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3. Field Installation Loss <br />It is common practice to assume that the laboratory -measured value of a <br />partition can be 5 dB lower when it is installed in the field. <br />4. Average Room Absorption/Exterior Surface Area <br />The sound pressure level in the receiving (interior) room can be estimated <br />from the following expression: <br />L2 - L1 - (TL - 10 log S + 10 log A) -6 <br />If it is assumed that a typical exterior wall has an area of 15 square <br />meters, the effective laboratory -rated transmission loss (or STC here) <br />could be as much as 10 dBA lower than the laboratory value with a hard room <br />and the same for a very absorptive room. For purposes of standardization, <br />an average room was selected, giving room correction of -5 dBA. <br />5. Number of Walls Exposed <br />As noted in the Wyle study for the FHWA (Davy and Skale, 1977), the number <br />of walls can have a significant impact on the effectiveness of any exterior <br />wall. While three or even four exterior surfaces could exist in the most <br />unfavorable situation, a single wall was chosen for the standard case. <br />6. Angle of Incidence <br />The angles at which sound from aircraft flights might be expected to <br />hit exterior surfaces can be estimated for particular flight paths. This <br />could raise the effective transmission loss of the wall or its components <br />by as much as 5 dBA, which is the approximate difference between random and <br />field incidence (Beranek 1971). However, since the Model Ordinance is <br />intended to apply to a wide range of locations and structural orientations, <br />a random angle of incidence was selected for determining STC requirements. <br />1 <br />1 <br />45 <br />