Mass-spring equivalent of microbubbles

 P: 37 Hi, The Gilmore equation is associated with bubbles in gas. When equated to a mass-spring system, you get the following results. $$m_{effective}=4\pi R_{0}\rho$$ So the effective mass is given by the liquid volume that the bubble occupies. This is easy for me to see as it's effectively the spherical volume multiplied by the liquid's density. What I don't understand is the following $$k_{effective}=12\pi\gamma P_{0}R_{0}$$ where $$\gamma$$ is the ratio of specific heats, and $$P_{0}R_{0}$$ are ambient pressure and radius respectively. My notes say that this means the effective stiffness is provided by the compressibility of the gas. Could someone please explain how this is the case? Thanks.