Document 5L6oknvmyNzB21G83eY738Nm4

354 CHAPTER 25 'Table 3 . .. . Decibel Correction for Area A *9 * Deobef A Decibel A Decibel Addition tq ft Addition tq ft Addition 0.5 -3 `8 9 70 18 1 0 16 12 100 20 2 3 20 13 200 23 4 6 40 16 400 26 1960 Guide Values of the decibel addition for various values of A are given in Table 3. Example t: Assume a deflector vane type of grille-with a core area of 2 sq ft..Determine the sound power level in the speech interference bands for an air velocity of 2000 fpm. Solution: From Fig. 8, the Lwsi for a 1 sq ft deflection grille is 59. db. From Table 2, an addition of 3 more db is required, so that Lwbi 62 db. Diffuser Noise Aside from the information furnished by HHTnw manu facturers, to their users, on sound levels generated by air dif fusers, few other data are available. Recently, however, yimp test results on several types of ceiling air diffusers have yielded a satisfactory correlation of the sound generating characteris tics of these devices.11 The data, covering ceiling diffusers ranging in size from 4- to 18-in. neck diameter, show drat the overall sound power level (from 75 to 10,000 cps) can be found within 2 db by the equation: where Lw ~ 13 loguAAia + 60 logioV___ -- 2 db (20) Lwi " overall sound power level in the seven octave bands from 75 to 10,000 cps, decibels re 10~ia watt. Ann. " minimum flow area for the air passing through the diffuser, square feet. For undamped flow the mini mum area will normally occur in the diffuser neck, whereas for damped flow it will occur at the damper position (see Fig. 9). =* maximum air velocity (j/60 Aaia), feet per second. For ceiling diffusers of cone-type construction used in conjunction with deflecting vanes .nH a damper, Fig. 9 presents the approximate octave band spectrum relative to the overall sound power level as given by Equation 20. The curves of Fig. 9 agree within approximately 3 db of the octave hand FREQUENCY BMC - CYCLES PER SECOND Fig. 9 .... Octave Band Spectrum of Ceiling Diffusers data collected on 8-, 12-, and 18-in. neck diameter filing dif fusers. NOISE GENERATED BY OTHER SOURCES Objectionable noise may be produced by fan motors, pumps, compressors, etc. The designer must give attention to these sources of noise. It is not possible to predict the noise, because units from different manufacturers differ greatly in the noise produced. Objectionable noise is also generated in the duct system by excessive air velocity or poor duct fittings. No systematically taken data have been published from which conclusions can be drawn. The reader is referred to Chapter 21 Air Duct Design for good design practice. SOUND ATTENUATION IN DUCTS Unlined Straight Sheet-Metal Ducts The attenuation of sound in straight sheet-metal ducts is a function of the length, shape, and size of the duct. If a ther mal insulating material is applied to the outside of the duct this will also affect the attenuation characteristics.u This was shown in an investigation11 conducted at the Massachusetts Institute of Technology where two sizes of ducts were tested, a 12 x 12 in. and a 12 x 24 in. These ducts were investigated not only bare, but also with two types of glass fiber thermal insulation applied to the out- Table A .... Octave Band Attenuation of Bare and Externally Insulated Straight Sheet-Metal Ducts Attenuation--db per ft Duet Size indies Type ef Insulation Density Frequency Band--Cycles per S^ond none 1-in. gloss fiber blanket 1-in. glass fiber blanket none t \s 1-in. glass fiber blanket 1-in. glass fiber board _ 0.75 6.0 _ 0.75 6.0 20-75 75-100 150-300 300-600 6001200 0.26 0.40 0.66 0.30 0.38 1.04 0.26 0.40 0.66 0.36 0.44 0.56 0.20 0.32 0.50 0.16 0.22 0.28 0.08 0.16 0.26 0.06 0.10 0.16 0.08 0.08 0.08 0.12 0.12 0.12 1200- 2400- 48002400 4800 10,000 0.12 0.24 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.16 0.16 0.16 0.16 0.16 Sound Control Table 5 .... Natural Attenuation of Air Frequency cps Attenuation db/tt 40-850 1700 3400 6900 10000 neg. 0.003 0.008 0.022 0.035 355 Table 6 .... Attenuation Data for Typical 1-in. and J4*in. Thick Duct lining Board I-bid) fhrdcness V-lnd> TMdcneu Cydesftir Absorp Second tion Co efficient a Attenuation db Absorp tion Co efficient a "''4 Attenuation db p 128 0.12 0.051 0.64 lj 0.09 0.034 0.43 side of the duct. The first type used was a 1-in. thick glass fiber insulation with a density of 0.75 lb per cu ft. The second type was a glass fiber semirigid 1-in. board with a density of 6 lb per cu ft. Attenuation values were determined by plotting L9 readings made at 3-ft intervals over a total duct length of 40 ft. The results are presented in Table 4. . It should be noted from Table 4 that the addition of ther mal insulation to the outer duct walls had little or no effect on the attenuation in the four octave bands above 600 cps for the ducts tested. In the lower octave bands, however, the effect of added thermal insulation on attenuation is indeed striking. More data on larger size ductwork are needed to confirm these results. As duct gis increased, the attenuation rate decreases toward the natural rate in free air which is shown in Table 5 for 68 F air at 60 percent relative humidity.14 Duct Lining Procedure In the past, the most common method of obtaining sound absorption in ventilating systems has been to line the duct with absorbing material. It is usually more convenient to line all four sides of the duct, but a lining on one side over a longer length of the duct will, in general, give the same effect for the area of applied acoustical material. Subject to certain restrictions, the attenuation, in decibels, of a fully lined duct to angle-frequency sounds may be expressed by the approxi mate Equation 21." R = 12.612*" (21) where R ** attenuation, decibels. t -- length of lined duct, feet. P = perimeter of duct inside the lining, inches. Ab 9 cross-sectional area of duct inside the lining, square inches. a *= absorption coefficient of lining (a function of fre quency).1* This formula was empirically developed for a set of duct mags ranging from 9 x 9 in. to 18 x 18 in.; for cross-sectional Himp-nsinn ratios of 1:1 to 2:1; for frequencies between 256 and 2048 cycles; and for absorption coefficients between 0.20 and 0.80. The duct lining material used was 1-in. rock wool sheet. The absorption coefficients of a material of this type in one-half pnd one-inch thicknivw are listed in Table 6. It is alun possible to calculate the absorption by a very com plicated mathematical theory.17u Such calculations are in substantial agreement with Equation 21. This equation may be in error when applied to other types of duct lining and to duct sizes and shapes greater than those specified. With every application, the use of sound absorptive mate rial should be considered in the dual function of insulation and p 256 0.38 0.26 3.2 l ~ 0.25 0.15 '9`l 512 1024 2048 0.70 0.60 7612 P 0.80 0.73 9.2 0.79 0.72 -'5 0.40 0.2S 3.5 I A 0.72 0.63 7.9 0.78 P 0.71 8.9 l -j- A sound absorption. It haa been shown theoretically that the reduction (in decibels per linear foot) of sound transmitted along 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 characteristics of the lining. Experimental evidence likewise mdw-fiira that there is no simple formula involving the varia bles which will apply accurately to all cases. However, it may. be stated generally that the attenuation in decibels at a given frequency 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. It bu been shown1* that the attenuation rate is red uced in the presence of an air stream of appreciable velocity. 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 tiie 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 result, particularly in the low-frequency range, from the flexural vibrations of the duct. In the selec tion r-ttH application of the absorptive material, the following points should be considered: 1. For the absorption of the low-frequencies below 500 cps the material should be 2- to 12-in. thick. Thin materials, par ticularly when mounted on hard solid surfaces, will absorb only the high frequencies. 2. In order to provide as much low-frequency noise absorp tion as possible by means of flexural vibration, it is desirable to fasten the absorptive panels discontinuous!y. This result may be attained to some extent by spot cementing, but better re sults are obtained when it is possible to fasten the absorptive panels to furring strips, leaving an sir space behind. However, the exact resonance characteristics of the panels, and thus their ! ]|