Document zdmneBB1Mgb04Y0Moxya3kdb7

852 CHAPTER 40 1950 Guide Tablb 5. iAttenuation op Elbows* Elbow '' . ,./ Size, iN.b , 3 to 15 r 15 to 36 . 36 plus Attenuation pee Elbow, db 3 2 1.5 1 "The attenuation in varied elbows shouId.be considered the-same aa-iu elbows bavins the same dimen* eions as the radius of curvature of the vanes. If the vanes are lined for the purpose of damping any vibra tions in them, one third may be added to the attenuation values listed. bThese attenuation values are based oh elbows having a center line radius 1.5 to 2 times the diameter or width of the duct. The attenuation will be greater if the ratio 1$ less than 14> and less when the ratio is greater than 2. This attenuation is a function of the total grille area (supply and return) and the total sound absorption of the room in sabins. (The sound absorp tion of a room'in sabins is the summation of the productsof each surface of the room measured in square feet multiplied by its corresponding absorption coefficient. The sabin is a unit of sound absorption equivalent to the .absorption of one square foot of, a totally sound-absorbent surface). The attenuation is {pyenjin Equation 5 as : , . .. / Attenuation between'V , Total Room Absorption mSabins . `"'V grilles and room J" 10 "?' ;. . ^otalGrillo Area' '7 . W Values in Table 7 approximate the attenuation for various' rates' of air change, and general types of room surfaces. ; . . DUCT SOUND ABSORBERS The difference, between the required sound attenuation and the natural attenuation must be supplied by the proper sound treatment of the ducts. Selection of the Absorptive Material .' When a sound wave impinges oh the surface of- a porous material, a vibrating-motionris 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 waveis called, the ; absorption, coefficient. . Considerable .absorption;-may, also result,-particularly in the low-frequency.range,'from.the,flexural vibrations of the duct. In the' selection and application of the absorptive material, the.following points shouldffie considered:. : Table 6. Attenuation at Duct-: Branches or ;Outlets ... Ratio Branch Duct 4- Outlet Area `Supply-Duct Area - - Sum of Branch Areas ' Supply Duct Area 1.00 1.20 . 1.35 1.50 ;i.75 2.00 Attenuation PER Transformation, db 0.0 0.8 1.3 1.8 2;5 ' 3.0: Sotnid Control 853 l;. For the absorption of: the low frequencies below 500 cycles-per. second 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 provide as niuch low frequency noise absorption.as possible by means, of flexural vibration, it is desirable to fasten the-absorptive panels discontinuously; This result may be attuned to some extent by spot cementing, but better results are obtained when.it is possible, to fasten the absorptive panels to furring strips,.leaving an air-space behind. _ However, the exact resonance characteristics of' the panels, and thus their absorption, are so unpredictable that'flexural vibration cannot be relied upon for a specific value of attenuation. Requirements -for a good .sound absorption material are: (1)- high absorption at low frequencies,5 (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 Table 7. Approximate Attenuation Between Grilles and Room Outlet .. .Velocity FPM - * Ats Change - - ,, - '-r 500 - - 5' 10 15 20 .... g " 10 750 15 20 Live Room* ' = 0.05 ' *' db Medium a -> ai db u : 16 . 14 .19 16 ` 21 17. - . . . . 22 13 :: is " 16 21 18 23 19 24 Dead . .Roow*._ a = 0.25 db 18 21 23 24 20 23 25 26 . 1000 ' .-- - . 1250 5 10 15 .20- 5 10 15.. . 20 14 . 17 . . 19 20 J5 18 : 20 " 21 19 22;-. . .24 .. 25 20 . - 23 25 . -. 26 21 24 . .26 , .28 " - 22' 25 ,27 28 ,. ^Average absorption coefficient for the i*oom. bLlve room-everage absorption coefficient 0:05. Bare wood or concrete floor--hard plaster walls Vnd celling--minimum of furniture. Medium room-average absorption coefficient 0.15. Carpeted floor, nphnlatpred furniture,', hard plaster waUs and ceiling or bare room with acoustically treated ceiling. dDead room-average absorption coefficient 0.25. Heavy carpeted floor. Walts and treated. Upholstered furniture. acoustically * and algae, (6) low surface coefficient of friction, (7) particles should not fray off at the higher design velocities, and (8) freedom from odor when either diy 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 theoretically5 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 accu rately tq, all cases: However, it may be stated generally that the attenuar taon in decibels, at a given frequency is directly proportional to the length:of