Duct Sizing
The equal-friction method: pick a pressure loss per metre and size every duct to it. Around 0.8–1.0 Pa/m is the usual starting point for commercial low-pressure work. Then check the velocity — friction rate alone will happily hand you a duct that whistles.
Roughness 0.09 mm is ASHRAE’s medium-smooth galvanised value; flexible duct is far rougher (around 3 mm fully stretched, and dramatically worse compressed). Velocity, not friction rate, is what makes a duct audible — keep branch runs near occupied spaces under about 5 m/s.
Design-stage estimates. A real project needs a full load calculation to the ASHRAE/CIBSE procedure, and ventilation rates are code-mandated — check the standard and jurisdiction that apply to your building. A helper, not a substitute for engineering judgment.