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Bone-Rattling Sound: New Speakers That Are Made From Bone

Bones have amazing electrical conductivity properties and, as one artist recently found out, can vibrate at the right frequencies to make a lovely macabre speaker set.

Turned on its head, bone's response to physical stress can be used to produce music---or at least musical tones. That's what artist Boo Chapple discovered during the course of a year-long collaboration at the University of Western Australia's SymbioticA lab, the only research facility in the world devoted to providing access to wet labs to artists and artistically minded researchers.

When Chapple began this project, she knew that extensive scientific literature suggested bone had what are known as piezoelectric properties. Basically, when a piezoelectric material is bent, compressed, or otherwise physically stressed, it generates an electric charge. Conversely, applying an electric charge to a piezoelectric material can change its shape. This has made piezoelectrics the backbone of countless environmental sensors and tiny actuators.

While many crystalline and ceramic materials have piezoelectric properties, bone behaves a little differently. Rather than coming from its primary mineral--hydroxyapatite--bone's piezoelectric properties appear to derive from the collagen that holds bone together and prevents it from being brittle.


What's more, some scientists hypothesize that the piezoelectric properties of bone are an essential part of the body's internal signaling mechanisms. In response to stresses like exercise, bone thickens at the points that experience the most strain. It's possible that bone "knows" what these points are because it's producing an electrical signal every time it's stressed. Scientists have exploited this property to speed the healing of bones, by applying an electrical current to them that, at least in theory, mimics the current the bone itself would generate if it were stressed.

Poring through this literature, Chapple realized that applying a current to bone at just the right frequency should make it vibrate like the diaphragm in an audio speaker. And because bone retains its piezoelectric properties even when it's no longer living, it should be fairly straightforward to transform any old bone into the world's most outre audio component.

Because Chapple is an artist and not a technologist, her goal wasn't to pursue this technique until it yielded a new product. Rather, the point was to accomplish what all good art can: "making strange" otherwise familiar objects. Here's how Russian writer Viktor Shklovsky describes it:

"The purpose of art is to impart the sensation of things as they are perceived and not as
they are known. The technique of art is to make objects “unfamiliar,” to make forms difficult, to increase the difficulty and length of perception because the process of perception is an aesthetic end in itself and must be prolonged."

The ultimate product of Chapple's work is a transformation perfectly in line with Shklovsky's notion of "defamiliarization," since it starts with an object we all know--red cuts of fresh meat from the butcher--and, with little more than some cleaning, polishing, and affixing of wires, arrives at an audio speaker.

Using a laser interferometer to determine the exact nature of the vibrations of the bone speakers that Chapple ultimately crafted from cleaned, dried slices of cow femur, she discovered that they had a frequency response of about 300 to 3,000Hz, or cycles per second. That's on the low end of the range of what a human can hear, which is typically between 20 and 20,000 Hz, so it's doubtful that even the most over-the-top high-end audio company is going to be advertising speakers made from once-living tissue.
To complete her work, Chapple needed to produce an installation that a casual observer could participate in. Looking at squiggly traces on an oscilloscope hooked to a laser interferometer was fine for the lab, but not suitable for a public exhibition. The problem was that the vibrations produced by the bone simply weren't powerful enough to be audible.

Having cast about for months for a solution to this problem, Chapple and her student William Wong stumbled into the idea of simply touching their bone speakers with a stethoscope. As she recounts in her paper on the speakers (PDF), "The sound was very soft but clearly audible. As I moved the frequency dial on the function generator I could hear the sweep of the tone coming from the bone. By this stage I was suspicious of anything that seemed to be a positive result, so I spent several days testing the set-up before I allowed myself to become excited."

Ultimately, the stethoscope worked, suggesting that a bio hacker who was so inclined could turn just about any slice of bone into a workable substrate for an in-ear speaker bud.

[Images: Flickr user dmitri_66, Boo Chapple]