: 20 Hz - 20 kHz.

  • Most sensitive: 400 Hz - 7,000 Hz.

How We Hear

  1. Sound enters the ear.

  2. Eardrum vibrates

  3. Ear bones vibrate

  4. Fluid in the cochlea moves.

  5. Nerves carry energy to the brain (frequency).

  6. Brain interprets these signals as sound.

How We Make Sound

  • Air from the lungs flows through the windpipe to the voice box.

  • Air pushes vocal cords apart, causing vibration.

  • Vibrations create sound waves that exit through the mouth.

  • Vocal cord shape changes sound and pitch.

Out of Range Frequencies

  • Infrasonic: < 20 Hz (felt, not heard).

    • Examples: sub-woofers, thunderstorms, earthquakes, elephants.

  • Ultrasonic: > 20 kHz.

    • Examples: sonar, medical ultrasounds, bats, whales, dolphins.

  • Echolocation: Uses echoes to determine distance.

Doppler Effect

  • Apparent change in frequency due to relative movement.

  • Approaching source: higher pitch.

  • Receding source: lower pitch.

  • f' = f \frac{v \pm v{observer}}{v \pm v{source}}

Sonic Boom

  • Occurs when the source moves at the speed of sound or faster.

  • Sound waves pile up into a shock wave.

  • Supersonic: faster than sound.

  • Subsonic: slower than sound.

Mach Travel

  • Mach speeds are at or above the speed of sound.

  • Mach 1 = speed of sound.

  • M = \frac{v}{v_{sound}}