Sound attenuation apparatus and methods

A sound attenuation apparatus for arrangement on a mounting surface of a duct, comprising:

• a body with an interior extending between a bottom surface facing the mounting structure and a top surface;
• and
• a stacked cavity structure comprising
• a structure inlet formed in the top surface of the body;
• a passage extending from the structure inlet into the interior of the body to a base surface within the interior of the body;
• a first cavity with a first cavity inlet fluidly coupled to the passage and being formed by a first arm, a first side wall within the interior of the body with a first side wall longitudinal axis, and the base surface, wherein the first arm extends from the first side wall and includes at least two first arm segments oriented at different angles relative to the first side wall longitudinal axis;
• and
• a second cavity with a second cavity inlet fluidly coupled to the passage and being formed by a second arm, the first side wall, and the first arm, wherein the second arm extends from the first side wall and includes at least two second arm segments oriented at different angles relative to the first side wall longitudinal axis.