*One example, which is particularly apposite here, is a form of distortion known as 'ringing' which may occur in pressure measurements with conventional catheter-manometer systems It consists of high frequency oscillations (which may be mistaken for events within the ventrical) and is caused by exactly the same mechanism as the heart sounds themselves. There is an elastic membrane (the manometer diaphragm plus the catheter walls) coupled to a mass (the fluid in the catheter), and when this mass is set in motion by a sudden change in pressure at the catheter-tip the diaphragm is displaced, overshoots, and the system oscillates briefly. Damping is again supplied by the viscous properties of the fluid.