Physics - Chapter 14

14.1 §

• Sound wave is formed when an oscillating object creates alternating regions of compressed and expanded air
• Speed of sound wave depends on what it moves through
• The speed of sound depends on the “stiffness” of the material it’s passing through
  • Stiffer material = faster sound wave
• Speed of sound is same for all directions of travel and for all frequencies
  • v = $\lambda$f
  • speed = wavelength x frequency
• Pitch - pitch of a sound is the frequency of the corresponding sound wave
• Infrasonic - Sound frequencies less than 20 Hz
• Ultrasonic - Sound frequencies greater than 20,000 Hz
• Beats - Result of two waves interfering with one another
• Beat Frequency- Difference in frequency between two original waves
  • f${}_{beat}$ = | f${}_1$ - f${}_2$ |
  • SI Unit: 1/s = s${}^{-1}$

14.2 §

• Standing Wave in bottle must have a node at the bottom and an antinode at the top
• First harmonic/Fundamental Frequency = f${}_1$ = $\frac{v}{\lambda }$ = $\frac{v}{4L}$
• Second harmonic = 3 $\frac{v}{4L}$ = 3 f${}_1$
• Third harmonic = 5 $\frac{v}{4L}$ = 5 f${}_1$
• Standing Waves also form in pipes that open on both ends
  • They must have an antinode at each end of the pipe

14.3 §

• Doppler Effect - Source of sound moves toward an observer and the frequency heard is higher than the frequency produced by the source
  • Occurs with all types of waves not just sound waves
  • Doppler effect is basically that waves will be stronger around the source
  • f${}_{observer}$ = $\frac{f_{source}}{1\pm \frac{v_{source}}{v_{sound}}}$
    • Doppler effect for a moving source
    • • for moving away from the observer
    • • for moving toward the observer
    • SI Unit: Hz = s${}^{-1}$
  • Observer of doppler effect hears higher frequency when approaching the source and a lower frequency when moving away from the source
  • f${}_{observer}$ = f${}_{source}$ (1 $\pm$ $\frac{v_{observer}}{v_{sound}}$ )
    • • for observer moving toward the source
    • • for observer moving away from the source
    • SI Unit: Hz = s${}^{-1}$

14.4 §

• Intensity - Amount of sound energy passing through a given area in a given time
  • intensity = $\frac{energy}{area\ast time}$
  • intensity = $\frac{power}{area}$
    • SI Units: $\frac{W}{m^2}$
• Intensity decreases with distance from the source
  • Sound spreads out over a larger area as it moves away from it’s source hence reducing the intensity
• Increasing by a factor of 10
• Decibel (dB) - Unit used to measure loudness (1/10th of a bel)
• Doubling loudness of sound increases intensity by factor of 10 corresponding to an increase of 10 dB
• Intensity Levels - Loudness of a sound given in decibels