The Ripple Tank, ca.1975 – Introduction
An appropriate complement to any theoretical discussion is a demonstration. In wave theory the classic demonstrator is the “ripple tank.” Development of a form of this wave visualizer appropriate to the study of acoustics is the project which this paper reports.
The ripple tank is an optical device by which the ripple wave action of a transparent liquid becomes visible via shadows and bright spots cast upon a viewing screen. An illustration of the arrangement used for this ripple system is Figure 1.
Light is passed through a glass bottomed pan and is refracted by the sloping water surfaces of the ripple wave leaving a shadow directly above on the viewing screen. The wave peaks and troughs have a flat surface and impinging light is not refracted but passes directly through to create bright spots on the overhead viewing screen. In practice, the ripple tank produces a broad and bright splay of light while the trough produces distinguishable but relatively narrow and dim splays.
Wave Generation
Early ripple tanks employed the vibrating tips of tuning forks to provide a regular vibration which would last some length of time before it would die down and have to be struck again. They provided single and multiple point sources. With the advent of electricity, cycling solenoids and eccentrically loaded rotating shafts driven by motors were used to provide variable frequencies and parallel wave generation. Initial experiments in this project found such mechanical type wave drives to produce not only the wanted frequency but an assortment of others which produced confused visual images, particularly at the lower frequencies.
A pneumatic wave drive was developed, as shown in figure 2, which provides a clean frequency response throughout the relevant frequency range of 1 to 40 Hz.
The driver of the pneumatic wave drive system is a hollow chamber whose open end is set just below the surface of the water thereby entrapping a quantity of air. As the air pressure inside the chamber is varied, it raises or lowers the water level inside the chamber which changes the local water level just outside the walls of the chamber, creating ripples.
The air chamber is connected by a rubber hose to a pipe which passes through a mounting plate. On the other side of the plate is mounted airtight a highly compliant acoustic speaker. Variations of air pressure due to speaker cone displacements are conducted by the rubber tube to the wave drive chamber.
The speaker is driven by an amplifier which is controlled by a signal generator. There is a “volume” control which regulates the intensity of the power to the speaker.
Wave Damping
Once a water ripple has served its viewing purpose it continues to travel about the ripple tank by reflecting off the walls. In a short time the view screen would be full of these reflecting waves if not for the ripple damping system which is installed around the perimeter of the tank. A sectional view of the tank wall and damping system is illustrated in figure 3.
The damping system consists of an inclined slope which is covered with fibrous material. A slope length of 2-1/2 in. is inclined 10° and positioned so that its upper edge is 1/4 in. above the mean water level, providing effective damping of even the strongest low frequency waves. Sheets of 1/4 in. industrial scouring material glued onto the slope provide the necessary energy dissipation of the waves.
– AMN 1975
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