Imagine a duct,
6"x6"x10' (15cm x 15cm x 3 meters). 6"x6" of airflow.
Build the duct with a perforated metal sheet absorber (sheet steel with thousands of 1/8" holes (3mm), with 2" (5cm) to 4" (10cm) of rigid rockwool behind it, then a couple layers of drywall, another 2" of rockwool and then plywood, but having 6"x6" of air path).
If there are no bends or curves,
and you speak into one end,
can a friend at the other end hear you?
I'm guessing yes. But it's been asserted that they can't.
It's been asserted that the drywall increases attenuation through the assembly.