Infrasonicon icon. Click here for home page.INFRASONICON: Time-Compressing Infrasonic Recordings to Discover New Sounds, by Clark Huckaby

Overview, Part 4 (of 6):

Water Theremin

My water surface wave transducer (Figure 9) is based on the theremin. Electronic music pioneer Leon Theremin (1896-1993) invented the basic technology in 1919 (Ref. 7). His namesake was the first and most famous musical instrument designed to be played without physical contact. The distance between a theremin's pitch antenna and a player's hand ("ground") acts as a variable capacitor which determines the output frequency.

Figure 9. "Water Theremin"
Figure 9: The "Water Theremin," a transducer for water surface waves. Cover is removed to show guts. Some of the labeled parts are not discussed in this Overview, but are covered in the WT's Technical Page.

The main difference between the "Water Theremin" (WT) and a musical theremin is antenna deployment: The WT's antenna is insulated and points downward into the water (a minute drop of epoxy cement seals the bottom tip). In capacitor terms, the antenna's conductor is one plate, its insulation is the dielectric, and water touching the insulation is the other plate. The deeper the water, the greater the capacitance. With enameled copper wire, the transducer's amplitude limit increases with decreasing diameter. I used 36-AWG wire, which allowed a maximum amplitude of 30 mm crest-to-trough. In static tests, non-linearity was about six percent within this range. The WT's Technical Page presents the unit's schematic diagram and gives details about WT theory and performance.

The WT's antenna must be clean and smooth and the water free of oily film or pond scum. Also the water must contain some charge carriers (ions). Tap water and any natural water body has enough ions and performs about like brine. The WT's metal support stand makes a necessary ground connection to the water. The WT ignores distilled water, where it responds instead to the distance between antenna and any ground reference--like its musical cousin does. (This answers the question: Can you play a theremin under water? Yes, but only under de-ionized water.)

The WT is shown picking up signals on a peaceful body of fresh water in Figure 10. The corresponding time-compressed recording is Sound Example 5. To my knowledge, this is the first time water surface ripples and wavelets have been made into sound (but see Ref. 8). This recording finally transposes (reverses the direction of) a time-honored analogy and teaching tool--that sound waves are like water surface waves. The analogy is not perfect, of course; see my sidebar "Water Surface Waves Versus Sound".

Figures 10 (left) and 11 (right)
Figure 10 (left): Water Theremin in calm Lagoon. See Sound Example 5.
Figure 11 (right): Water Theremin in sink. See Sound Example 7 and Figure 12.

The WT needs refinement, however. For surface waves resonating in a sink (Figure 11), a close look at the waveform suggests distortion (Figure 12). Crests appear clipped in the low-amplitude part of the decay envelope. Maybe this helps the sped-up recording (Sound Example 7) to sound like a cheap mic on a badly tuned tom-tom (although "tuning" probably depends more on sink geometry). Antenna-water adhesion may be at fault: Using magnification while poking 36-AWG enameled wire through a water surface, I saw the wire attracting a small positive meniscus. Water climbed about 0.2 mm up a stationary wire, but was less during insertion and more during withdrawal. This and other concerns about the WT are discussed on its Technical Page. Also, my sidebar on the physics of water surface waves gives some background relevant to the WT.

Figure 12
Figure 12: Waveform of water resonating in sink. Corresponds to Sound Example 7. There is a disparity between presumed and recorded crests of low-amplitude waves. See also WT technical page.

Overview continues in Part 5==>

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References Cited in Overview Part 4:

7. Theremin:

8. Ocean Tide Historical Data Converted to Audio:

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