Phy 202: General Physics II

1. Phy 202: General Physics II Ch 16: Waves & Sound Lecture Notes

2. What are waves? • Waves are traveling disturbances or vibrations • Waves carry energy from place to place • There are 2 types of waves: • Transverse (example: light, electromagnetic waves) • Disturbance is perpendicular to direction of wave propagation • Longitudinal (example: sound waves) • Disturbance is parallel to direction of wave propagation

3. Properties of Periodic Waves • All waves share 3 properties: • The propagation speed of the wave, vwave • The spacial length of a wave from crest to crest is called its wavelength, l • The rate (or frequency) of vibration describes how fast a wave oscillates, f • The propagation of a wave is related to its wavelength & its frequency: vwave = l.f • The speed of wave depends on the properties of the material (medium) where the wave travels. • For a wave on a string: • Where: • F = tension in the string • m = mass of the string • L = length of the string

4. Sound • Longitudinal wave • Produced by vibrations in a medium • The disturbance is the local change in pressure generated by the vibrating object • It travels because of the molecular interactions. • The region of increased pressure( compared to the undisturbed pressure) is called condensation • The region of lower pressure is called rarefaction. • The maximum increase in pressure is the amplitude of the pressure wave. (measurable) • Frequency of the sound 20Hz to-20000Hz. • Pressure waves below 20 Hz are called infrasonic waves • Pressure waves over 20,000Hz are called ultrasonic waves.

5. The Speed of Sound • Speed of sound depends on the compressive properties of the medium. • Because of the high frequencies, the compression/expansions are fast and no heat is exchanged (adiabatic). • Sound can travel in gases, liquids and solids. • The speed of sound in gases: • The speed of sound in liquids: • The speed of sound in solids:

6. Sound Intensity • Waves transport energy without transporting mass. The amount of energy transported per second is called the power of the wave (W) (power of the sound) • The power of the wave is determined by the source. The power is distributed (spreads) in all directions. Far away from the source, the power is spread over a greater area. • Intensity is a measure of power transmitted by a wave per unit area: I = P/A • The human does not sense sound intensity linearly but rather logarithmically • The decibel scale has been developed to “linearize” the human ear response to sound intensity b = (10 dB) . log (I/Io) where Io is the minimum intensity of sound the “average” human ear can detect

7. Doppler Effect • The frequency of the source producing the wave equals the number of cycles per second. • The frequency measured by an observer is the number of crests (condensations) encountered per second. • When both the source and the observer are at rest, the 2 frequencies are equal. • When one or both are in motion, the 2 frequencies are different. • The difference between source and observed frequency is called Doppler shift

8. Doppler Shift(Moving source, observer at rest) • The wavelength is changed because of the relative motion of source and observer Source moving toward observer Source moving away from the observer Notes: vs = speed of source v = speed of sound fs = frequency of source

9. Doppler Shift(Moving observer, source at rest) • The time between encountering 2 crests changes for observer Observer moving toward source Observer moving away from source Notes: vo = speed of observer v = speed of sound fs = frequency of source