Speakers haven't changed much over the past few decades, and speaker technology is one of the most mature and stable technologies available to the electronic musician. In particular, the radiation patterns of various drivers are well known and difficult to manipulate, which has led designers to try many different configurations of cones, domes, ribbons, and horn-loaded compression drivers to create speakers that have optimized directional characteristics.
One approach to controlling the directivity of a single driver is to construct it from a thin, flexible sheet of plastic that can be shaped in various ways. However, this presents two problems: finding a type of plastic that responds to electrical signals, and then finding a way to permanently attach electrodes to the plastic.
Koh Seok-keun, president and CEO of South Korean startup Plasma & Ionbeam Corporation (www.plasma-ion.com), may have a solution to both problems. In the 1980s, when he was working on his Ph.D. in the United States, Koh began working on drivers made from thin sheets of plastic. He used a type of plastic called polyvinylidene fluoride (PVDF), which exhibits piezoelectric properties. A material that has such properties can be made to vibrate when exposed to an electric current, but he was unable to reliably affix electrodes to the plastic film.
Koh dropped his research for a time and instead worked at the Korean Institute of Science and Technology on other projects, including the study of ion-assisted reaction (IAR) surface-modification technology. This technology immerses a material in a plasma gas at temperatures exceeding 10,000° Celsius and irradiates it with low-energy ion beams to change the characteristics of the material's surface.
FIG. 1: A cylindrical plastic-film speaker exhibits an even radiation pattern at all frequencies.
IAR was initially used to improve the efficiency of heat exchangers in air conditioners, but Koh soon realized that the process could also be used to change the adhesion characteristics of PVDF to allow platinum electrodes to remain affixed to it. Specifically, the surface becomes hydrophilic — that is, it exhibits an affinity for water molecules — which plays an important role in bonding the plastic to the metal atoms in the electrodes.
Koh established P&I Corporation in 2000 to commercialize the IAR process for various applications, including plastic-film speakers. As with other planar-speaker designs, such as electrostatic panels, the P&I plastic-film speakers can't reproduce frequencies below 50 Hz, but they are said to match the sonic performance of conventional speakers above 300 Hz.
There are many applications for plastic-film speakers. They can be made transparent and placed in front of computer monitors or TV screens; they can even be made into banners or kites with graphics or text printed on them. The flexible plastic film can be shaped and molded as needed; for example, it can be formed into a cylinder with a very uniform radiation pattern (see Fig. 1), allowing it to be incorporated into a light sconce and other architectural design elements.
FIG. 2: A semicylindrical midrange/tweeter driver could be mated with a conventional woofer to cover the entire frequency spectrum while radiating the mids and highs evenly along the vertical axis.
Of most interest to electronic musicians is how this technology might be applied to studio monitors. One of P&I's prototypes is a speaker that has a conventional woofer and a semicylindrical PVDF midrange/tweeter (see Fig. 2). Given the uniform radiation pattern of a cylindrically shaped film driver, such a design could allow the mids and highs to be evenly dispersed throughout the vertical axis. Alternatively, the PVDF driver could be mounted the other way to disperse the sound horizontally.
Koh anticipates that plastic-film speakers could be seen in commercial products as early as this year. I look forward to seeing how this technology might be applied to benefit electronic musicians everywhere.