It’s All About the Noise

For noise reduction applications, the interior pipe is perforated with thousands of 3/32" holes, which progressively reduce downstream noise while still protecting the insulation from air stream erosion. Dual-wall, insulated spiral pipe can also handle significantly higher air temperatures.

Round...

...Rectangular

Double-wall duct can be either round or rectangular. When it’s specified on a job, it’s done for one of two reasons: thermal or acoustic. In most cases, it's the latter—acoustical. Sound attenuation, which is a term for noise reduction, is achieved by using a perforated metal sheet that serves as the inner wall, wrapped in unfaced liner, and slid into the outer solid metal duct wall.

The small holes serve an important function: they help reduce noise traveling down the ductwork by drawing it into the insulation, something a solid inner wall would not achieve.

SMACNA specifications require the inner wall of perforated duct to be 22-gauge galvanized metal with 3/32" holes spaced 3/16" apart in a staggered pattern. For round ducts, it’s sold as a 5.394" slit coil that slides into spiral pipe. For rectangular ducts, it comes as a 60" x 120" sheet or a 60" wide coil, formed into a section of rectangular duct.