In the paper Acoustic metasurface-based perfect absorber with deep subwavelength thickness by Yong Li and Badreddine M. Assouar, is presented a “a metasurface-based perfect absorber capable of achieving the total absorption of acoustic wave in an extremely low frequency region” [cit.]

I designed and 3D printed the stl file of the model present in the study, out of curiosity.

My model has a slit where a holed sheet of plastic can be inserted to complete the acoustic absorber.