Noise Radiated by Circular Ventilation Ducts

Noise remains a major concern for building occupants, both in their home and workplace. Ventilation system is one of the noise sources in buildings. Usually, the main issue is the resulting noise level in the room. It is generated by the fan and the ductwork components, travels inside ducts, and is then radiated into the room by air diffusers, air inlets, and air outlets. But ducts also go through other indoor spaces. Airborne noise will pass through the duct wall and radiate in the surrounding space. This can be an issue for occupants. 
There is today no data available from manufacturers on noise radiated by ducts, no standard to determine the noise passing through duct wall and a lack of experience from acoustic consultants, despite results published in the 80's on breakout characterisation of rigid ducts. 
CETIAT performed a wide experimental campaign to characterize ventilation ducts, most of them with a 160 mm diameter, with varying parameters such as material (metal, plastic, PU), shape (round, oval, rectangular, corrugated), stiffness (rigid, flexible), and presence of an acoustical or thermal insulation layer. The experimental approach consists in producing sound inside the duct, and measuring both injected sound power level and sound power level radiated by the portion of duct under test.  
Test results show a wide variety of acoustical behaviour, from high insulation to high transmission, with in some cases ducts as transparent as if there were no duct. 
Beyond these results, several issues should be considered: how to make the measurement more reliable, how to define a metric to express the sound insulation ("breakout" is well suited for high insulation duct, but not for low insulation ones), how to deal with duct performance in between high and low insulation?