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There are two aspects of sound: one is the physical aspect which involves the physics of the production, propagation, reception, and detection of sound; the other, which is the sensation of sound as perceived by the individual, depends upon physiological and psychological effects. It is not desirable to separate the two aspects of sound completely, but the main emphasis in this book must necessarily be on the physical aspect. In this chapter we shall consider mostly musical sounds. A vocabulary has been developed to describe the sensation experienced when a musical sound is heard. Such terms as the pitch of a sound, its loudness, and its tone quality or timbre are used to describe the musical sound. The physicist, on the other hand, speaks of the frequency of the sound, its intensity, and the number and intensities of the overtones present in a musical sound. Unfortunately, there is not a one-to-one correspondence between the terms used by the physicist and the terms used by the musician. A great deal of progress has been made in recent years as a result of tests involving thousands of persons which attempt to correlate the sensation of sound with the physical properties of sound. Some of these results will be mentioned at appropriate places in this chapter.