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768 CHAPTER 41 1948 Guide tt = temperature of air leaving duct, Fahrenheit degrees. U = temperature of air surrounding duct, Fahrenheit degrees. *' XJ = thermal transmittance coefficient, Btu per (hour) (square foot) (Fahrenheit degree). V = fluid velocity, feet per second. Fa ~ velocity of air in duct, feet per second. Vm = velocity of fluid, feet per minute. W =* weight of air through duct, pounds per hour.. x = thickness, inches. * REFERENCES 1--See Chapter X for definition of standard air. *--A.S.H.V.E. Research Paper--A New Friction Chart for Round Ducts, by D. K. Wright, Jr. (A.S.H.V.E. Transactions, Vol. 51. 1945, p. 303). --Friction Factors for Pipe Flow by L. F. Moody {AS.MJL, Transactions, VoL 66,1944, p. 671). --Friction Charts for Gases Including Correction for Temperature. Viscosity and Pipe Roughness. bOyctRob. eDr.. 1M9a46d)is. on and W. R. Elliot (A.S.H.V;E. Journal Section. Heating, Piping and Air Conditioning, 5--A.S.H.V.E. Research Report No. 1211--Pressure Loss Caused by Elbows in 8-inch Round Vents* lpa.t3in3g5)D. uct, by M. C. Stuart, C. F. Warner and W. C. Roberts (A.S.H.V.E. Transactions, Vol. 48.1942, --Loss of Pressure Due to Elbows in the Transmission of Air Through Pipes or Ducts, by F. L. Busey CA.S.H.V.E. Transactions. Vol. 19.1913, p. 366). Pressure Losses in Rectangular Elbows, by R. D. Madison and J. R. Parker (Ecatint, Piping and Air Conditioning, July. p. 365, August, p. 427, September, p. 483,1936). BIBLIOGRAPHY Method of Determining Rectangular Equivalents and Weights of Ducts, by Peter Frainck (A.S.H.V.E. ber, 1940). Journal Section, Heating, Piping and Air Conditioning, Decem The Flow of Liquids, by W. H. McAdams {Refrigerating Engineering, February, 1925, p. 279). Air Conditioning and Engineering (American Blower Corp.). Fan Engineering (Buffalo Forge Co.). Heat Power Engineering, by W. N. Barnard, F. O. Ellenwood, and C. F. Hirshfeld, Part III (John Wiley and Sons). Mechanical Engineers' Handbook, by Lionel S. Marks (McGraw-Hill Book Co.). Frictional Resistance to the Flow of Air in Straight Ducts, by F. C. Houghten, J. B. Schmieler, J. A. Zaiovcik, and, N. Ivanovic (A.S.H.V.E: Transactions, Vol. 45, 1939, p. 35). Analysis of Factors Affecting Duct Friction, by J. B. Schmieler, F. C. Houghten, and H. T. Olson (A.S.H.V.E. Transactions, Vol. 46, 1940, p. 193). The Flow of Fluids in Closed Conduits, by R. J. S. Pigott {Mechanical Engineering, Vol. 55, 1933, p. 497). Mechanical Similitude and Turbulence, by T. von Karman (translated and reprinted as Technical Memorandum N.A.C.A. No. 611, 1931). Turbulent Flow in Pipes, with Particular Reference to the Transition Region between the Smooth and Rough Pipe Laws, by C. F. Colebrook {Journal, Institute of Civil Engineers, Vol. II, 1938-39, p. 133). Evaluation of Boundary Roughness, by H. Rouse (Proceedings Second Hydraulics Conference, University of Iowa Bulletin 27, 1943). The Flow of Air in Ducts, by E. Kemler {Healing and Ventilating, May 1936, p. 38). Performance Tests of Asbestos Insulating Air Duct, by R. H. Heilman and R. A. McArthur (AS.H.V.E. Transactions, Vol. 44, 1938, p. 197). A Study of the Data on the Flow of Fluids in Pipes, by E. Kemler {A.S.M.E. Trans actions, Vol. 55, 1933, Hydraulics, p. 7). .A Rational Method of Duct Design, by L. G. Miller (A.S.H.V E. Transactions, Vol. 43, 1937, p. 71). Chapter 42 SOUND CONTROL Unit of Noise Measurement, Apparatus for Measuring Noise, General Problem, Kinds of Noise9 Noise Transmitted Through Ducts, Design Room Noise Level, Noise Generated by Fan, Natural Attenuation of Duct System9 Duct Sound Absorbers, Air'Supply Noises, Grille Selection, Cross Transmission Between Rooms, Controlling . Vibration from Machine Mountings IN ventilating and air conditioning a building or a room, the effect of the mechanical system employed must be considered on the acoustics of the space conditioned. It is important to consider also that the use of air conditioning often permits keeping the windows closed, thus giving relief from certain external noises, but at the same time increasing the ' necessity- of-providing adequate sound .control. It is assumed that in a given space the architect and acoustical engineer have produced a room or rooms which are satisfactory for speech, music,, or other uses. The ventilating engineer's sole function is to ventilate and air condition these rooms properly so that they will be physically comfortable without adding any acoustical hazards. UNIT OF NOISE MEASUREMENT According to an international standard, two terms are used for noise measurement. The decibel (db) is the physical unit for expressing in tensity or pressure levels. The phon is the unit of loudness level. The loudness level, in phons, of any sound is by definition equal to the in tensity level in decibels of a thousand cycle tone which sounds equally loud. The decibel is defined by the relation N = 10 logio -y-, where N is the *0 number of decibels by which the intensity flux 7j exceeds the intensity flux J0. The intensity flux is the measure of the intensity of a sound wave and is defined in terms of watts per square centimeter passing through a unit area of wave front in a freely traveling plane wave. It is usually more convenient to select an arbitrary reference intensity for J0 and express all other intensities in terms of decibels above that level. For this purpose a reference intensity of 10"1* watts per square centimeter has been selected. This intensity is slightly less than the threshold of audibility for the average ear at a frequency of 1,000 cycles per second. This reference level also corresponds to a pressure of 0.0002 dynes per square centimeter for sound in air at usual room temperatures. A stated sound level in decibels, unless otherwise defined, will thus be related to a threshold of I0-16 watts. For example, a level of 60 db above this reference threshold is 10*10 watts. In a similar manner, when sound measurements are given in actual intensity or energy units, they can be converted to decibels by this relation. Since the decibel is based on a ratio, it can only be employed when related to a reference threshold level as given. Noise levels, which vary with frequency as well as intensity, must not only be related to this reference threshold level, but also to a reference frequency, which is taken as 1000 cycles. These terms and procedures may be found in Standards1 published by the American Standards Association. 769