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590 ' ____________ , _________ Chapter 32 , ~ - 1945 Guide. . Selection of the Absorptive Material When a sound wave impinges on the surface of a porous material, a vibrating motion is set up within the small pores of the material by the alternating sound waves. As the ratio of the cross sectional area of the pores to their interior surface is small, the resistance to the movement of , air in. the pores is large. This viscous resistance within the pores of the. material converts a portion of the sound energy into heat. The decimal fraction representing the absorbed, portion of the incident sound wave is called the absorption coefficient. , Considerable absorption may also Table 5. Approximate Attenuation Between Grilles and Room Outlet Velocity FPM 500 Air Change Min. 5 10 15 20 Live Room1* g* 0.05 db ii 14 16 17 Medium. Room* o = 0.15 db 16 19 21 22 Dead.' Room* . a - 0.25 .db 18 .21 23 24 , 750 5 10 15 20 13 18 20 16 21 23 18 23 25 19 24 . ' . 26 1000 5 10 15 20 14 T9 21 17 22 24' 19 24 26 20 25 . 28 1250 5 10 15 20 15 20 22 18 23 25 20 25 27 21 26 ' 28 Average absorption coefficient for the tootn. bLive room average absorption coefficient 0.05. Bare wood or concrete floor--hard plaster walls and - ceiling--minimum of furniture. Medium room average absorption coefficient 0.15. Carpeted floor, upholstered furniture, hard plaster walls and ceiling or bare room with acoustically treated ceiling, *Dead room average absorption coefficient 0.25. Heavy carpeted floor. Walls and ceiling acoustically treated. Upholstered furniture. result, particularly in the low frequency range, from the flexural vibra tions of the .duct. In the selection and application of the absorptive material, several points should be considered. 1. For the absorption of the low frequencies the material should be at least 1 to 2 in. thick. Thin materials, particularly when mounted on hard solid surfaces, will absorb the high frequencies and reflect the low. < 2. In order to take advantage of low frequency noise absorption by panel vibration, it is advisable to fasten the absorptive sheets to stripping so that the panels themselves may vibrate. However, the exact resonance characteristics of the panels and thus their absorption are so unpredictable that panel resonance cannot be relied upon for a specific value of attenuation. Requirements for a good sound absorption material are: (1) high absorption at low frequencies4,*(2) adequate strength to avoid breakage, (3) fire resistance and compliance with national and local code require ments, (4) low moisture absorption, (5) freedom from attack by bacteria and algae, (6) low surface coefficient of friction, (7) particles should not 4For coefficients of commercial sound absorbent materials see Bulletin Acoustical Materials Association, 910 No. Michigan Ave^ Chicago, 111. Sound Control ' 591 fray off at the higher design velocities, and (8) freedom from odor when either dry or wet. . With every application the use of sound absorptive material should be considered in the dual function of insulation and sound absorption. It has been shown theoretically6 that the reduction, in decibels per linear foot, of sound transmitted through a duct lined with sound absorbing material . is related in a rather complicated manner to the size and shape of the .duct, to the frequency of the sound, and to the sound absorbing char acteristics of the lining. Experimental evidence likewise indicates that, there is no simple formula involving the variables which will apply accurately to all cases. However, it may be stated generally that .theattenuation is directly proportional to the length of lined duct. It decreases as the cross-sectional area increases, and increases as the aspect ratio is increased. The noise reduction varies to a considerable extent with the frequency of the sound.. In calculating noise reduction, consideration should be Fig. 1. Absorption Plenums With and Without Sound fmia given both to the comparative efficiency of the duct lining material at different frequencies, and to the frequency distribution of the noise to be quieted. In the case of fan noise, it is recommended that calculations be based on the frequency 256 cycles, since most of the. noise energy, is in the region of this frequency,. In quieting noise due to air turbulence and eddy currents, in which the high frequencies predominate, the frequency 1024 cycles should be used.' _ Plenum Absorption \ In systems, where individual ducts are directed to `a number of rooms and sound treatment is required in every duct, a sound absorption plenum on the fan discharge as shown in Fig. 1, will often prove the most-eco nomical arrangement. The absorption in the plenum may be approxi mated by Equation 4. db (Attenuation) = 10 logw-------- ^ Fan Di^ge' ^ (4) The area of the plenum should be at least ten times as great as the fan discharge area. The plenum should be lined with 2 in. of muslin covered rock wool blanket or 1 in. sound absorbing board preferably nailed to wood strips on the inside of the plenum. With such a lining the plenum 0 Hot.h30^1112 Materiala- by L- J- Sivian Aiousticcl