Sound: Absorption, Propagation and Perception - Abstract and Review (text only - minus images, tables, etc.) In addition to the geometric decrease in intensity caused by the inverse square law, a small part of a sound wave is lost to the air or other medium through various physical processes. One important process is the direct conduction of the vibration into the medium as heat, caused by the conversion of the coherent molecular motion of the sound wave into incoherent molecular motion in the air or other absorptive material. Another cause is the viscosity of a fluid medium (i.e., a gas or liquid). These two physical causes combine to produce the classical attenuation of a sound wave. This type of attenuation is proportional to the square of the sound wave's frequency, as expressed in the formula [{alpha}] /f[{sup 2}] , where [{alpha}]is the attenuation coefficient of the medium and f is the wave frequency. The amplitude of an attenuated wave is then given by where A[{sub o}] is the original amplitude of the wave and A(x) is the amplitude after it has propagated a distance x through the medium. Table 5 gives sound-absorption coefficients for several gases. The magnitudes of the coefficients indicate that, although attenuation is rather small for audible frequencies, it can become extremely large for high-frequency ultrasonic waves. Attenuation of sound in air also varies with temperature and humidity. Because less sound is absorbed in solids and liquids than in gases, sounds can propagate over much greater distances in these mediums. For instance, the great range over which certain sea mammals can communicate is made possible partially by the low attenuation of sound in water. In addition, because absorption increases with frequency, it becomes very difficult for ultrasonic waves to penetrate a dense medium. This is a persistent limitation on the development of high-frequency ultrasonic applications. Most sound-absorbing materials are nonlinear, in that they do not absorb the same fraction of acoustic waves of all frequencies. In architectural acoustics, an enormous effort is expended to use construction materials that absorb undesirable frequencies but reflect desired frequencies. Absorption of undesirable sound, such as that from machines in factories, is critical to the health of workers, and noise control in architectural and industrial acoustics has expanded to become an important field of environmental engineering. This abstract and review was obtained from OnLine subscription services - courtesy of Charles Turley.