Audio Microscope by Joe Davis
Part of my installation at Ars Electronica is undertaken in collaboration with Katie Egan and pertains to a question about two "singing plants". At a point in time now almost exactly two years ago, a young pre-med student approached me with an interesting question. She had recently returned from South America where she had carried on field work in the Ecuadorian rain forest. There she had encountered a Native American brujo or "medicine man". The brujo had told her that a given species of plant in the mountains sings a different song than the same species of plant in the valley. The student wanted to know if it was possible to "listen" to plant cells.
All acoustic phenomena, including "sound", are the result of mechanical movements of physical objects within or upon the surface of a solid, gaseous, or liquid acoustic medium such as steel, air, water, etc.. In the case of the acoustic phenomena we call "sound", the movement of physical objects occurs at or close to audio frequency so that the resulting waves or pattern of waves passing through an acoustic medium do so at audio frequency. When these audio frequency waves impinge on the human listening apparatus (the inner ear) the result is that "sound" is perceived in the human brain.
To begin with, it seemed to me that the problem wasn't that cells are naturally "mute". Many of them - and their flagella, cilia, pili, etc., - are normally at least partly engaged in activities that appear to occur at audio frequency. Further, no non-dormant living organisms are known to exist in vacuum or otherwise outside of an acoustic medium.
At the time, there were to my knowledge no existing microphones of sufficient sensitivity to register microacoustic signatures of individual (microscopic) cells. The function of conventional microphones generally depends on the mechanical motion of crystals or diaphragms that react to impinging sound waves. Sound waves generated by individual cells or microorganisms are simply too weak to effect such movements in mechanical listening apparatus.
Conventional microphones translate audio frequency sound waves into audio frequency electrical (electromagnetic) signals. These electrical signals may then be routed through amplifiers, equalizers, and other electronic audio equipment and eventually into speakers or earphones where electric signals are transduced back into "sound". At the speaker, sound is created when a electromagnetically-driven diaphragm or crystal produces corresponding sound waves in surrounding air .
At the turn of the last century Alexander Graham Bell built what was probably the world's first optical transducer of sound waves. He called it a "photophone". Instead of translating sound into electrical signals, Bell built an apparatus that turned sound waves into audio frequency pulses of light. He also built "detectors" that would convert audio frequency pulses of light into electrical signals that could then be converted into sound. To construct my audio microscopes I also used optical detectors and specially illuminated stages and microscope slides that allow only light reflected from the surfaces of specimens to enter the objective lens of microscopes . These optical signals are then transduced into electrical signals via detectors mounted on the microscope eyepiece. The electrical signals are subsequently routed through more or less conventional audio equipment so that they may then be perceived as sound in the ear/brain of the user/observer.
At early stages of this work I was surprised to find a wide range and diversity of information in the microacoustic world. At lab we find organisms on almost a daily basis that we have never seen or listened to before. We therefore now routinely listen to organisms for the first time. Different organisms make different sounds in the way that say, the sounds of horses are perceived as different than the sounds of sheep. My experiments with spectrum analysis tend to reinforce that notion. I found that slightly different acoustic signatures corresponded to slightly different species of microorganisms. Paramecium multimicronucleatum for instance, has a slightly different audio signature than Paramecium caudatum. The signatures of a given species however tend to be uniquely distinct to that species. So as it turns out, the two plants of the same species must indeed "sing the same song", unless perhaps the Ecuadorian brujo knows of some exceptional organism unlike those we have observed to date.