Speakers: Parts Is Parts - PVDF Applications in Headphones, Microphones, and Pickups (Part 2)

This article explores Polyvinylidene Fluoride Film (PVDF), and how it is used in various transducers for headphones, microphones and loudspeakers. 

The first part of this article explored polyvinylidene fluoride (PVDF) piezoelectric film loudspeakers. Now, our exploration of this technology focuses on its many other successful applications — from headphones, microphones, to guitar pickups, and more recent developments in PVDF electro-active polymer (EAP) products for haptic vibration in headphones, which creates an extreme bass sensation. Aside from these applications, we will discuss why PVDF film transducers are relevant for audio design including their lighter weight without neomagnetic structures, the piezoelectric effect without the use of lead (which is now used now in piezo ceramics), and the availability of semiconductors for driving the high voltage needed for high-polymer films.

What we have is a transducer technology with unique capabilities and design advantages. Lightweight and flexible, piezoelectric film transducers can serve as highly reliable low-cost alternatives to more expensive speakers, headphones, mics, and musical instrument sound pickups.

Headphones

The flat frequency response over a wide range is a consequence of the PVDF polymer film’s softness, which eliminates the self-ringing found in brittle materials such as piezoelectric ceramics. In the late 1970s, Pioneer introduced a well-reviewed series of PVDF film headphones — the SE-300, the SE-500, and the SE-700 (see Photo). These products were advertised as being “the world’s first high molecular polymer stereo headphones.”

Photo 1: The Pioneer SE700 Headphone was among the first PVDF film headphones on the market.

PVDF film headphones have three practical downsides. First, they are barely efficient enough to play from the average receiver headphone jack let alone a smartphone’s headphone output. Second, a piezoelectric device is electrically a capacitor, and some amplifiers have difficulty with reactive lossy loads.

This aspect is a real deal breaker for mainstream acceptance today. High voltages are required and PVDF film transducers aren’t very efficient. However, the classic step-up transformers aren’t pocket portable. On the other hand, a purpose-built high-voltage swing amplifier that can drive lossy capacitive loads is readily achievable, but it requires integration into the headphone. Since PVDF requires high voltage and not high power, the power amplifier necessary to drive PVDF film loudspeakers could be designed with much lower power/current requirements yielding a more efficient, compact, and economic system.

Today, DC-DC and boost converters, voltage doublers, and the like are inexpensive, readily available high-efficiency power integrated circuit (IC) devices. Third, PVDF stretches and it’s difficult to obtain enough excursion from the semi-tensioned PVDF film diaphragm for it to reproduce bass at high output. In the 1980s, Sony also had a development project for PVDF film. Toshitaka Takei, the team’s lead developer, designed  a loudspeaker using piezoelectric film. Because of the difficulties driving them with existing amplifiers, development at Sony was halted. While Sony never commercialized the product, Takei eventually left Sony and founded TakeT around 2003, a low-production and expensive headphone manufacturer that uses PVDF film (see Photo).

Photo 2: The TakeT H2 headphones’ design resolves the low frequency problems of limited excursion by using a piezoelectric air motion transformer.

Totally unique, TakeT’s approach was to form the PVDF film into a variation of Dr. Oskar Heil’s air-motion-transformer invention. This is a brilliant idea as it provides both stiffness through topology and more excursion and compliance though the corrugations (much like a spider/damper gets its excursion from the corrugations rather than stretching the fabric itself). Besides its headphone line, TakeT also manufactures a PVDF film super tweeter.

Digital Loudspeaker and Microphones

Digital loudspeakers and headphones that can provide direct conversion of sound from digital sources have been theorized for decades. PVDF has the bandwidth and may lend itself to this. Another “science project application” for PVDF film is focused ultrasonic beam parametric array speakers that can place audio in a specific location over long distances. This category has a few players, but Woody Norris’s ATCO (now LRAD, which stands for long-range acoustic device) probably made the most hype.

Currently, this technology was spun off to Parametric Sound. This unique field includes HyperSonic Sound (HSS) from TK Geomedia (now owned by Turtle Beach Corporation) (see Photo) and Audio Spotlight from Holosonics. A few others have dabbled in this esoteric field including Sennheiser’s AudioBeam and Mitsubishi Engineering. Others have patented variants but not commercialized their inventions. Implementations have included arrays of piezoelectric mics (used as ultrasonic radiators), electrostatics, and PVDF film.

Photo 3: An HyperSound HSS highly-directional speaker provides an effective means of projecting highly directional sound beams.

Back in the 1970s, the Allen Clark Research Centre in England reported on the design of a pressure-operated PVDF microphone. Later, the Plessey Company reported on a noise-canceling first-order pressure-gradient-operated microphone using PVDF elements in a bimorph edgeclamped configuration. As with electrets, the mass was small and exhibited high immunity to vibration pickup. The advantage of the PVDF design was its immunity to stray magnetic fields, eliminating more costly assemblies.

Extensive research in PVDF transducer technology was also conducted by the Matsushita (Panasonic) organization in Japan. At the Audio Engineering Society (AES) Convention in 1977, Naono, Rikow, et al, presented the most comprehensive report to date concerning PVDF microphone applications.

They detailed curvature mode of operation PVDF elements with design equations and experimental models. PVDF mics were theoretically and experimentally analyzed with cylindrical and spherical diaphragms at the Technical University of Darmstadt in

Germany. Researchers demonstrated that geometries were possible with PVDF that offered good mechanical and thermal stability. The PVDF also made it possible to vary the diaphragm tension and damping with a back plate/support mechanism.

Read the complete article, originally published in audioXpress, May 2013, now available online >>>


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