1. The problem statement, all variables and given/known data Assuming that an OWC device is floating/moored in monochromatic waves. When there isn't air pressure distribution inside (atmospheric condition) the device behaves as an undambed body (with the oscillating chamber open to the atmosphere). When there is air pressure oscillation inside the OWC which is the natural frequency of the device? 2. Relevant equations For an undamped moored body, the equation of heave motion is: (-iw(M+A33)+B33+i/w(C33+K33))*U=Fz where c33 is the restoring spring coeff equals ρgAwl; K33 is the vertical mooring stiffness; M is the body's mass; A33 and B33 is the heave added mass and damping coeff; U is the vertical velocity; Fz is the exciting wave forces and w the wave frequency. Thus the natural frequency in heave motion equals to w=sqrt[(c33+K33)/(M+A33)] For a moored OWC device the equation of motion equals to: (-iw(M+A33)+B33+i/w(C33+K33))*U+F*P=Fz Here F is the complex damping coeff due to air pressure oscillation inside the OWC and P is the air pressure head. The air pressure term, P, from equation of volume flows, can be written as: P=(Qd+Qr*U )/(Λ-Qp), where Qd is the exciting volume flow (due to diffraction problem); Qr is the motion radiation volume flow (due to motion radiation problem) and Qp is the pressure radiation volume flow (due to pressure radiation problem or 7th degrees of freedom radiation problem). 3. The attempt at a solution Assuming that the air inside the OWC is incompressible, which is the natural frequency of the OWC device? It should be different from the undamped body, but can't figure out how to calculate it.