Document 6b03n8Oj0XoebxGyJqz05ydom

598 Chapter '32 1945 Guide\ '-- Where more than one supply'opening must be considered, the problem . .is more complicated; If a similar supply opening is added in a far corner 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. 5. Although this method may place an unwarranted limit on velocitywhen 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. 5. 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 6f 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 the equipment room air may be trans mitted through the wall of the duct, thus reaching the air stream and be 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 ofNo. 22 gage metal is 24 db. When a.sheet of rock wool insulation 1 in, thick 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 insula tion value unless the material is dense, or unless it is surfaced with another gound 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 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 ejbows, dampers and similar obstructions; and noise transmitted from room to room where there is a common duct system. Sound Control . 599, CONTROLLING VIBRATION FROM MACHINE MOUNTINGS - It is impossible to select equipment which will operate without pro ducing 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. " - M uch 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 mounting 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 air borne vibrations known as sound, there is one basic law which is important in the solution of the problem. That is the law of transmissibility as governed by the equation: (12) where T = transmissibility of the support, to = frequency of the vibratory force.., ton = 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 w = wa the transmissibility becomes infinite.' This is not true impractice 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 w/wB = V^2 it again has the value of unity. Beyond this point true isolation is first accomplished. At a ratio of 3 to 1 for w to wn the. isolation is effective enough for practical application, and experience and economical design.have shown that a ratio of 5 to 1 is good. For high speeds, higher ratios for w to wn are easily attained and give better results for effective vibration control but for the lower speeds as experienced with compressor work the higher ratios become uneconomical. For a given installation the speed of the compressor is fixed by the specifications, therefore the'value of w is fixed. That leaves only wn to be determined and that is accomplished by. the choice of mounting material and design for the support of the machine.-- It is well to keep in mind that