Document RKxY9MMg6VOZq2OOYwkkGwx7

American Society of Heating and Ventilating Engineers Guide Chapter 18--Sound Control possible to design filters which will suppress the ordinary noises ' to the ventilating or air conditioning of buildings4.. Inci4 In general, the motion of air resulting from the ventilating of r not sufficient to introduce any appreciable difficulty in auditoriums00111^ where noise may originate from the issuing of high-speed air from ' However, by proper stream-lining of .the nozzles, it is possible to0*-' with speeds which are adequate for all practical purposes without*^ ducing any disturbing noises. Since sound is propagated with a velJEHl of more than 1100 fps, the velocity of the air would have to attain JSSi of at least 20 to 30 fps before. these wind velocities would have^* appreciable influence upon the propagation of sound. If there is to be any appreciable motion of air in an auditorium advantageous to have the upper layers of air moving in a direction full the stage toward the audience, as this will tend to refract the sound down toward the audience. However, unless the speed of the air ;s^ great as 20 or 30 fps, the amount of refraction will not be noticeabi Therefore, as a rule themotion of air in an auditorium does not have lj appreciable effect upon the acoustical properties of the room. EFFECT OF HUMIDITY UPON ACOUSTICS Recent experiments6 have shown that both the humidity and the perature of air have a marked influence upon the rate of absorption high-pitched sounds. Perfectly dry air is less absorptive than air cofi taining any amount of water vapor. At relative humidities of 5 to 25 peg cent, the air is highly absorptive but becomes less and less absorptive aa the humidity is increased. High-frequency sounds are propagated! better in cold humid air than in hot dry air, and since high-frequency! sounds are particularly important for the preservation of- good quality! in speech and music it is advantageous to maintain the air in a room afq relatively high humidity, not less than about 55 to.60 per cent. On the] other hand, where it is desirable to. absorb all frequency components oil sound, as for the reduction of noise in offices, it is advantageous to maiii| tain relatively dry air. The time of reverberation in a room is given by the following equation! where t - 0.049 V 4mV -- 5 loge (1 -- a) V -- volume of room in cubic feet. 5 = interior surface of room. a = average coefficient of sound-absorption of the interior surface of the room. m = the absorption coefficient of the air in the room. The coefficient m depends upon the frequency of the sound and the; humidity (and probably the temperature) of the air. At a temperature of| 70 F, and for sound waves having a frequency of 4096 vibrations per| second, m = 0.0027 at 25 per cent relative humidity, 0.0018 at 54 per/ - Moois at 82 per cent. It will be seen, therefore, that the absorp- jn the'air is twice as great at a relative humidity of 25 per ^r^Wat a relative humidity of 82 per cent. This explains why 1^'^tKe Pen trave' s0 much better on humid days than they do on !r-in ? Although this dependence of absorption upon humidity is m df low-frequency as well as high-frequency sound, the actual absorption in the air is negligible for frequencies below about ^^'ijfations per second. However, the absorption of the higher the air is a significant factor, and its. dependence upon calls for careful consideration in planning the air-conditioning - gSt for buildings. Mir ^ PROBLEMS IN PRACTICE ,V?i|i?VFibat are the requirements for good hearing in a room? from noise, adequate loudness of speech or music, uniform distribution of .IffiTthroughout the room,, freedom from echoes and sound foci, no pronounced reso- "-and proper reduction of reverberation. jjga|6|S - \thy do modern improvements in the acoustics and air conditioning ofbuildings present new acoustical problems to the heating and ventilating engineer? /[IScoustically treated rooms, both outside and inside noise are reduced, and conse^fnoStiy the noise of ventilating equipment becomes more noticeable. The closed ^windows in air conditioned buildings exclude outside noise, which makes all inside noise ./fnjmvmechanical equipment seem louder. -.>3 'Name the acoustical problems which should be solved in connection with ' the installation of heating or air conditioning equipment. `/Selection of quietly operating equipment; adequate insulation of walls surrounding the 'Veqnipnient room; mounting of all vibrating equipment on flexible supports which will id!inmate solid-borne vibrations; design of suitable sound filters to reduce the trans- , imssion of noise through ventilating ducts; the use of suitably low air speeds and stream- SfiSg, where necessary, to prevent eddy noises. if-'-f/l . - F; 1 * Are good heat insulators also good sound insulators? fesAt* rule, no. Blankets and felted materials offer considerable insulation for sounds of high frequency, but very little for soundsoflow frequency.- pH?-- :hV3# What is the principal consideration in the selection of elastic supports for ^insulation of machinery vibration?. . fc' Thesupport should besocompliant that the natural frequencyof the massof the machinery |.* on its elastic support will be low in comparison with the vibrational frequencies which are /' to be insulated. ^ 6 # What means should be utilized for preventing air-borne noise from the b ventilating equipment from being transmitted through the walls, ceiling, or ] floor of the equipment room? 1 "'tenor walls and ceiling of the equipment room with absorptive material; see How Sound is Controlled, by V. O. Knudsen (A.S.H.V.E. Transactions, Vol. 37. 1931). ^ jp^that all doors and windows to the equipment room fit tightly in their frames; and use Effect of Humidity upon the Absorption of Sound in a Room, by V. O. Knudsen (Journal Acottstud . Society of America, July, 1931). Also see report presented at the May, 1933. meeting of A. 5. of A. * *], and floor and ceiling partitions which have .an insulation value of not less than 50 db~ 312 313