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852 CHAPTER 42 1949 Guide) ample acoustical treatment. Lagging material similar- in -character -to acoustical board, when placed on the outside of ducts, serves to prevent' noise'originating outside the ducts being carried inside the ducts and into the air stream. . A case where outside lagging is. desirable occurs when ducts originate at the fan in the equipment room and pass through this room on the way to the room being1 conditioned or ventilated. Unless the ducts are lined some of the mechanical noise from theequipihent'room air may be trans mitted through the wall of the duct into the air stream and thereby carried into the room. In such cases,-that portion of. the duct which is exposed to '' Fio. 6.- Room Absobption Correction Chart : the-spiinds in the equipment room should be'lagged with material such as cork, pipe covering or other sound1 damping material to prevent the sound from-eritering the duct at this point. Numerical data are not available to permit a simple and practical calculating procedure'to determine-thickness of-'covering ;which should be used for thus purpose.: ; .....-r.,- Laboratory measurements have shown that the loss through a sheet1 of, Nol-22 gage-metal- is-24 db;' When a sheet of rock-wool'insulation l;:iri.r thick :and weighing 1.4 lb per square foot is added to this, the1 insulation value is: increased to 29'db. In general, howeverj-addingia layer-of'insula tion or pipe covering' does noti materially increase the-sound insulation value unless the material is dense, or unless it is surfaced with another sound impervious layer such as metal or board: Standard-reference books should be consulted for sound insulating properties of various materials. , Inside lining'material vised in the case previously mentioned would: seiwe as .an Sound Control 853 absorber of the sound transmitted through the duct walls, and thus act as a means of preventing the transfer of . noise into the air stream. Inside lining may also be used in ducts to-absorb noise, which reaches the air stream from equipment such as fans, sprays and coils; noise due to eddying currents set up by elbows, dampers and similar obstructions; and noise transmitted from room to room where there is a common duct system. CONTROLLING VIBRATION FROM MACHINE MOUNTINGS . It is impossible to select equipment which will operate without producing some mechanical noise and, since the equipment must be mounted in a building, it is probable that a part of this noise will be transmitted to the building to such a degree as to make noisy conditions in the rooms which are to be air conditioned. Much of this noise may be transmitted by the duct if it is rigidly con nected to the fan outlet. It.is common practice to make the connection between the fan and the duct with a canvas sleeve which effectively restricts noise at this point. Noise may also enter the building through the mount ing of the motor and the fan. Flexible mountings should be provided in all installations but these mountings must be carefully designed so that they will actually reduce the energy transmitted between the machinery and the supporting floor. If a flexible material is used, it is desirable to investigate the installation so that it is not short-circuited by through bolts which are improperly insulated and by electrical conduit which is not properly broken and is attached both to the equipment and to the building. The flexible mounting, if improperly engineered, may actually increase the energy transmitted, between the equipment and the floor upon which it is supported. In the proper isolation of vibration, which is usually in the lower range of frequencies and does not include the airborne vibrations known as sound, there is one basic formula-which is important in the solution of the problem. It is the formula of transmissibility as governed by the equation: where T = transmissibility of the support. / = frequency of the vibratory force. /u = natural frequency, of the machine unit on its support (Damping = 0). Equation 12 shows that the transmissibility approaches unity for disturbing frequencies considerably lower than the natural, frequency of the mounting. As the disturbing frequency is. increased the, transmis sibility is also increased until at the resonant frequency, where / = /n thetransmissibility becomes infinite. This is not true in practice because all materials have some internal damping effect. However, operating at or very close to the resonant frequency is always serious as forces and stresses may be multiplied 10 to 100 times. As the disturbing frequency becomes greater than the natural frequency the transmissibility becomes a smaller quantity and at the value of ///,, = \/2 it again has the value of unity. Beyond this point true isolation is first accomplished'. At a ratio of 3 to 1 for / to /,, the isolation is effective enough for practical application, and