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. 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.