Sound Conduction

SOUND CONDUCTION, the property of bodies to serve as a transmission medium for acoustic waves. Apart from the attenuation of sound with distance due to the distribution of energy over an expanding surface, the intensity of sound passing through a medium decreases due to the conversion of the energy of sound vibrations into the energy of thermal movements (sound absorption). Sound conduction of a substance depends on its density and elasticity, increasing with the density of the conducting medium and with the increase of the speed of sound in it. Therefore, solid bodies possess the greatest sound conduction, and gases the least. Steel, glass, wood, gutta-percha, cork, and rubber represent a series of solid bodies with decreasing sound conduction (in wood, sound conduction along the fibers is greater than across them). In liquids and especially in gases, inhomogeneities of the medium have a great effect on reducing sound conduction; in liquids and gases, low tones can propagate over greater distances. The intensity of sound passing through a narrow tube—from 0.6 to 2.6 cm in diameter (the stethoscope belongs to such tubes)—is J = C " d⁴ / l, where d is the diameter, l is the length of the tube, and C is a constant depending on the material of the tube and the sound source. Upon transition from one medium to another, the greater the attenuation of sound, the more significant the difference in the "acoustic resistances" of the media (acoustic resistance is equal to the product of the density of the medium and the velocity of sound in it). With equal acoustic resistances, sound is transmitted from one medium to another without attenuation. Therefore, composite screens made of substances with different acoustic resistances (e.g., iron-felt-plywood) are most advantageous as sound insulators. To the receptive organ of the inner ear, sound is delivered through the air and through the bones of the skull; the loudness of a sound generated in the air and perceived by the cranial integuments is insignificant due to the large difference in the sound resistances of air and cranial integuments. In premises (factories, hospitals, etc.), sound is transmitted through the air (doors, ventilation pipes) and directly through walls. To attenuate the sound conduction of ventilation pipes, sharp changes in the width of the passage are made in them, or membranes are installed. In walls, sound transmission is possible through the pores of the walls and due to the vibrations of the entire wall (like a membrane). Literature: see bibliography to the article Sound.