7. Direct visualization of vibrations

As early as the 17th century, vibrations were the subject of analysis, and laws were formulated concerning the mechanics of the periodic manifestations of stretched strings, pendulums, beams and plates.

The German physicist Chladni (1756-1827) was the first to discover that by placing fine sand, or better still dried lycopod powder, on a vibrating plate, vibratory patterns, even of high modal rank, are easily visualized by the grouping of these fine grains drawing nodal lines, the locus of minimum vibratory amplitudes (figure 10 ).