confusion

[asdf1]

Why is the density of standing waves in a cavity
G(f)df=((8*pi*f^2)/(c^3))*df?
Also that formula looks weird... doesn't look like it has anything to do with density...

[asdf1]

??? can somebody please help?

[Giantbab]

it relates to density afrer u change a bit in the formula itself.

[alibaba]

thats not true answer