Document omdJqnwz26QRvbG5B5X0JZZZ8

American Society of Heating and Ventilating Engineers Guide7i93j fan. in a certain high school auditorium was nearly as loud as aver speech in a large auditorium. Such an amount of noise is devastating good acoustics; in fact, it is impossible to hear speech in the presence ^ such a noise. ` db. 100 Boiler Factory 90 ""Electric Power Substation 80 Ventilating Room for Large Hotel (Very Noisy) Average Loudness of Music in Room-* 70 Conversation in a Small Room-* w-lnside of Duct, near Large Low Speed Fan +-tquipment Room I Average Condition l - 60 '*- Fan Room for School Building fRather Quiet) Speech in a Small Auditorium -* Guest Room. Large Hotel on Noisy Street (Windows Open) bO Speech m a Large Auditorium-* ^ Near Outlet of Ventilating Duct in High School Auditorium < Very Noisy. no"Filters" in Ductl 40 <*- Fan Noise in Theater < Poor Control of Noise) 30 20 Fan Noise m Theater (Proper Control of Noise I 10 _^_Near Outlet Of Ventilating Duct in M; G. M. Sound Studio < Planned Control of Noise i --------- -- Fig. 1. Chart Showing the Equivalent Loudness (in Decibels) of Speech, Music, and a Number of Noises Incident to the Ventilating of Buildings3 Acoustical Problems in the Heating and Ventilating of Buildings, by V. O. Knudsen (A.S.H.V.E. Transactions, VoI. 37, 1931). In every problem of noise reduction in buildings it is necessary to know how-much noise can be tolerated. The noise levels given in Table 1 may be regarded as completely inoffensive. They represent what might be termed ideal conditions, not often realized in existing buildings. How ever, they represent conditions which can be attained by proper control of noise, and the heating and ventilating engineer should aim to provide the degree of quiet specified in the table. In considering the tolerable room noise level due to heating,, ventilating, or air conditioning apparatus, not only must the absolute value of the noise be considered but also its relation to the room noise level without the apparatus running. This is necessary since a large increase of noise subjects the apparatus to serious criticism even though the level may be low:. It must also be borne in mind that the noise produced by the ap- 302 Chapter 18--Sound Control s is additive to that, of the room without apparatus. Thus if the P*ra " gqual, when combined the noise level will be 3 db higher. For areasons the room noise caused by the apparatus should not exceed jEfrther room noise. Noise Control Essential to the design of a satisfactory system are: first, a knowl, of the nature and intensity of the noise generated by the various rts of the equipment; second, a knowledge of how to vary the noise P ei between the apparatus and the conditioned room if need be; third a knowledge of the acceptable level of apparatus noise in the con ditioned room. Besides these, the engineer must be able to deal with other noises whichTnight enter the room-when openings are made into it; such as cross talk between rooms connected with common ducts, and noise Table 1. Acceptable Noise Levels Apartments, hotels, homes, small private offices........................................ Theaters churches, auditoriums, classrooms, libraries............................ Talking picture theaters, small clothing stores.......................................... Large public offices, banking rooms, upper stones of department Main floor of department stores..................................................................... 6 to 8 db 8 to 10 db 8 to 12 db 10 to 15 db 10 to 20 db 12 to 24 db 15 to 25 db 20 to 30 db 25 to 35 db 30 to 50 db 35 to 45.dD 40 to 50 db transmitted to portions of duct systems outside the conditioned room and thence to its interior. The problem of apparatus noise is receiving the study of equipment manufacturers who are aiming at both noise reduction and standardiza tion. Some manufacturers now have noise ratings available- for their equipment, while some pass each unit of equipment of certain types through sound tests during the course of manufacture. The problem of noise reduction from apparatus to room must take into consideration and treat separately the three modes of travel of noise to the room: first, from the apparatus through the air to the walls of the room and thence to its interior; second, through the building structure to the room; third, through ducts or openings to the room. Because the noise entering by each of these three channels is susceptible to quantitative analysis, solutions are available. Along with the transmission of sound through the building structure, the engineer must also consider the transmission of vibration, which may also be objectionable. The solution is not complete, however, until the effect of the noise entering the room on the room noise level is determined. ROOM NOISE LEVEL, COEFFICIENTS OF ABSORPTION One of the most effective means of reducing noises in ventilating equip ment is accomplished by the proper covering of the interior walls and 303