Document 71aaLe97B3VXVp1Q9LaMDpYao

permissible velocity becomes that corresponding to a total sound level of 36.3, or% approximately 800 fpm. If the room had been highly reflective with an absorption of less than 100, the 1 correction would be much more important. For instance, for a room of 35 sabins, '?* a correction of minus 3 db should be made, and .the maximum velocity corresponding to the 32 db total sound level would be approximately 600 fpm. j- Where more than one supply opening must he considered, the problem is more complicated. If a similar supply opening is added in a far comer of a highly absorbent room, the change in noise level at the 5 ft station at the first supply opening is small; however, if the room is small, or highly reverberant or both, the intensity at the 5 ft station may be almost doubled and the noise level increased nearly 3 db thereby. The simplest method of handling this problem is to treat the room as though all the air were being supplied by one supply opening. Thus, if two outlets, each supplying 1000 cfm are used, the value 2000 cfm should be used with Fig. 9. Although this method may place an unwarranted limit on velocity when used in a large room, it 'is seldom that such a room has a noise level low enough to justify a more complicated, though more exact procedure. In general, return grilles are selected for velocities about half the supply velocity, and when this is done, they may be neglected in sound computa tions. However, if supply and return grilles are the same size, resulting in the same face velocity, they must be treated as two supply openings. That is, if 1000 cfm are supplied and exhausted through grilles of the same area, 2000 cfm must be used in the solution with Fig. 9. CROSS TRANSMISSION BETWEEN ROOMS Ducts serving more than one room permit cross talk between the rooms and should be lined with acoustical material. Where the rooms are close together and the ducts short, the ducts should be sub-divided to provide 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 being conditioned or ventilated. Unless the ducts are lined, some of the mechanical noise from air in the equipment room may betrans-. | 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 the sounds in the equipment room should be lagged with material, such as cork, pipe covering or other sound damping material, to prevent the sound from entering 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 this purpose. Laboratory measurements have shown that the loss through a sheet of No. 22 gage metal is 24 db. When a sheet of rock wool insulation 1 j11thick and weighing 1.4 lb per square foot is added to this, the insulation value is increased to 29 db. In general, however, adding a layer of insulation or pipe covering does not 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, used in the case previously mentioned, would serve as an absorber of the sound transmitted through the duct walls, and thus act as a .Sound Control 949 Fio. 10. Room Absorption Correction Chart 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. 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 e ween the fan and the duct with a canvas sleeve which effectively restricts oise at this point. Noise may also enter the building through the mountinstli mo*'or anfi th fan. Flexible mountings should be provided in all will ns' *->ut ^ese mountings must be carefully designed so that they i actually reduce the energy transmitted between the machinery and the floor. If a flexible material is used, it is desirable to investigate installation so that it is not short-circuited by through bolts which are Properly insulated, and by electrical conduit which is not properly broken m attached both to the equipment and to the building.- The flexible tr ntl?g> if improperly engineered, may actually increase the . energy nsnutted between the equipment and the supporting floor. n ^e proper isolation of vibration, which is usually in the lower range