Sound absorption coefficients differing with frequency

[gex]

Hi there, my understanding of absorption coefficients is that harder materials generally reflect more sound similarly to how a harder material will result a ball to bounce higher than a softer material. What I am having trouble finding research on and understanding is the reasoning behind the absorption coefficients of certain materials differing with the frequency of the sound wave.
-Thank you in advance :)

http://soundproofyourhome.com/absorption-coefficient-chart/

 

[Vailanor]

Absorption coefficients are a measure of how well a material absorbs sound. Different materials absorb different frequencies of sound differently, so the absorption coefficient will vary depending on the frequency of the sound wave. This is because different materials have different densities and composition, which affects their ability to absorb sound vibrations. Harder materials, such as concrete or metal, will generally reflect more sound than a softer material like foam or fabric, but the absorption coefficient of a material also depends on the frequency of the sound wave. Higher frequencies are more easily absorbed by dense, solid materials, while lower frequencies are more easily absorbed by more porous, lighter materials.