Duct sound absorber

A fluid duct sound absorption system comprising:

• a duct having at least one side wall for directing flow of a fluid and having a longitudinal axis defining upstream and downstream directions;
• and
• a low frequency (LF) coupled pair of Helmholtz resonators comprising an upstream LF Helmholtz resonator and a downstream LF Helmholtz resonator positioned on the duct and separated by a first coupling distance, each of the upstream LF Helmholtz resonator and the downstream LF Helmholtz resonator having a first resonance frequency;
• a high frequency (HF) coupled pair of Helmholtz resonators positioned downstream from the LF coupled pair of Helmholtz resonators comprising an upstream HF Helmholtz resonator and a downstream HF Helmholtz resonator positioned on the duct and separated by a second coupling distance, each of the upstream HF Helmholtz resonator and the downstream HF Helmholtz resonator having a second resonance frequency that is greater than the first resonance frequency;
• and
• an internal sound reflector formed of a planar acoustically reflective material positioned inside the duct to reflect sound waves emanating from any of the Helmholtz resonators back to the same Helmholtz resonator,
• the upstream LF Helmholtz resonator and the upstream HF Helmholtz resonator being covered and at least partially filled with an acoustically absorbing medium.

An air duct for a motor vehicle, the air duct comprising:

• a duct having at least one side wall for directing air flow and having a longitudinal axis defining upstream and downstream directions;
• and
• a low frequency (LF) coupled pair of Helmholtz resonators comprising an upstream LF Helmholtz resonator and a downstream LF Helmholtz resonator separated by a first coupling distance, each of the upstream LF Helmholtz resonator and the downstream LF Helmholtz resonator comprising:
• a LF chamber portion bounded by at least one enclosure wall defining a LF chamber volume;
• and
• a LF neck connecting an aperture in the at least one enclosure wall to an aperture in the at least one side wall and thereby placing the LF chamber portion in fluid communication with the duct;
• and
• a high frequency (HF) coupled pair of Helmholtz resonators positioned downstream from the LF coupled pair of Helmholtz resonators, and comprising an upstream HF Helmholtz resonator and a downstream HF Helmholtz resonator separated by a second coupling distance, each of the upstream HF Helmholtz resonator and the downstream HF Helmholtz resonator comprising:
• a HF chamber portion bounded by at least one enclosure wall defining a HF chamber volume;
• and
• a HF neck connecting an aperture in the at least one enclosure wall to an aperture in the at least one side wall and thereby placing the HF chamber portion in fluid communication with the fluid duct,
• the LF neck of the upstream LF Helmholtz resonator and the HF neck of the upstream HF Helmholtz resonator being covered and at least partially filled with an acoustically absorbing medium.