Lesson 2
Announcements
 Waves HW #3 due in class (maybe check today?)
 Waves HW #2 due next Monday
AP Physics B Standards
IV.A.1. Traveling waves
Students should understand the description of traveling waves, so they can:
c) Understand qualitatively the Doppler effect for sound in order to explain why there is a frequency shift in both the moving‐source and moving‐observer case.
e) Describe qualitatively what factors determine the speed of waves on a string and the speed of sound.
Lesson Objectives
Students will be able to
1.
2.
predict the change in pitch of a sound depending on the motion of the source or the observer.
describe sound from supersonic sources.
Pure Sounds
 Sounds are longitudinal waves, but if we graph them right, we can make them look like transverse waves.
 When we graph the air motion involved in a pure sound tone versus position, we get what looks like a sine or cosine function.  A tuning fork produces a relatively pure tone. So does a human whistle.  Later in the period, we will sample various pure sounds and see what they “look” like.
Graphing a Sound Wave
http://frank.mtsu.edu/~wroberts/purecomp.html
http://frank.mtsu.edu/~wroberts/fourier_3.htm
Complex Sounds
 Most sounds in the real world do not resemble pure sine or cosine functions.
 Most real world sounds are composed of multiple
frequencies, or harmonics.
 These additional frequencies change the shape of the wave, but not the period. We call this complex shape the timbre.
BONUS Reading: http://www.asel.udel.edu/speech/tutorials/acoustics/time_domain.html
The Oscilloscope
With the Oscilloscope we can view waveforms in the “time domain”. Pure tones will resemble sine or cosine functions, and complex tones will show other repeating patterns that are formed from multiple sine and cosine functions added together.
The Fourier Transform
We will also view waveforms in the “frequency domain”. A mathematical technique called the Fourier Transform will separate a complex waveform into its component frequencies.
Introducing MacScope
 Let’s use the laptop and MacScope to examine the properties of sound.
Doppler Effect
 The Doppler Effect is the raising or lowering of the perceived pitch of a sound based on the relative motion of observer and source of the sound.  The speed of sound is constant, while the frequency and wavelength change.
Demo: Doppler Effect
Time for the Doppler ball!
 Can you think of examples from everyday life when you’ve noticed the Doppler effect for sound?
Stationary Source
http://www.kettering.edu/~drussell/Demos/doppler/mach1.mpg
Animations courtesy of Dr. Dan Russell, Kettering University
Doppler Shift
 As the source travels v
from left to right, the sound waves get bundled closer to one another, shortening the wavelength.  Remember that frequency is inversely proportional to wavelength.
Moving Source
http://www.kettering.edu/~drussell/Demos/doppler/mach1.mpg
Animations courtesy of Dr. Dan Russell, Kettering University
Doppler Shift
 If the source is moving TOWARD you, the wavelengths are shortened and you perceive a HIGHER pitch.
v
Doppler Shift
v
• If the source is moving AWAY from you, the wavelengths are lengthened and you perceive a LOWER pitch.
Supersonic
 If the source travels faster than the speed of sound, a CONE of sound forms.
v > 343 m/s
• The CONE is also called a SHOCKWAVE. • When it reaches your ears, it is called a SONIC BOOM.
Supersonic Source
http://www.kettering.edu/~drussell/Demos/doppler/mach1.mpg
Animations courtesy of Dr. Dan Russell, Kettering University
Comparison
v =  f
Stationary source
Moving source
Supersonic source
http://www.kettering.edu/~drussell/Demos/doppler/mach1.mpg
Animations courtesy of Dr. Dan Russell, Kettering University
Width of the Cone
 The faster the 2x
4x
jet or rocket, the narrower
the cone will be.
Video Clip
 F‐14 and F‐18 jets forming cones of water vapor