Document BvME9rDZxqKgG83Eng34j3zVw

1034 CHAPTER 40 1958 Guide Selection of the Absorptive Material When a sound wave impinges on the surface of a porous material, a vibrating motion is set up within the small pores of the material by the alternating sound waves. As the ratio of the cross-sectional area of the pores to their interior surface is small, the resistance to the movement of air in the pores is large. This viscous resistance within the pores of the material converts a portion of the sound energy into heat. The decimal fraction representing the absorbed portion of the incident sound wave is called the absorption coefficient. Considerable absorption may also re sult, particularly in the low frequency range, from the flexural vibrations of the duct. In the selection and application of the absorptive material, the following points should be considered: Sound Control 1035 the duct termination (grille) must be converted to sound pressure levels in the room, because it is sound pressure to which the ear and the sound level meter respond. The sound pressure level at a distance r from the grille opening and at an angle 8 with respect to an axis perpendicular to its surface is given by the equation18 L,, = PWL + 10 log,. (j^-t + + 0.5 dbe (24) where PWL = sound power level emitted at the duct termination and is the power summation of the attenuated fan noise and the grille noise in dbe (See Example 8). Fig. 15. Four Typical Means for Terminating a Ventilating Duct in a Room A. Duct projecting in the room. B. Duct in the center of and flush with the wall. C. Duct in the center of one edge. D. Duct in the corner. The listener L is at distance r and angle & from the duct opening. 1. For the absorption of the low frequencies below 500 cps the material should be 2 to 12 in. thick. Thin materials, particularly when mounted on hard solid surfaces, will absorb only the high frequencies. 2. In order to provide as much low frequency noise absorption as possible by means of flexural vibration, it is desirable to fasten the absorptive panels discontinuously. This result may be attained to some extent by spot cementing, but better results are obtained when it is possible to fasten the absorptive panels to furring strips, leaving an air space behind. However, the exact resonance characteristics of the panels, and thus their absorption, are so unpredictable that flexural vibration cannot be relied upon for a specific value of attenuation. Requirements for a good sound absorption material are: (1) high ab sorption at low frequencies;10 (2) adequate strength to avoid breakage; (3) fire resistance and compliance with national and local code requirements; (4) low moisture absorption; (5) freedom from attack by bacteria and al gae; (6) low surface coefficient of friction ; (7) particles should not fray on at the higher design velocities; and (8). freedom from odor when either dry or wet. DETERMINATION OF ROOM LEVELS In the previous sections on fan and grille noise, sound power levels were given in dbe referred to 10"watt. These quantities' as they appear at FREQUENCY t LENGTH x to'* = et/>000 IN cpa - INCHES 16. QFig. Directivity Factor for the Four Duct 15Configurations of Fig. B W Mfor (A, exceptthe valueof q isgiven forV=45 deT ^ requency / is in cps and the duct is assumed to be square with an area L* i square inches. Q = directivity factor and is a dimensionless function of 0. r = distance from the duct opening, feet. a = the room constant, square feet. (R increases with the amount of ac- coustical absorption in the room and generally depends somewhat upon frequency.) In Fig. 15 four methods are shown for terminating a ventilating duct in a room. In each of these cases, the noise power radiates into the room. Low frequency sounds radiate equally in all directions. High frequency sounds tend to "beam" in the direction the duct opening is facing. The magnitude f this beaming effect is described mathematically by the di rectivity factor Q. After the sound at any frequency has reflected from wall, it generally travels around the room many times to produce reerberant sound. This fact appears in Equation 24, wherein the first t>rm m the parenthesis described the direct sound (prior to reflection) and timis)COn^ *rm Ascribed the reverberant sound (the result of many reflec- thJd6 directiYity factor Q is of importance only when the listener is near the fiUC+ ^'creation, i.e., when r is small. At large distances, i.e., when hears only the breecvoemrbeesrannetglsigoiubnled. compared to the second, the listener