Abstract: A sheet metal media free sound attenuator reduces noise from machinery and gas flow in duct systems, that has a solid outer shell box and at least one flow passage made of perforated sheet. The flow passage can have a gap called expansion chamber and supports that divide cavity between a wall of the perforated passage and a wall of the outer shell box. Entrance transition and exit transition of the sound attenuator provides smooth flow into the attenuator, reduces pressure drop, and increases attenuation. In the preferred embodiment, the perforated sheet is a perforated spiral sheet.