SOUND AND LIGHT WAVES

Because seismic waves are similar to sound waves in many of their properties, it is useful to consider the characteristics of sound waves.

When a tuning fork is struck, the vibrations of its prongs produce alternate compressions (pushes) and dilations (pulls) of the adjacent air, setting up the sound waves. Such sound waves are are transmitted by longitudinal vibrations of the air; which means the displacements of the air are always in the direction in which the wave is travelling. In a uniform gas, the wave front will spread out at an equal speed in all directions, forming a spherically expanding surface.

Irregular sound waves generally constitute noise; regular waves such as those produced by the tuning fork, give rise to musical notes of definite pitch. Pitch is determined by the frequency of the sound waves, and loudness by their amplitude or wave energy. Many of the concepts and terminology used in the analysis of music carry over to seismological studies.

A pure musical tone consists of a single pitch or frequency. However, most musical tones are complex summations of various pure frequencies - one characteristic frequency, called the fundamental, and a series of overtones or harmonics. Any complex tone from a musical instrument can be distinguished by the trained ear or electronic equipment from a similar tone from another instrument because harmonics tones are produced in different combinations of amplitudes by different instruments. The display of the component tones in terms of individual frequencies an amplitudes is called the wave spectrum. Pictures of the spectra of complex wave forms can be very useful not only in acoustical studies but also in seismology.

Sound waves have the same variations of other wave forms. As they travel out from their source they diminish in loudness as a result of the process of geometrical spreading and frictional attenuation. When they pass from a medium of one density into a medium of another density, they are refracted. When they encounter obstacles, they are reflected as echoes. The backwards scattering that occurs when they strike small obstacles is a major cause of attenuation. All types of waves suffer attenuation by scattering as they propagate through matter environments containing obstacles, boundaries, cavaties and layers of different materials. The blueness of the day sky is a result of the scattering of sunlight by air molecules.

Light waves in contrast to sound waves (which only have longitudinal charactersistics) vibrate in a plane perpendicular to the direction in which the light is travelling. The wave properties of sound and light can be closely related to the characteristics of ground vibrations caused by earthquakes.

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