 Two different modes can, on the other hand, have coincident nodal diameters if the ratio of their indices m is an integer number.A single pressure point different from the centre identifies therefore a mode only having a diameter and a circle passing through that point.The points which “discriminate” better the frequency modes are, however, those near the centre, because the nodal circles become densely packed towards the perimeter and a single point therefore tends to have many of them very near to it.