For noise reduction applications, the interior pipe is perforated with thousands of 3/32” holes, which progressively reduce downstream noise and still protect 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 done , it’s done so for one of two reasons: Thermal or Acoustic.  In most cases, it’s the latter of the two, Acoustical. Sound Attenuation—a fancy term indicating noise reduction—is achieved by means of a Perforated metal sheet that serves as an inner wall, wrapped in an Unfaced Liner and slid into the outer solid metal duct wall.

The small holes serve an important function: they aid in reducing noise traveling down the ductwork by pulling it into the insulation, something that a solid inner wall would not accomplish.

SMACNA specs the inner wall of Perforated to be a 22 ga Galvanized with 3/32” wide holes on 3/16” staggered centers. For Round, it’s a Slit Coil is sold as 5.394” that will slide in top Spiral Pipe. For Rectangular, it’s a 60” x 120” sheet or 60” wide coil formed into a section of Rectangular duct.

Wrapped with insulation before sliding into the solid section to the right of it