In this lecture we again pick up the simplest case, one-dimensional waves propagating in a pipe. What we've found from these lectures, acoustic waves are dominated by three essential parameters. One is pressure, acoustic pressure, and then the other one is velocity, particle velocity of medium, and the last one is the density of fluid. In this case, this is fluctuating density, and then this is acoustic pressure, in other words, fluctuation, fluctuating pressure. And these three elements, not independent, but related each other, okay, P and v related by Euler's equation. And the P and rho is related by a state equation. And v and rho is related by conservation of mass. Okay, and these three variable, Are related by three equations. Therefore, we can have equation that governs acoustic wave propagation. Because it has three variables and the three equations, therefore we can get one equation that governs everything that is a wave equation. By seeing the details of these relations, we can find out how the pressure of velocity and density is related. And also, because those are governed by wave equation, that means if you solve this wave equation, depending on the boundary condition and the initial condition, the solution essentially provide us how the wave is propagating in space and time. In the next lectures, we will talk about the units of these three variables, pressure, velocity, and rho, because without knowing the units it does not have a physical meaning.