Strutt|Mechanical Services|Duct Breakout|Flex Duct Breakout inserts the breakout (in dB) for 3 m (10 ft) sections of non-metallic flexible duct into the active row of the worksheet.
Radiated duct breakout for flexible duct (according to AHRI) is not directly proportional to length. Most breakout occurs in the first 1-2 ft [0.3-0.6 m] of the duct. The calculated losses are for 10 ft [3 m] of length but can be used for any length up to 10 ft [3 m].
The breakout values are given by the formula in Table D4 of AHRI 885-2008:
`0.00179 f + 10.79 - (0.0000563 f + 0.41419)D`
where,
`f` is the ocatave band mid-frequency, Hz
`D` is the diameter of the flex duct, in inches (the lower bound of `D` is 100 mm (~4 inches) and the upper bound of `D` is 406.4 mm (16 inches)).
Note that the flexible duct breakout values are effectively insertion losses (i.e. difference between Lw,in and Lw,out) not true transmission losses.
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