Speakers: Parts Is Parts - High-Polymer Film, The Lost Speaker Technology (Part 1)
Exploring the Potential of High-Polymer Piezoelectric Film
In another article from the "Speakers: Parts Is Parts" series, written by Mike Klasco and Steve Tatarunis, the history of Polyvinylidene Fluoride Film (PVDF), a piezoelectric polymer used in various transducers for headphones, microphones and loudspeakers, is revisited.
The high-polymer element is polyvinylidene fluoride film (PVDF) and its application possibilities seem less familiar to speaker hobbyists than any other type of drivers. In fact, it seems that the audio engineering community at large is either not familiar with PVDF film or misunderstands PVDF piezoelectric film technology. Hopefully, this article will serve as an introduction (or reintroduction) to PVDF technology.
In 1985, Jesse Klapholz (a former Journal of the Audio Engineering Society editor) presented a paper, “High Polymer Piezoelectric Film in Electroacoustical Transducer Applications,” at the 79th Audio Engineering Society (AES) Convention. Klapholz’s introduction stated: “Since the discovery of piezoelectricity by the Curie brothers in 1880, there has been continued research and development because of piezoelectricity’s practical and economical applications as well as scientific importance.
Piezoelectricity, simply defined, is ‘pressure electricity’ and is a property of certain materials that alter their shapes under the influence of an electrical field, or vice versa. Many different types of piezoelectric transducers have been manufactured over the years, for example: phonograph pickups, microphones, strain gauges, vibration pickups, underwater sound sensors, and tweeters.”
More than 28 years have passed since Klapholz’s talk, and while there have been a half-dozen relatively technically successful efforts involving high polymer film transducers for audio (e.g., speakers, microphones, headphones, and even air-motion transformer high polymer tweeters) only the first launch — Pioneer Electronics’s HPM speakers in the mid-1970s — made much of a commercial impact.
There are several reasons to revisit PVDF film today. Its lighter weight is better for autosound applications. It can be used as an alternative piezoelectric solution, eliminating the lead issues found in most ceramic piezoelectric speakers. The electronics for the high-voltage drive is possible now with projected sound from ultrasonic beam speakers and the potential for direct digital-sound reproduction.
Past PVDF Speakers
More than a half-dozen PVDF speakers have been commercialized. But, for the most part, these speakers have not made it to the mainstream users, so people might be unfamiliar with all these valiant PVDF designs.
PVDF technology was introduced to the audio world when Pioneer Electronics announced a series of viable products in the mid-1970s. Masahiko Tamura and his co-workers at Pioneer’s Acoustical Engineering Research Laboratory in Saitama, Japan produced high-quality PVDF film microphones, phono cartridges, stereo headphones, and tweeters. Their work was a turning point for PVDF piezoelectric film because it showed its commercial viability in audio applications. The Pioneer HPM-100 was a four-way system designed by Bart Locanthi and his team of former- JBL engineers, all of it that is, except the super tweeter, which came from Pioneer’s Japanese research group (see Photo).
HPM stands for “high-polymer molecular,” in reference to Pioneer’s then newly developed high-polymer molecular film “PVDF” super tweeter. The film was shaped semi-cylindrically to project 180° sound. This driver’s frequency response ranged from 12 to 25 kHz. Locanthi continued to support Pioneer’s high-polymer film speaker development, but he also lead its development of the Technical Audio Device (TAD) pro group studio monitors, beryllium-diaphragm compression drivers, and concert-sound speaker systems. Pioneer’s Japanese team continued to develop the HPM series.
After the initial introduction of the semicircular high-polymer super tweeter, most new implementations were cylindrical and omnidirectional, with at least a horizontal approximation of a pulsating sphere. The HPM-200 was another four-way design, but it boasted high-polymer drivers operating at 2,000 Hz and higher.
The HPM-150 used a new version of the PVDF tweeter, which was then horn loaded, but it retained its omnidirectional characteristics (see Photo). By 1981, Pioneer had discontinued the HPM series, perhaps due to the high cost of the sophisticated designs and issues with the metallization corroding (attributed to the high levels of air pollution in many US cities 30 years ago). Another issue was extending the response below 2,000 kHz did not lend itself to practical implementations. PVDF film response at the bottom-end tends to require a larger radius, and getting down to the lower midrange proved difficult. Today, the idea of a 2.1 or 5.1 system that uses all PVDF drivers with a common subwoofer is a distant dream. Matsushita (now Panasonic), Mitsubishi Electric, Sony, Samsung, and others also had active PVDF film speaker projects through the 1980s. While some of these efforts yielded technical papers and patents, the programs never resulted in commercialized products from these brands.