Document Lxz86v094Mnp4Rx29Q1zvqMX

HEATINC VENTILATING: AIR CONDITIONING GUIDE 1944 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 Table 5. Approximate Attenuation'Between Grilles and Room Outlet . .Velocity FFM .. 500 750 1000- .1250 :'v ; , Aik Change; Min. . . 5" 10 15 20 ' 5 10 15 20 5 10 15 20 5 10 15 r 20 Live Room*> a* = 0.05 db ii 14 16 17 13 16 18 19 14 17 19 20 15 18 20 21 Medium Roomo ' a - 0.15 db 16 19 21 22 18 . 21 23 24 19 22 24 25 20 23 25: 26' ' Dead Room4 o - 0.25 db : 18 21 23 24 20 23 25 26 21 24 26 28 22 25 27 28 Average absorption coeffipent-for the room. ! j.kLive room average absorption coefficient 0.05. Bare wood or concrete floor--hard plaster walls and ceiling--minimum of furniture.' 1 `- eMedium room average absorption coefficient 0.15. Carpeted floor, upholstered furniture, hard plaster walls and ceiling or bare room' withamustically-treated failing, ; dDead room average absorption coefficient 0.25.. Heavy, carpeted floor. Walls'and'ceiling acoustically treated. Upholstered furniture. may vibrate. However, the .exact resonance characteristics of the panels and thus their absorption is so unpredictable that panel-resonance'cannot bie relied upon for a specific value of attenuation. Requirements for a gocid sound absorption material are: (i) high absorption at low frequencies4, (2) adequate strength toi. 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 Mctidri, (7) particles'shbiild hdt 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 beeu shown theoretically5 that the reduction, in' decibels per linear foot, * : *Fbr coefficients of commercial sound absorbent materials see Bulletin Acoustical Materials Association. 919, No. Michigan Ave..Chicago, DL . `Sound Propagation in Ducts Lined with Absorbing Materials, by L. J. Sivian (Journal Acoustical Society qf America, VoL 9, p: 1937). ' * ' ' 626 CHAPTER 33: SOUND CONTROL 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. Experimnetal 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 the attenuation 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 given both to the comparative efficiency of the. duct lining material at different frequencies, and to the frequency distribution of the noise to be TtJteoff 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. . ... Plenum Absorption in Sabines , db (Attenuation) = 10 logio Area Fan 'Discharge ; (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 stripSiOn the inside of the plenum. With such a lining the plenum is particularly.effective in reducing low frequency fan noise. The absorp tion of the plerium in sabines is the sum of the products of-each: interior 627