http://www.youtube.com/watch?v=vpxEmD0gu0Q – human echolocation
7.6 The Reflection of Sound Waves
Sound waves follow the same laws of reflection as light waves.
Recall some of those laws and attributes below:

Angle of incidence ___________ angle of reflection.

Concave/Converging
Echoes are reflected sound waves. The distance between the observer and the reflecting surface must
be at least 17 m for an echo to be heard. Bats, dolphins, and Orca whales all use echolocation to
navigate, hunt, or communicate.
Ultrasound imaging has many applications in the medical field. Waves reflect off tissues and
computerized images are created based on density.
Example 1:
Two mountain-climbers, part way up a mountain, clapped their hands and heard the echoes return from
the mountain face opposite them in 6.0s. If the air temperature was -5.00C, how far away was the
mountain face?
Example 2:
A ship sends out a pulse that returns from the floor of the sea in 3.42s. If the speed of sound through
salt water is 1531 m/s, what is the distance to the bottom of the sea?
Seatwork:
Read more about echolocation and ultrasound on pages 256-257
Answer page 257 #1-5, page 258 #1-3, 6
7.10 The Doppler Effect and Supersonic Travel
The Doppler Effect is noticed when source of sound approaches an observer and then moves away from
the observer.
List some examples of the Doppler Effect.
Draw a figure to explain the Doppler Effect.
The wavelengths in front of the source are becoming shorter than those behind.
Since v = λf , we can rearrange to get f = v/λ,which shows that frequency and wavelength are
inversely proportional. That means that if the wavelength gets smaller, the frequency gets larger and
vice versa.
Thus, if a source is moving, the waves in front of the source have a shorter wavelength and a higher
frequency. The waves behind have a longer wavelength and a lower frequency. That is the Doppler
Effect and is why a police siren approaching you sounds different than one going away from you.
http://www.youtube.com/watch?v=xbsJ1GNSC6g (at 0:50 – Doppler effect)
http://www.youtube.com/watch?v=Y5KaeCZ_AaY (big bang theory costume)
The following relationship shows how frequency can be calculated:
Where v is the speed of the sound in the medium, and vs is the speed of the source through the
medium. We use v + vs if the source is moving away from the observer and v – vs if the source is
approaching the observer.
This is in part how radar works. In the case of the police speed radars, the speed of a car can be
determined by comparing the frequency of the initial wave sent by the radar gun and the frequency of
the reflected wave from the car.
Similarly, meteorologists use radio waves to determine wind velocities during storms. The waves are
reflected off raindrops and the frequencies are measured and compared. This lets them know how fast
the raindrops are moving horizontally (due to the wind) and therefore the velocity of the wind.
Example 1:
A police car, traveling 100 km/h, passes a stationary pedestrian standing on the sidewalk. The police
siren has a frequency of 4000 Hz and the air temperature is 00C. What is the frequency of the sound
waves reaching the pedestrian as the car approaches and after it has passed the pedestrian?
Supersonic Travel
http://www.youtube.com/watch?v=-d9A2oq1N38 – Sonic boom
http://www.youtube.com/watch?v=GvtAElaDVz8 – sonic boom mythbusters
Subsonic speeds are speeds less than the speed of sound in air.
Supersonic speeds are speeds greater than the speed of sound in air.
Mach number is the ratio of the speed of the object to the speed of sound.
Example 2:
The speed of sound at sea level at 00C is 332 m/s , or about 1200 km/h. At an altitude of 10 km, the
speed of sound is about 1060 km/h. What is the Mach number of an aircraft flying at an altitude of 10
km with a speed of 1800 km/h?
Breaking the Sound Barrier

Read pages 270 – 272 and answer the following:
What is the Sound Barrier and how is it created?
What must an aircraft be capable of doing in order to break through the sound barrier?
Name an aircraft that can do this.
What is a sonic boom?
Seatwork/homework:
Page 269 #1,2 page 270 #3-5, page 272 #1-6