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As a form of energy, sound can disappear from an enclosed space in which it is produced only by transmission or by absorption. The latter consists in the dissipation process of transforming the regular vibrational motion of sound into the random, molecular motion of heat, and occurs only when work is done against dissipative forces, such as by non-elastic compression or flexure of soft materials or by motion to and fro in the minute channels of porous materials. When sound is incident upon a solid surface, part of its energy is reflected, part transmitted through the solid, and part absorbed. For purposes of the interior acoustics of rooms, it is unimportant whether the energy disappears by absorption or by transmission, and since in general the latter is a relatively small proportion of the unreflected energy, we include it in the absorption. The absorption coefficient depends in a marked degree upon the pitch, to some extent upon the quality, that is whether the sound is pure tone or one with harmonics, and, probably upon the intensity, intense sounds being absorbed more strongly than faint sounds.
From the foregoing, it is apparent that sound once produced within a room will persist for an appreciable length of time after the source has ceased emitting it, due to the fact that it requires time for the multiple reflections necessary to reduce its intensity to inaudibility. This persistence is technically called reverberation and is measured by the time required for sound to decrease to 1/1,000,000 of its initial intensity. The phenomena of reflection, absorption, and reverberation can all be illustrated by means of photographs of sound pulses taken by means of electric sparks.
Another phenomenon that occurs in rooms is that of interference. This results from the undulatory nature of sound. As a simple illustration consider the case of a train of waves incident upon a plain surface. At points at which the reflected wave train travels an odd number of half wave lengths further than the oncoming, the condensation of one will coincide with the rarefactions of the other and assuming equal intensity the pressure change at such points will always be zero. For an even number of half wave lengths difference, the two pressure changes will reinforce each other. Thus we have in a room with a sustained source of sound of fixed frequency a point to point variation in the intensity, which changes with changing pitch at the source, with the position of the source, and with any shift in the position of reflecting surfaces within the room. As a general rule this inequality of distribution is not an important factor in the acoustic properties of a room. Concentration of reflected sound by extended curved contours, may accentuate these inequalities to the point of being a real acoustical defect.
Echo is the distinct repetition of a sound of short duration, by reflection from some solid surface or surfaces. In order to separate two sound
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