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206 CHAPTER 14 Table 16.... Approximate Levels of Sound Power Generated by Air Rowing Through BbowS* WHh Turning Vanes at 2000 fpm.' Duct Stz* SqH 0.25 . 0.5 1 2 Octave Sand Canter, 106 212 425 650 1700 3400 6900 52 52 51 48 44 38 31 57 56 55 . 51 47 41 34 62 60 58 54 50 44 37 73 66 62 57 53 47 <0 125 250 .500 1000 2000 4000 6000 0.25 52 52 50 47 43 36 29 0.5 '57 56 54 50 45 39 32 1 62 60 67 53 49 42 35 2 . 71 65 61 56 52 45 38 * Valid for SO to 90 de* elbow tarn engtue 1965 Guide And Data Book take-off (lines 3 and 5 of Table 18) is considered, it is dear that this source will be of no more significance in determining required attenuation than was the 24 X 36 in. elbow. Accordingly, Ime 20 of Table 18 shows the total required attenuatioo as identical to that calculated as required for the fan alone. In the quiet part of the system, between the sound treat ment and the air-conditioned room, even relatively low air flow velocities may regenerate excessive noise. In order to achieve low design goals, duct velocities in this part erf the sys tem'skould be correspondingly low, andfittings should be designed for low turbulence. As a precaution, it is desirable to line the last five feet-of duct before each air outlet in low velocity systems with one inch thick fiber blanket, in order to attenu ate any unforeseen fitting noise. as long as the vanes are Sufficiently rigid and braced to pre-: vent vibration. For- air Sow velocities other than 2000 fpm, add velocity correction factors from Table 17.- Technical Committee 1.7 of ASHRAE is sponsoring a re* search project for establishing design information on air flow' noise in other duct elements. The following discussion and the procedure for applying air flow noise data illustrated in Example 4 should be helpful when more comprehensive data become available. Table 17 shows that even a moderate decrease in air velocity will reduce the leVcl of the air flow noise considerably. If space permits, it is therefore usually possible to avoid air flow noise problems simply by sizing the duct and fittings so that the air flow noise level is well below the fhn noise level at any point in the system. This is illustrated in Example 4- Example 4'. Calculate the required duct attenuation for the system shown in Fig. 15, using the same fan as in Example 3. Assume that the permissible octave band power levels are 3 db lower th*n cakttilwtad inExample t, to allow for an equal amount of sound coming through the return duct. Solution: An examination of Fig. 15 shows three possibly sig nificant Bound sources: the fan, the 24 X 36 in. v&nea elbow, and the 12 X 18 in. branch take-off with vanes. FV-K of these sources should be treated separately.-and the - requirements for added attenuation calculated to reduce the sound from each source to the design goal in the room. The atten uation requirements should then oe added In accordance with In high-velocity, high pressure air distribution systems, the ;duct velocities and static pressures are such that the-air flow and valve noises have to be controlled by sound attenuating terminals. Although sound attenuation is one of their most Table 2 to give the total required attenuation. It is most impor- a important features,-commercial high velocity terminals are tant that the actual attenuation required for each be determined -- primarily selected on the basis of-their sound power ratings, as' the algebraic difference between the sound power level from that source and the allowable level (this difference may have as described in Step 2. This is so because the sound power' either positive or negative values), and that they always be added rating.expresses the net effect of the sound generated by the in accordance with Table 2. " Table 18 presents the calculation for the fan sound output and that'of thb vanfis in the branch take-off. Note that the attenua tion.required for sound from the vanes is over 20 db less than that'required for sound from the fan. The former, therefore, has mixing and throttling' valves minus the attenuation provided by.;ths attenuator section of the unit. Separate attenuation ratings for terminal units are used only to determine what, if any, additional duct attenuation is needed to take care of the no effect when added to the latter in accordance with Table 2. Air flow' velocity in the 24 .X 36 in. elbow la 1000 ft/min, and in the branch take-off is 800 ft/min. According to Table 17, this difference Ln velocity amounts to a 6 db difference in eound generation. When the attenuation of the large'duct Ond of the branch low. frequency part of the fan sound. Frequently overlooked sources of noise in high velocity sys tems are the walls of ducts and fittings, as well as fan noisetransmitted through the return duct. The sound treatment Table 17.... Effect of Air Row Velocity on Notse Generated by Air How in Duct Fittings* Vefoafy, fpm sod ; 1000 1200 1500 Decibels to be added to noise gene-, rated at 2000 fpm ... -24 -18 . -13 -7.5 * To interpolate between velocity value* lifted, add 1 db far each 4 percent laera** fit velocity. 2000 .0 2500 - 3000 4000 +6 - +10- . +18 Sound Control 207 Table 18 .... Sample Calculation of Duct Attenuation Required to.Reduce Fan and Fitting Noise for Estampls 4 tin* ' ' 106 Octave Bend Center, cps ; 212 425 850 1700 3400 6900 1 Sound Power Output of: Fan 2 Power Divisions to Terminal, 5% Area From Ex. 3 Table 12 - 89 89 79 74 69 64 59 13 13 13 : 13 - 13' 13 '-13- 3 Natural Attenuation, 24 X 36 in. Duct, 20 ft long Table 13 X Length 4 Attenuation of 36 in. Elbow, with Vanes Tables 14 and 15 5 Elbow Attenuation of 18 in. Take-Off, with Vanes Tables 14 and 15 4 2 1 1 _1 1 1. 2 4 4 3 ` 3 '3 3 0 2 5 .3 3 3. 3 6 Natural Attenuation, 12 X 18 in. Duct 20 ft long Table 13 X Length 4 3 2 '2 2 ' 2 2 7 End Reflection for Size 12 X 16 in. Q -- 4 Figure 13 5 1 0 . 0 .0 >.o- 0' 8 9 10 11 Total Natural Attenuation in Branch Sound Power Level per Terminal without Sound Treatment Permissible Sound Power Level per Terminal, Allowing 3 db for Return Duct Min. Attenuation of Sound Treatment to be designed for 12 X 18 in. Duct Sum of Lines 2 to 7 lane 1 Minus Line 8 line 18 of Step 2, , Minus 3 db Line 9 Minus line 10 ?fi 25 25 22 22 22 22 61 64 54 52' 47- 42 '37-' 52 45 39. 34 33 (31. 31 9 19 15 ' 18 14 11 6 12 Sound Power Output of: Vane9 of Take-Off (800 FPM) From Tables 16 + 17 -44 39 36 32- -28 22 15* 13 Power Divisions to Terminal, 25% Area Table 11 or 12 6 6 6 6 -6 . 6 6 14 Natural Attenuation,-12 X 18 in. Duet, 20 ft long Table 13 X Length 4 3 2 2 - 2- 2- 2 15 End Reflection for Size 12 X 18 in. Q ^ 4 Figure 13 5 1 0 0 O' - 0 0 16 17 ' 18 19. .20 Total Natural Attenuation in Branch Sound Power Level per Terminal ' without Sound Treatment Permissible Sound Power Level per Terminal, Allowing 3 db for Return Duet Min. Attenuation of Sound Treatment to be designed for 12 X 18 in.' Duet' -Total Attenuation Required Sum of lines 13 to 16 - - 15 10 8 8 - 8 8 8- line 18 of Step 2, Minus 3 db 29 29 28 24 20 14 7 52 45 39 34 33 31. . 31' -- ' Line 11 Added to Line 19 Per Table 2 --23 -16 -11 -10 -13 -17 -24 159 19 18 14 11 6 required-for the return duct system rfmuM be calculated fol lowing the procedure illustrated in Example 4. . To prevent excessive noise radiation from the walls of high velocity ducts, all fittings should be smooth and designed to avoid abrupt changes of direction or velocity. Whenever pos able, high velocity ducts and terminal units should be' in noncritical areas, such as above corridors. Plaster ceilings or tightly sealed window stools are good sound barriers, but perforated ceiling tiles are not.0 The space above the acoustical tile ceiling of a quiet room should not be used to install a group of high velocity terminal units serving surrounding rooms. Cross.Transmission Between Rooms Radiator enclosures, etc., should not extend through parti tions, and all openings around pipes and ducts should be sealed tightly to prevent sound leakage between rooms. Barriers should be provided in suspended acoustical epflinga above the partitions.0 To eliminate cross transmission through the duct system, sound attenuation should be provided in ducts between air outlets in adjacent rooms.** The required duct attenuation toO depend upon the degree of sound isolation (transmission loss, TL) for which the wall is designed. Normally, the sound coming through the duct should not add more than 1 or 2 db to the sound'coming'through the wall.* On this basis, the required duct attenuation can be calculated from Equation 15. Attenuation, db -- (TL) +6 -- 10 logi/S^/S, (15) where ... (TL) -- transmission loss of the,partition, decibels.-. Sp = area of the partition, square feet. - & -- area of outlet, square feet. Transmission loss, ratings for typical wall constructions in the speech frequency range (600-4800 cps) may be obtained from References 9 or 55. The factor 10 logu (Sp/S,) can be found from Fig. 1 for any ratio of-the partition area <S,, to.the'outlet area S,. Example 6 illustrates the use of Equation 15. Example 6: Calculate the attenuation required in a duct be tween two rooms, each having two 12 X 12 in..air.outIetA The partition between the rooms has an area of 200 so ft and an aver- traosmissioa loss of 40 db in the speech frequency range to 4800 cpa). * Solution; From Fig. 1: 10 log,, (8,/8J - 10 logj, (200/2) - 20 db From Equation 15, the required attenuation is40 + 6 -- 20 -- 26 db. . . ` , To provide the required duct attenuation, duct lining, splitters, cells, outlet absorbers, or prefabricated silencers may be used. In high-velocity supply systems with-separate terminal units for each room, the combined attenuation of two terminal units in series is adequate for practically all cross transmission problems, even in music practice rppms. \: -