A sound attenuating baffle mounts within the duct section of an air distribution HVAC system, a ventilation system, or other air movement system for either air or gas streams, in which the duct section can be mounted directly to a fan or incorporated into a transmission duct. The sound attenuating baffle is typically oriented in the flow direction through the duct, and includes an outer casing containing sound absorbing material therein. An internal mass layer formed of a sound barrier material is suspended within the sound absorbing material which fills the outer casing. The mass layer increases the low frequency sound attenuation of the sound attenuating baffle.