In places where there is a fracture or where the layers of material have come loose from one another, the sound will be different from the sound made where the hull is intact. Though widely used, this procedure is a costly, time-consuming and inaccurate way of detecting damage. In this test, which is called ESPI, vibration is induced in the entire hull and measured using a combination of laser and video techniques. Fractured or damaged areas will vibrate more strongly than intact areas. This method of testing is much faster and more reliable than the coin tapping method.
Vibration is induced in a panel similar to a ship's hull using pressurised air.
Using a laser, the vibration patterns over the entire panel are recorded in real-time .