1. Reflection Superposition Interference
Wave Behavior
Reflection
Superposition
Interference

2. Reflection
Waves are reflected when they encounter a boundary of the medium
Example: Rope tied to a wall - Reflection of Waves from Boundaries
free boundary  Rope can move up and down
fixed boundary  Rope cannot move at all
At a free boundary, waves are reflected upright
At a fixed boundary, waves are reflected inverted

4. Superposition
Two waves can occupy the same medium at the same time (called superposition)
They affect the medium independently
The resultant wave is the sum of the displacements of the individual waves

5. Interference
When two waves approach each other from opposite directions they combine in a way called interference
When the displacements are in the same direction, constructive interference occurs
When the displacements are in opposite directions, destructive interference occurs
Transverse Waves-2

6. Sound

7. Sound Waves Sound waves are created by something vibrating:
A sound wave is simply a pressure variation that is transmitted through matter
Compression is where density and pressure are at a maximum (crest)
Rarefaction is where density and pressure are at a minimum (trough)
vibrating surface
pressure waves

8. Sound Waves (cont.) Sound waves are longitudinal waves
Sound waves usually propagate in three dimensions
They create spherical wave fronts

9. Sound Waves (cont.)
If you graph air pressure versus time, it looks like this:

10. Sound Waves (cont.) The speed of sound depends on the medium
Air (25C): 346 m/s
Water: 1490 m/s
Sound waves cannot travel through a vacuum

11. Sound Waves (cont.) The speed of sound in air depends on temperature.
The speed of sound in air at sea level at room temperature (20°C) is 343 m/s.
Example:
If you are 100 m from the crack of a bat striking a baseball, how long before you hear it?
t = d/v = (100 m)/(343 m/s) = 0.29 s

12. Sound Waves (cont.)
The loudness of sound is determined by the amplitude of the pressure wave.
The pitch of sound is determined by the frequency of the wave.
Humans can hear sounds between 20 Hz and 20,000 Hz (audible sound)
Less than 20 Hz is called infrasound
Greater than 20,000 Hz is called ultrasound

13. Beats Sound waves at slightly different frequencies produce beats
The beat frequency is fbeat = |f1  f2|

14. Sound Intensity
Intensity (I) is the rate of energy flow through a given area
Intensity = Power/Area
Intensity has units of W/m2
For a spherical wave:
I = 
where P = power output of the source
r = distance from the source P
4 r2

15. Relative Intensity: Decibels
The sensation of loudness is logarithmic with respect to intensity
The decibel (dB) is a measure of the relative loudness of sound
The reference intensity is the threshold of hearing, I0 = 1.0  W/m2
Decibel level (dB) = 10 log[I/I0]
0 dB is the threshold of hearing
120 dB is the threshold of pain

16. Doppler Shift
Doppler shift is an apparent change in frequency caused by motion of the source relative to the receiver.
When source and receiver are moving toward each other the sound has a higher frequency
When source and receiver are moving away from each other the sound has a lower frequency.

17. Doppler Shift (cont.) Doppler shift is used to detect speed.
Bats use the Doppler shift of sound to detect the speed of flying insects.
Radar guns use the Doppler shift of radio waves to detect how fast someone is driving.
Astronomers use the Doppler shift of light from distant galaxies to measure their speed and infer their distance.

18. Resonance
All structures (strings, buildings, bridges, columns of air) have natural frequencies
The natural frequencies correspond to standing waves in the structure
The smallest natural frequency is called the fundamental frequency
Resonance occurs when a periodic force acts on a structure at or near one of its natural frequencies
A resonating structure can have very large oscillations and even break (Tacoma Narrows Bridge)

19. Standing Waves
Standing waves form when a periodic wave interferes with its own reflection
Standing waves form at only certain frequencies
Nodes form where the displacement of the medium is zero
Antinodes form where the displacement of the medium is maximum

21. Harmonics
Harmonics are natural frequencies that are multiples of the fundamental frequency
Each harmonic corresponds to a standing wave
Example: vibrating string
fn = n  n = 1, 2, 3, …
Harmonics account for sound quality, or timbre v 2L

23. Standing Waves in an Air Column
Standing waves can form in a tube of air
If both ends are open, all harmonics are present
fn = n  n = 1, 2, 3, …
If one end is closed, only odd harmonics are present
fn = n  n = 1, 3, 5,… v 2L v 4L