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chapter_14 - The characteristic sound of any instrument is...

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chapter_14 - The characteristic sound of any instrument is...

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458 14 CHAPTER Sound OUTLINE 14.1 Producing a Sound Wave 14.2 Characteristics of Sound Waves 14.3 The Speed of Sound 14.4 Energy and Intensity of Sound Waves 14.5 Spherical and Plane Waves 14.6 The Doppler Effect 14.7 Interference of Sound Waves 14.8 Standing Waves 14.9 Forced Vibrations and Resonance 14.10 Standing Waves in Air Columns 14.11 Beats 14.12 Quality of Sound 14.13 The Ear P atrick Ward/CORBIS Sound waves are the most important example of longitudinal waves. In this chapter we dis- cuss the characteristics of sound waves: how they are produced, what they are, and how they travel through matter. We then investigate what happens when sound waves interfere with each other. The insights gained in this chapter will help you understand how we hear. 14.1 PRODUCING A SOUND WAVE Whether it conveys the shrill whine of a jet engine or the soft melodies of a crooner, any sound wave has its source in a vibrating object. Musical instruments produce sounds in a variety of ways. The sound of a clarinet is produced by a vibrating reed, the sound of a drum by the vibration of the taut drumhead, the sound of a piano by vibrating strings, and the sound from a singer by vibrating vo- cal cords. Sound waves are longitudinal waves traveling through a medium, such as air. In order to investigate how sound waves are produced, we focus our attention on the tuning fork, a common device for producing pure musical notes. A tuning fork consists of two metal prongs, or tines, that vibrate when struck. Their vibration dis- turbs the air near them, as shown in Figure 14.1. (The amplitude of vibration of the tine shown in the figure has been greatly exaggerated for clarity.) When a tine swings to the right, as in Figure 14.1a, the molecules in an element of air in front of its movement are forced closer together than normal. Such a region of high mo- lecular density and high air pressure is called a compression . This compression The characteristic sound of any instrument is referred to as the quality of that sound. What is it about the sound from the tuba that allows us to distinguish between it and the sound from a flute?
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14.2 Characteristics of Sound Waves 459 moves away from the fork like a ripple on a pond. When the tine swings to the left, as in Figure 14.1b, the molecules in an element of air to the right of the tine spread apart, and the density and air pressure in this region are then lower than normal. Such a region of reduced density is called a rarefaction (pronounced “rare a fak’ shun”). Molecules to the right of the rarefaction in the figure move to the left. The rarefaction itself therefore moves to the right, following the previ- ously produced compression. As the tuning fork continues to vibrate, a succession of compressions and rar- efactions forms and spreads out from it. The resultant pattern in the air is some- what like that pictured in Figure 14.2a. We can use a sinusoidal curve to represent a sound wave, as in Figure 14.2b. Notice that there are crests in the sinusoidal wave at the points where the sound wave has compressions and troughs where the sound wave has rarefactions. The compressions and rarefactions of the sound
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