That is really the crux of the matter for this conversation: using your eyes to perceive what sound waves are doing in a room. This week we read a description of the mechanical apparatus used in this innovative approach to bridging the sensory divide that renders acoustic wave behavior sometimes difficult to understand.
The light rays incident to the glass bottom, as shown in Figure 1, are parallel and perpendicular to the glass bottom. This is an ideal condition which should be approximated as closely as conveniently possible. Apparent distortions of the true wave form due to divergent light source or misalignment may not be so critical to a general demonstration as to an experiment in which data are being taken from the images on the viewing screen.
The typical light source for the ripple tank should be a heavy duty slide projector with remote control and two brightness levels. As the projector is moved farther away from the ripple tank the divergence of the light is reduced but so is its intensity. A utility compromise of 20 feet proved functional. At that distance the projected area of light is at least 5 feet across and blinding to the ripple tank operators. An opaque slide with a small hole inserted into the projector narrows the light beam to only the area of the mirror below the tank.
A Strobotac set next to the projector with similar light shielding provisions and frequency synchronized to the output of the amplifier provides a stop motion of the wave action which is often useful. The two light sources have different colored lights and simultaneous operation is also useful.
The Ripple Tank
The ripple tank is a rectangular glass plate sealed in a 2 by 3 foot and 7 in. deep wooden frame which is finished with marine boat paint. It is removable from the rigid angle iron frame which holds it in place. The wooden frame is held together with screws and sealed with caulking so that the glass can be removed if necessary. It is supported by a rollable carriage which is illustrated in Figure 4.
Centered beneath the tank is a 2 by 2 foot mirror which is mounted on a hinged and adjustable backboard. The mirror is of the expensive type which is optically flat. It is usually set to an angle of 45°.
Above the ripple tank is a 2 foot square glass plate in a wooden frame which is adjustable in height. Tracing paper is placed on it for wave analysis and is removed if the image is to be projected onto the ceiling. The nominal height of 1 foot above the water surface provides focus of ripple images. The lower the frequency the higher the focus height will be.
The Ripple Tank System
The ripple tank system consists of three discrete and coordinating subsystems: optical, ripple tank, and wave generating. As depicted in the block diagram of Figure 5 the fundamental system source is a sine wave generator which can handle the very low frequency range of 0.5 to 50.0 Hz. The signal amplifier which can handle this low frequency range is in practice a DC amplifier. It feeds into an impedance of the DC value for the speaker which is nominally 4 ohms and must be able to supply a minimum of 5 watts. The Strobotac requires a 2 volt signal to operate.
A problem at low frequency generation occurs because the 2 volt trigger level of the Strobotac results in over-driving of the waves and visual confusion of images is the result. A heat dissipating L-pad used for remote volume control of a speaker should be added to the input of the speaker. Addition of a valved side branch which opens to the air acts as a pressure control bleed for the pneumatic drive system.