# Consider three sound waves and The Walker School

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```					                        Warm Up
Wave A

Consider three sound waves: A, B, and C.

Pressure
• Wave B has a higher pitch than the other
two.
• Wave C has the softest volume.                        Wave B
• Wave A and C have the same pitch.

Pressure
• Wave A and B have the same volume.
Accurately sketch the three sound waves.

Wave C
Challenge:
Pressure
• Draw Waves A and C so that they
interfere destructively.
Videos
•   Fire Truck
•   Big Bang Theory
•   Breaking Sound Barrier
•   Sonic boom animation
Discussion Question 1
As a fire truck approaches a listener, what
happens to its sound waves, and what will you
hear?
a.   Waves spread apart; Pitch goes down
b.   Waves spread apart; Volume goes down
c.   Waves get closer; Pitch goes up
d.   Waves get closer; Volume goes up
Discussion Question 2
A fire alarm on the wall suddenly starts ringing. Is
this an example of the Doppler effect?

Challenge: Would the sound change if you
panicked, and started running towards the alarm?
Discussion Question 4
•     Two fire trucks have sirens that emit waves of the same
frequency. As the fire trucks approach a person, the
person hears a higher frequency from truck X than from
truck Y. Which of the following statements about truck X
can be correctly inferred from this information
I. It is traveling faster than truck Y
II. It is close to the person than the truck Y
III. It is speeding up, and truck Y is slowing down.
(A)   I only
(B)   III only
(C)   I and II only
(D)   II and III only
(E)   I, II and III
Discussion Question 3
Doppler Effect
• Refers to the change in pitch
of sound due to the motion of
the source or the listener
• If source and listener moving
towards each other, then the
pitch is _______.
• If the source and listener
moving away from each other,
the pitch is _______.
• Physlet:
http://www.astro.ubc.ca/~schar
ein/a311/Sim/doppler/Doppler.
html
Mach 1
• Sound barrier broken
1947 by Chuck
Yeager.
• “Demon in the sky.”
• Today, NASA space
shuttle Mach 25
F-18 at Mach = 1.4, altitude = 35,000 ft, NASA Dryden, pilot: Ed
Schneider.
T-38 at Mach = 1.1, altitude = 13,700 ft, NASA Wallops.
• Object creating
Sonic Boom     disturbance travels
faster than speed of
sound.
• Multiple overlapping
crests = constructive
interference
Discussion Question 4
The four figures below represent sound waves emitted by a
source. Which picture(s) represent(s) a source moving at
less than the speed of sound?

E) more than 1
Discussion Question 5
• Did Chuck Yeager hear the Doppler effect from
the X-1’s engine when he exceeded mach 1?
• Did he hear a sonic boom?

Doppler   Boom
(A)    yes      no
(B)    yes      yes
(C)    no        no
(D)    no       yes
Demo: Drive-by
Prepare to go outside.

```
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