Vibrations of loudspeaker membranes cause air pressure waves, which our hearing system perceives as sound. As long as they are travelling through the air, these sound waves remain invisible to human beings. But as Ernst Chladni already describes in his book Entdeckungen über die Theorie des Klanges, in 1787, sound can be visualised by bowing a surface lightly covered with sand. So-called Chladni patterns are produced in this way. In her piece Touche Nature (2017) I-lly Cheng creates a phenomenon similar to Chladni patterns by placing transparent bowls filled with water on loudspeakers. The sound waves are visualised as water waves:
A contact microphone is attached to the bowl and amplifies the bowl itself and the water movements in the bowl. Four percussionists each play a bowl filled with 1,000 ml. of water. During the performance the percussionists produce sounds by moving the water with their hands, rubbing the bowl, throwing stones in the bowl, or pouring more water into the bowls. These sounds are all picked up by the contact microphones and then amplified through the PA system. The water sound itself is also picked up by four condenser microphones and amplified directly through the loudspeakers in the hall.
As the first page of the score shows, the sound is processed with a pitch shift effect in the computer and there is also some feedback used. The feedback is created by sending the signal of the contact microphones back to the four loudspeakers underneath the bowls. There is no direct feedback happening; but two loudspeakers and two microphones together produces one big feedback loop. The sound picked up by the contact microphone of player 1 is sent to the loudspeaker of player 3, which is then picked up by the contact microphone of player 3, creating a feedback loop by sending it to the loudspeaker of player 1. The same feedback is created between player 2 and 4.
You can hear different kinds of effects, including some feedback, at the very beginning of the piece, performed here by We Spoke:
During the piece I-lly explores the sonic world between natural water sounds and more abstract percussive rhythms. At the end of this fragment some pre-produced sounds are played through the loudspeakers, creating big movements in the water. These movements cannot only be seen but the water sound caused by these movements is amplified through the condenser microphones. The loudspeakers seem to become liquid themselves: