and simplifying, we obtain the sound power level at the reference end in terms of the break-in transmission loss
where Lwoi = sound power breaking into the duct at a given point (dB) Lwo = sound power level incident on the outside of the duct (dB) ALTLoi = sound transmission loss from the outside to the inside of the duct (dB) D = duct loss correction term (dB) Ver (1983) has developed simple relationships between the breakout and break-in transmission loss values based on reciprocity. Above cutoff where higher order modes can propagate,
and below cutoff, af A LTLio - 4 + 10 log - + 20 log —
A LTLoi = the larger of
Note that for round and square ducts, the duct dimensions a and b are equal. The lowest cutoff frequency is given in Eq. 8.21, in terms of the larger duct dimension, a c fco = 2- (14.59)
The sound power impacting the exterior of the duct will depend on the type of sound field present in the space. Where the reverberant field predominates, the sound power level incident on the exterior is
where Lp is the sound pressure level measured in the reverberant field and the dimensions are in feet.
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