Hydromonochord: Visualizing String Vibration by Water Swirls

The hydromonochord is a horizontal vibrating string that just makes contact with the surface of a water bath. The motion of the string sets up a pattern of swirls on the surface of the water, thus complementing the usual pattern of nodes and antinodes. The device is based on the traditional monochord.1 A water basin (Fig. 1) has two slits in the opposite walls on the left‐hand side, with the width of the slits equal to the diameter of the string. Consequently, the slits function as fixed bridges and form the nodes of a standing wave. The string is bowed or plucked on the part outside the basin, and the frequency is controlled by the position of the variable bridge on the right‐hand end of the string. If the position of this bridge is related in a simple way with the length of the basin, patterns of swirls will occur on the surface of the water, visualizing the vibration of the string. We will present a series of experiments and show how to integrate them into the very first stage of teaching acoustics.