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Noise ReductionThe sound-level, or noise, meter is useful for making quantitative studies of noise reduction. Noises produced within or transmitted into a room will remain audible until their sound energy is absorbed by the surfaces and objects within the room. In studying reverberation, it was shown that the intensity to which a sound (or noise) increases within a room is determined in part by the total amount of sound absorption present and is inversely proportional to it. If the sound output of the disturbing noise source remains constant, then doubling the absorption will halve the intensity, and so on. Thus, the noise level in a room can be reduced by the addition of sound-absorbing material.
The sound reduction in decibels is10
where a1 is the total units, or sabines, of sound absorption present, before treatment, and a2 is the total amount after treatment. From Fig. 18 it appears that a reduction in the loudness level of the sound of 65 to 60 db reduces the relative loudness from about 6000 to 4000, or to about two-thirds of its original amount. Thus, but a small reduction in the noise level in decibels causes a marked reduction in the interfering effect of the noise. Typical noise levels which have been found to exist are shown in Fig. 23. Useful information regarding noise and vibration studies is given in reference 41.
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