Beats/Wave Packets

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Beats/Wave Packets
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• ConcepTest 33
• A note of frequency f1 is played on a
• xylophone. A different note f2 is played
• on the xylophone at the same time. The
• frequencies of the two notes are close
• together, but not identical.
• What will you hear?
• 1. Nothing.
• One note, then the other note, in a repeating
  fashion.
• 3. Two distinct notes at the same time.
• 4. A single note that gets louder and softer,
  repeating.
• 5. Bachs Concerto in D minor.

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ConcepTest 34 Consider a pipe of length L
that is open at one end and closed at the other.
The pipe can support standing waves of sound.
The wavelength of the fundamental (also called
the first harmonic or the n 1 mode) is
sketched. What is the wavelength of the next
higher harmonic?
1. L/2 4. 4L/3
2. 2L/3 5. 3L/2
3. L 6. 2L
If this standing wave gives a note of frequency
68 Hz, what is the length of the pipe? The speed
of sound in air is 340 m/s.
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ConcepTest 35
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Sources of Phase Difference?
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ConcepTest 36 Two speakers that generate
(sound) waves with frequency f 170 Hz are
driven by the same amplifier, so each wave is in
phase at the source. The point P is 2 m from one
speaker, and 2.75 m from the other speaker.
Assume the speed of sound is 340 m/s.
a) What is the phase difference between the two
waves at the point P?
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Two speakers that generate (sound) waves with
frequency f 170 Hz are driven by the same
amplifier, so each wave is in phase at the
source. The point P is 2 m from one speaker, and
2.75 m from the other speaker. Assume the speed
of sound is 340 m/s.
b) Which of the following best represents the
phasor diagram for the superposition of the waves
at the point P?
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Two speakers that generate (sound) waves with
frequency f 170 Hz are driven by the same
amplifier, so each wave is in phase at the
source. The point P is 2 m from one speaker, and
2.75 m from the other speaker. Assume the speed
of sound is 340 m/s.
c) One of the speakers is disconnected. What
happens to the sound at the point P? (Put up
one card from each column.)