Chapter 16 Sound and Hearing

Goals for Chapter 16 To study many aspects and variations of sound waves To relate intensity and sound intensity level To consider standing sound waves To view interference as it manifests in sound To study and calculate beats To view the many applications of the Doppler effect To contemplate sound shock waves

Introduction Listening to an iPod or MP3 files is not different than listening to cassettes or 8-track tapes. The way the sound is generated changes in tiny ways, but the method of hearing has not changed.

Longitudinal waves show the sinusoidal pattern A motion like the pulses of a speaker cone will create compressions and rarefactions in a medium like air. After the pulse patterns are seen, a sinusoidal pattern may be traced.

Sound waves may be graphed several ways See Figure 16.3 for different ways to graph sound wave information. Refer to Example 16.1.

Sound waves may be graphed several ways II While reading Example 16.2, see Figure 16.4 below.

Different instruments give the same pitch different “favor” The same frequency, say middle c at 256 Hz, played on a piano, on a trumpet, on a clarinet, on a tuba … they will all be the same pitch but they will all sound different to the listener.

Opening values along a coiled tube will change the tone

Speed of sound in liquids and solids The speed of sound will increase with the density of the material. Refer to Table 16.1 at right for examples. Consider Example 16.3 and Figure 16.8 below. Example 16.4 gives one more perspective.

The speed of sound in air Sound will travel in air at roughly 340 m/s. An exact speed would change slightly with humidity, temperature, and nature of the atmosphere. It still means you need to drive far too fast for our interstate highways to break the sound barrier in a car. (It has been done on a very long salt lakebed in Utah but it’s over 700 miles per hour.)

Sound intensity The in amplitude term in our wave equation can be related to the sound intensity, but perception of the listener often complicates the physics (location, weather, voice, or sound) in question. Study Problem-Solving Strategy 16.1. Follow Examples 16.6, 16.7, and 16.8.

The logarithmic decibel scale of loudness Table 16.2 shows examples for common sounds.

The decibel scale for front-row concert seats or for songbirds Example 16.9 reflects human reaction to “very” loud music, explosions, or perhaps walking amidst jet planes on a runway. Follow Example 16.10 and consider a much quieter situation. Figure 16.11 sets this stage.

Standing sound waves and normal modes Experiments often done in a first physics course laboratory will use common materials to reveal standing sound waves in resonance.

Sound wave resonance depends on the instrument The waveform must match the resonant container (open at both ends, one end, clamped at both)? Conceptual Example 16.11 uses Figure 16.14 to consider loudspeakers. Figure 16.15 shows how changing the resonator will change the frequency.

Cross-sectional views help us visualize the wave Nodes and antinodes will line up so that nodes are found where the resonator is closed and antinodes at an open pipe. The cross-sectional view helps to see the pattern.

Cross-sectional views reveal harmonic waves II

Cross-sectional views reveal harmonic waves III

The speed of sound can be revealed by a resonant pipe The frequency, speed of sound, and wavelength are all used to measure normal modes in a pipe Follow Example 16.12. Figure 16.19 is another way to consider the sound in an organ pipe.

The tone from one instrument can transfer Musicians playing near one another often notice that an organ pipe can cause a guitar string to resonate. Consider Example 16.13.

Wave interference … destructive or constructive

Sounds playing on a speaker system can interfere Refer to Example 16.4. Figure 16.23 illustrates the situation.

Slightly mismatched frequencies cause audible “beats”

The Doppler Effect II—moving listener, moving source As the object making the sound moves or as the listener moves (or as they both move), the velocity of sound is shifted enough to change the pitch perceptively.

The Doppler Effect III—Examples to consider Problem-Solving Strategy 16.2 will help guide work on a Doppler problem. Consider Example 16.15 and Figure 16.29 to concentrate on wavelength. Consider Example 16.16 and Figure 16.30 to concentrate on frequencies. Consider Example 16.17 and Figure 16.31 to keep the source at rest and move the listener. Consider Example 16.18 and Figure 16.32 to move both the source and the listener.

A double Doppler shift Consider Example 16.19 and Figure 16.33 below to guide your work.

Very fast aircraft can outrun the sound they generate A “sonic boom” can be heard when an aircraft’s speed overcomes the sound it generates. Before Chuck Yeager’s flight, designers were not sure the plane would survive. See Figure 16.33 and Example 16.20.