Reading Guide
for March 4

Chapter 20: Sound

pp. 380–381. Origin of Sound.   The key information in this section is the relationship of frequency to observed pitch.

pp. 381–382. Nature of Sound in Air.   Describes how air molecules move when sound travels.  The pulse depends on the interactions between molecules.  So, is sound a longitudinal or a transverse wave?

In the second paragraph on p. 382, the reference to Figure 20.3 is really to Figure 20.2, of course.  Later, figure 20.3 is discussed without reference.

pp. 382–383. Media That Transmit Sound.   Since solids and liquids have firmer connections between molecules than gases do, sound is transmitted faster and more efficiently in solids and liquids than in gases.  Sound does not travel at all in a vacuum!  (There is nothing there to oscillate.)

pp. 383–384. Speed of Sound in Air.   From this section, try to be able to identify the factors that affect the speed of sound in air and to rationalize why they have the effects they do.  Specifically, why do temperature and humidity have the effects they do?

p. 384. Reflection of Sound.   Skim.

pp. 385–386. Refraction of Sound.   All types of waves bend when they cross into substances where they have different speeds.  We will explore this in more detail in class, but it is important in understanding the physics of all types of waves.

p. 387. Energy in Sound Waves.   Skim.

p. 387. Forced Vibrations.   Most of the sound you hear from a string instrument is from air molecules set into vibration by the body of the instrument rather than from the strings!

p. 387–388. Natural Frequency.   Skim.  This is a lead-in to the next section.

pp. 388–389. Resonance.   Skim.  The basic idea is that applying a cyclic force to an object at its natural frequency causes the amplitude of its oscillation to steadily increase.  This property has uses and also poses problems.

pp. 389–391. Interference.   That sound waves combined can together be louder than the individual signals should be no surprise.  But they also can be quieter than the individual signals!

pp. 391–393. Beats.   Interference between sounds with exactly the same frequency can be louder or softer than the separate signals.  If their frequencies are close but slightly different, however, the combined signal varies between louder and softer.  Be able to answer:

• When is the combined sound louder?
• When  is the combined sound softer?
• What determines the frequency of the beats?

Reflection

Think about the various types of waves presented in this reading and in the one before it.  You know that waves travel through whatever medium they travel through, but why do they travel?  What forces and principles of physics make it happen?  (The answer is not the same for all types of waves.)