Help ASAP --- Missing Step --- Gas in a Tube I am dealing with finding normal modes of oscillations in a continuum I have no problems with the string example, but now I have a gas in a tube, one side of the gas has pressure= p_0 and the other side p_0+delta p, they're separated by a wall that is lifted at t=0. This is a tube closed in the extremes. So, I've started with the classic wave equation for the tube rho is the density, P is the pressure, psi the movement (second partial derivatire of psi on t = second partial derivative of psi on x, times - rho_0 )and proposing the typical solution psi(x, t)= A cos(kx)cos(omega*t+phi). I also know the relationship between the speed of sound and pressure and density, and the value of delta rho. I start with the boundary conditions to solve for k, and now it is time to write out the initial conditions, and from there solve for A and phi. According to the string example, one of these conditions will end up as a function f(x) to be solved by Fourier. The problem is, I can't see for the life of me which are the initial conditions in a gas! And I can't see what kind of function it would be either. In a string I just see where the string is at time=0 and what velocity it has (usually 0), and according to its shape, I find the f(x) to use Fourier with. Please, anyone answer this, I've been trying to find it for the last 5 hours with no luck, the library is closed and my exam is tomorrow morning. And I apologize for the lack of LATeX. Thanks, Sol.