真正影響個wave velocity 係complex refractive index. 佢除咗基本refractive index(n)仲考慮埋absorption coefficient (k).
而frequency 同溫度 越高 complex refractive index 越高.
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-20-S6-A856&id=242641
http://resource.npl.co.uk/acoustics/techguides/seaabsorption/physics.html