Abstract: A sound attenuation apparatus which is to be connected to an airflow duct and is to receive the air from the duct. The sound attenuation apparatus includes a mass of plates located in a stacked arrangement with peripheral gaps being formed between the plates. The airstream is to be conducted through an opening formed between the plates. The airstream is to be conducted into an opening formed within the center of the plates and then discharged into the ambient through the peripheral gap area. Each of the plates include a mass of holes.

Abstract: A sound attenuator for use in a rigid walled duct through which gas is to flow which comprises a cylindrical hollow shell within which is mounted a helical, sound absorbing, vane assembly. The interior of the wall of the shell is formed of a metallic wire screening. The shell itself is formed of a soft fiberglass mat for sound absorbing purposes. A similar fiberglass mat is included within each vane. Also, each vane includes a layer of lead.