As we saw in Lesson 16, mechanical waves are waves that require a material medium to exist. There are two types of mechanical waves: Transverse waves involve oscillations perpendicular to the direction in which the wave travels; longitudinal waves involve oscillations parallel to the direction of wave travel.
In this app, a sound wave is defined roughly as any longitudinal wave. Seismic prospecting teams use such waves to probe Earth’s crust for oil. Ships carry sound-ranging gear (sonar) to detect underwater obstacles. Submarines use sound waves to stalk other submarines, largely by listening for the characteristic noises produced by the propulsion system. Figure 17-1, a computer-processed image of a fetal head and arm, shows how sound waves can be used to explore the soft tissues of the human body. In this lesson we shall focus on sound waves that travel through the air and that are audible to people.
Figure 17-2 illustrates several ideas that we shall use in our discussions. Point S represents a tiny sound source, called a point source, that emits sound waves in all directions. The wavefronts and rays indicate the direction of travel and the spread of the sound waves. Wavefronts are surfaces over which the oscillations due to the sound wave have the same value; such surfaces are represented by whole or partial circles in a two-dimensional drawing for a point source. Rays are directed lines perpendicular to the wavefronts that indicate the direction of travel of the wavefronts. The short double arrows superimposed on the rays of Fig. 17-2 indicate that the longitudinal oscillations of the air are parallel to the rays.
Near a point source like that of Fig. 17-2, the wavefronts are spherical and spread out in three dimensions, and there the waves are said to be spherical. As the wavefronts move outward and their radii become larger, their curvature decreases. Far from the source, we approximate the wavefronts as planes (or lines on two-dimensional drawings), and the waves are said to be planar.
Fig. 17-2 A sound wave travels from a point source S through a three-dimensional medium. The wavefronts form spheres centered on S; the rays are radial to S. The short, double-headed arrows indicate that elements of the medium oscillate parallel to the rays.
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