FIG. 1: The Noise Shirt picks up ambient sound and displays its level so that wearers can avoid environments that might damage their hearing.
Sometimes research in one area leads to interesting applications in another. Among the most famous examples of this phenomenon was the accidental invention of the toy product known as Silly Putty, which resulted from the attempt during World War II to develop a rubber substitute.
I recently came across another example that, while not accidental, clearly demonstrates the potential of cross-pollination between disciplines. A group of researchers at Tampere University of Technology (www.tut.fi) in Kankaanpää, Finland, is working on the concept of wearable technology — that is, incorporating electronics into normal garments that can be washed and worn like any other clothes.
What caught my interest was the TUT group's proof-of-concept prototype, which they call the Noise Shirt (see Fig. 1). They encased a small microphone, some processing electronics, and a series of LEDs in a flexible polymer casting, which they attached to the inside front surface of a pullover shirt. The mic picks up ambient sound, and the LEDs indicate its level in dB SPL. The LEDs form a ladder-type meter with five steps from 65- to 100 dB, with the bottom three being green and the top two (85- and 100 dB) being red. This corresponds with the European Union guidelines for hearing protection, which state that continuous exposure to levels at or above 85 dB can cause hearing damage.
Of course, electronics are useless without power. Rechargeable batteries are the best answer, but waterproof connectors for a detachable power cord are large and expensive. Fortunately, the Noise Shirt doesn't require much power, so the group determined that wireless inductive power transfer was the best solution. This technology is currently used to recharge electric toothbrushes and the like. The concept is simple: an electric current running through a coil of wire creates a magnetic field, which induces a similar current in another nearby coil, and the induced current is used to charge a battery. The only limitation is that the coils must be in very close proximity for the process to be effective.
The TUT group realized that the simplest and most sensible vehicle for wirelessly recharging the Noise Shirt was a clothes hanger with a flat, spiral coil of wire attached to the center, which can be plugged into an AC outlet. A similar coil in the neck of the shirt is encased in flexible polymer, along with a lithium battery. The shirt's coil aligns with the powered hanger coil, recharging the battery.
After experimenting with hand-wound coils of wire, the group refined them by etching flexible circuit boards (FCBs) with a spiral coil, allowing the battery and recharging electronics to fit on one board. The current system can recharge an empty battery in three hours, and a fully charged battery will run the Noise Shirt for two to four hours, depending on the level of ambient sound.
In the current prototype, the mic/LED module and battery/coil module are connected with plasticized wiring sewn into the shirt. There are, however, other ways to connect different modules in such a garment, such as Bluetooth or another wireless system. The wearer's skin could also be used to carry low-current signals.
Although the TUT group is focused on researching the basic concept of wearable technology without much regard to specific purposes, the potential for musical applications is intriguing, and hearing protection is just the start. How about a system that modulates various synth parameters with different physiological indicators (heartbeat, breath, galvanic skin response, and so on) for a truly organic musical experience? Or maybe a biofeedback system that includes all the sensor and audio electronics in a shirt? I look forward to seeing what creative minds come up with in the future.