Acoustic cutoff frequency
= Acoustic cutoff frequency
{title2=$\omega_{\rm ac}\simeq c_s/(2H_\rho)$}
A stratified atmosphere can reflect low-frequency <acoustic waves> while permitting sufficiently high-frequency waves to escape. In a plane-parallel isothermal atmosphere, the angular acoustic cutoff is approximately $c_s/(2H_\rho)$, where $H_\rho$ is the density scale height. Its detailed form depends on the atmospheric profile and on the wave-variable approximation.