Document YDwjqymy9wownq51Mgm025nkE

918 CHAPTER 41 : `1954 Guide Sound Control 919 A. Unlined metal duct. B. Absorption lined duct (Case 1). C. Splitter plate type absorber (Case 2). (Channels 20 in. x 3.33 in. inside). - D. Cell type absorber. (Cells 5 in. x 3.33 in. inside). 1 in. thick absorption material in all cases.: < Sample Calculations for Duct Treatment Exdrriple l: An air conditioning installation is to be installed in a small theater. Determine the necessary sound treatment for the air distribution system to provide a satisfactory noise level in the theater utilizing these conditions:-. -Fan tip speed 4000 fpm, total pressure 1.25 in............................. 77J7. db Acceptable room noise level (Table 2). - - ^V_ d-b i , Required attenuation.................... ................. ................ ..., . -37 . dis solution: Natural attenuation of supply duct................................... , Sheet metal duct 50 ft long 48 in. x 36 in. (Table 4) 50 x 0.01........ 0.5 db Elbows, two sire 48 in. x 36 in. (Table 5) 2 x 1. 2.0 db , Attenuation grilles to theater air change 10 min (Table' 7) outlet velocity 1000 fpm..................... ........................ ......... .i........ 22.0 db . :i; Total natural attenuation . ........................ ..........24.5 db . Difference between required and natural attenuation, 37 minus 24.5, is 12.5 db. This attenuation must be supplied by sound treatment in the duet, either in the form.of - duct wall lining or rectangular cells of the plate or cell absorber arrangement. ' * , A similar analysis of the return duct system shows that 15 db attenuation is to be furnished by absorptive material. An inspection of the installation shows that the lining of the plenum on the suction side of the fan would prove the most eco nomical, where it would secure the dual function of heat .insulation and sound ab sorption. Example S:.A 10 x 20 in. duct is connected to a private office space in a quiet lo- cation. Determine the length of lining necessary to attenuate average .fan noise satisfactorily, using a lining material of a type to which Equation 6 applies, and having an absorption coefficient of 0.40 at 256 cycles. : Assume .that the auct is only 12 ft long as shown in Fig. 6, and that a 30 db reduction is required in this length. , Solution: Case I. (No splitters, duct lining only). From Equation 6, - -- 60 R = 12.6 x 12 X -- X 0.401* = 13 db. Table 9. End Reflection of Plate ob Cell Absobbers Percentage free area of absorber.............................. 50 40 30 25 Attenuation db............................................................. 1 . 2 4 5 20 0 Fig. 7. . Absorption Plenums With and Without Sound Cells Case 2. (Two 1 in. splitter plates, 3 channels each 20 im X ^ in.). From Equation 6, i- 46.7 > 3 = 12.6 X 12 X ~rr X 0.40M =4.29 db. . 66.7 ' Additional attenuation may be obtained by using additional-splitter plates or use of egg-crate arrangement of.absorbing material and application of Equation 6. 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. 7 will often prove the most economical arrangement. The absorption in the plenum may be approximated by Equation 7. : ' -.-i-;-- .' Plenum Absorption in Sabins tK> (Attenuation) = 101ogu> Area Fan Discharge ! (7) 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 is particularly effective in reducing low frequency fan noise. The absorp tion of the plenum in sabiris is the sum of the products of each interior area of the plenum measured in square feet multiplied by its corresponding absorption coefficient. Outlet Sound Absorbers , Outlet sound absorbers are rectangular or plate cells installed directly behind an outlet or they may be the lining of a pan or plaque outlet. They are particularly effective in the elimination of high frequency whistles which are generated by air flow in the ducts. They are also employed in large systems with long runs where only a few outlets near the fan require treatment. Frequently outlet cells are the only means of correcting existing noisy installations, as the duct sections directly behind the outlets "ay be the only sections accessible for treatment. (See Fig. 8). AIR SUPPLY OPENING NOISES When air is introduced into a room through a grille or register at a constant velocity, sound energy is being introduced into the enclosure at