The Hydra Cornelius is a laser light show that visualizes music by shining a laser beam through rippling water. The surface of the water is deformed by vibrations from a small speaker causing it to acts as a dynamic lens. The audio input does not drive the speaker directly, but rather modulates a carrier frequency which is output to the speaker. The patterns created by the Hydra are very similar to those created by sunlight reflecting off the surface of a swimming pool.
The optical path begins with a 100mW red laser. The beam is diverged and then passed vertically through the dynamic water lens. The dynamic lens is simply a 1mm layer of water on a sheet of clear plastic. Ripples are created in the water by a speaker connected to the plastic. The laser beam is diverged again after the water and before exiting the Hydra Cornelius. The optics are all vertical because the water lens needs to be horizontal, meaning that the pattern is always projected onto the ceiling.
The dynamic lens has several high-Q resonances. Only resonant frequencies will produce large ripples in the water surface, suitable for creating patterns. We only drive the speaker at the resonant frequency. At startup, an oscillator is manually tuned to one of the resonances. The audio input modulates the amplitude of the oscillator, which drives the speaker.
The optical system consists of a 100mW laser, beam expanding optics, and the dynamic lens. The laser and beam expanding optics are fairly straight forward. We needed a 100mW laser to make the pattern bright enough because it is expanded to several feet in diameter.
We used a short focal length concave lens to diverge the laser beam before it hit the dynamic lens. We used a similar concave lens to further diverge the beam after passing through the water. The complexity of the patter can be changed by changing the size of the beam entering the water. A bigger beam produces a more complex pattern. We opted for a beam diameter of about 1cm. The second lens expands the pattern so that it covers a large area of the ceiling. We used a concave lens followed by a meniscus. The second set of lenses can also be used to move the pattern around on the ceiling by moving them slightly off axis. Unfortunately, we do not know the focal lengths of any of the lenses we used.
We spent the majority of our time trying to understand and perfect the dynamic lens. The dynamic lens is simply a sheet of transparency film with about 1mm of water on the surface. The transparency is supported around the edges and is driven in the middle by a rod connected to a speaker.
The electronics are build around a LM13700 OTA used as an AM modulator.
We would like to thank Ivan Celanovic for his advice.