Monday, June 6,
2011
Announcements:
- Solutions to the multiple-choice section of Exam 2 are
now posted, as is a solution to the flywheel car problem; see class
notes from last Thursday.
- We are devoting more time and attention in class to
Chapters 13, 14, and 15, and less emphasis on Chapters 16, 17, and 18
because students typically find these first three chapters more
challenging than the last three (partly because of prior exposure to
thermodynamics in chemistry or physical science classes). However,
each of these chapters will be represented equally on the exams.
- Optional Web
Projects are due on the last day of class (Friday, June 10).
- Even if you do not
publish a website, you can still receive extra credit (~same as an
original RWP) for reviewing two Web Projects according to the specified
guidelines.
Assignments:
Chapter 14:
Sound Waves
A wave
is a propagating disturbance.
In a transverse wave, individual particles move at
right angles to the direction of propagation.
In a longitudinal wave, individual particles move in
the same direction as the wave.
Sound waves are longitudinal waves of compressions and rarefactions that
propagate through air, liquids, or solids.
Speed of sound in a medium = sqrt(elastic property/inertial property)
speed of sound in air = (331 m/s)*sqrt(T/273)
From this equation, we can see that the speed of sound in air increases
with temperature.
The frequency of a sound wave determines its pitch. Humans can
generally hear sounds in the range of 20 Hz to 20 kHz.
Sound waves propagate outward in all directions, so the intensity decreases
with distance I ~ 1/r^2
Intensity = Power/Area
Intensity level of sound (loudness) is measured in decibels (0 dB = threshhold of hearing, 160 dB = instant perforation of
eardrum)
Sound loudness is measured using logarithmic units since the range of intensity
that humans can hear spans 16 orders of magnitude.
A sound that seems twice as loud must have an intensity that is 10 times greater, or an increase in 10 dB.
dB = 10*log(I/Io), where Io = 1e-12 W/m^2 is the
Threshold of Hearing (TOH)
Other logarithmic units: Earthquake magnitudes
(Richter), pH, Stellar Magnitudes.
The change in frequency due to the relative motion between a source and
receiver is called the Doppler effect.
Waves that combine can interfere constructively when they are in
phase or destructively when they are out of phase. This adding of
waves is called superposition.
Standing waves result from sustained reflection between fixed endpoints
and are an example of resonance and interference.
Beats occur when waves of slightly different frequencies interfere,
producing a combined sound wave that varies in loudness according to the beat
frequency: f = |f1 - f2|
Ponderables:
- If you see a flash of lightning and hear the thunder 5
seconds later, how far away was the lightning strike?
- How much more intense is a 60 dB sound compared to a 40
dB sound?
- How much louder are two twins yelling compared to one?
Note: Doubling the sound intensity increases the intensity
level by 3 dB.
- How does the speed of sound depend on frequency?
- How does the speed of sound depend on the density of
the medium through which the sound travels?
- Does the speed of sound increase or decrease when the
air temperature rises?
- Which travels farther in air (and is therefore
attenuated less): high frequency or low frequency sounds?
- How can the sound barrier be broken in this classroon?
- How can the speed of an F-14
be determined by viewing the shock wave?
- Doppler effect: An
electronic buzzer emits a constant tone. Why does the pitch vary
when the buzzer is twirled horizontally but not vertically?
- Resonance in a half-closed tube occurs for a 256 Hz
(middle C) tuning fork when L = 32 cm. Why? What is the shortest
length of tube that will produce resonance for a 480 Hz tuning fork?
What is the next longest length of tube that will support a standing wave
with this same tuning fork?
- Why are car radio antennas about 3/4 of a meter long?
- Whirly tube: Why does the tube produce distinctly
different tones when twirled slow and then fast? What is the
mathematical relationship between these tones? How can the frequency
of the tones be determined by knowing that the tube is 80 cm long?
- When you blow over a bottle, it makes a tone with a
pitch that increases as the level of the liquid in the bottle rises.
However, the opposite is true for a glass of water that is rubbed or
struck. Why?
Demos
- Slinky and spring to show longitudinal and transverse
wave motion
- Whip crack
- Speakers to show beats and interference
- Whirly tube
- Water resonance tube
- Doppler buzzer
- Vibrating string - v, f, lamda,
tension, L, harmonics, mu
- Beats with tuning forks
- Sympathetic vibrations and how the ear works
- Breaking a wine glass with resonance
- http://www.youtube.com/watch?v=PMg_nd-O688