The D term in Eq. 14.39 can be ignored in short sections of duct, particularly when the duct is unlined and unwrapped. However, for lined duct it should be included. In internally lined ducts the attenuation term is usually larger than the breakout term. In flex or fiberglass ducts the breakout term may dominate, though transmission loss data for these products is difficult to obtain. The breakout sound power can never exceed the internal sound power.

Once the sound has penetrated the duct walls it radiates into the room at high frequencies as a normal line source. Equation 8.85 can be used to predict the expected sound pressure level in the room. Alternatively if the duct is long and unlined it radiates like a line source at high frequencies and the sound pressure level is given by where K is 0.1 for SI units and 10.5 for FP units.

At low frequencies if the duct is oriented perpendicular to two parallel walls it may excite resonant modes in the room, in which case the simple diffuse field condition does not exist (Ver, 1984).

Transmission Loss of Rectangular Ducts

The duct transmission loss for breakout of rectangular ducts is divided into regions by frequency that are similar to those discussed in Chapt. 9 for flat panels (ASHRAE, 1987). The transmission loss behavior with frequency is first stiffness controlled, then mass controlled, and finally coincidence controlled. Figure 14.11 shows the general structure of the loss for rectangular ducts.

For all but very small ducts the fundamental wall resonance falls below the frequency range of interest. In this region there is a minimum transmission loss that is dependent on the duct dimensions (a, b in inches and l in feet)

At higher frequencies, in the mass controlled region, there is a crossover frequency, below which the transmission loss is affected by the duct dimensions, and above which it follows

Figure 14.11 Interior to Exterior Transmission Loss for Rectangular Ducts

(ASHRAE, 1987)

Figure 14.11 Interior to Exterior Transmission Loss for Rectangular Ducts rf\

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