An acoustic panel structure includes a perforated front sheet, a back sheet and a porous core between the front sheet and the back sheet. The acoustic panel structure also includes a non-perforated back cover which overlaps a portion of the back sheet. A portion of the back sheet is perforated and another portion of the back sheet is not perforated. An acoustic chamber is formed at least in part by the space between the back sheet and the back cover. The acoustic chamber may also be formed in part by the space between the front sheet and back cover. The space between the back sheet and the back cover is generally elongated in the x-y direction of the acoustic panel structure. This relatively thin acoustic panel structure is configured to attenuate long wavelength and low frequency noise.