Wen-Ching Ko

A Thin Light Flexible Electromechanically Actuated Electret-Based Loudspeaker for Automotive Applications

An electromechanically actuated electret-based loudspeaker made with an organic material membrane is presented, which is small in volume, extremely thin, and lightweight for light vehicle applications. As obtaining good sound quality in a vehicle is a complicated and difficult task, the electrostatic thin-film actuator can be cut into any shape and then assembled into an electret-based loudspeaker for placement at any location within the vehicle. The sound field in a vehicle can be optimized by properly tailoring the electret-based loudspeaker performance and choosing the most appropriate location for the loudspeaker for optimal sound performance. In addition, it is possible to design a novel 3-D sound field. Our newly fabricated 100-mm-by-100-mm electret loudspeaker weighs only 4 g and is 1 mm thick. It only consumes 0.15 mW to generate a sound pressure level at 88 dB at 1 kHz, which is very energy efficient for electric car applications. In summary, our newly flexible electret-based loudspeaker has tremendous potential for future automotive applications due to its excellent advantages over current vehicle loudspeakers.

On the improvement for charging large-scale flexible electrostatic actuators

Recently, the development of flexible electret based electrostatic actuator has been widely discussed. The devices was shown to have high sound quality, energy saving, flexible structure and can be cut to any shape. However, achieving uniform charge on the electret diaphragm is one of the most critical processes needed to have the speaker ready for large-scale production. In this paper, corona discharge equipment contains multi-corona probes and grid bias was set up to inject spatial charges within the electret diaphragm. The optimal multi-corona probes system was adjusted to achieve uniform charge distribution of electret diaphragm. The processing conditions include the distance between the corona probes, the voltages of corona probe and grid bias, etc. We assembled the flexible electret loudspeakers first and then measured their sound pressure and beam pattern. The uniform charge distribution within the electret diaphragm based flexible electret loudspeaker provided us with the opportunity to shape the loudspeaker arbitrarily and to tailor the sound distribution per specifications request. Some of the potential futuristic applications for this device such as sound poster, smart clothes, and sound wallpaper, etc. were discussed as well.

Simulation and experimental study of flexible electret-based loudspeaker vibration modes by electronic speckle pattern interferometry

Electret-based electrostatic devices have been used in the electro-acoustic field for decades. Recently, the improvement of its charge retention has been of interest for application to the field of smart materials. Hence, the flexible electret-based loudspeaker has become an important research topic for futuristic applications such as 3C (computers, communications and consumer electronics) and smart curtains. The volume velocity and the on-axis sound pressure level (SPL) of an electret loudspeaker are the key parameters of interest. To study the vibration characteristics of an electret diaphragm, a finite element analysis (FEA) was introduced to facilitate the design. To validate the finite element analysis (FEA) model, an out-of plane full-field non-destructive optical detection method which incorporates electronic speckle pattern interferometry (ESPI), was applied to determine the vibration mode shape of thin film. By driving the electret loudspeaker at different frequencies, a corresponding vibration mode of interest was detected with the ESPI set-up. Both the simulations and the experimental results obtained on the measurement platform are detailed in this paper.

A double-sided electret polymer film-based electrostatic actuator

A solution made from blending cycloolefin copolymer (COC) and polystyrene (PS) was proposed to create a double-sided electret polymer film. This electret polymer was then sandwiched to form an electret-metal-electret structure by using the MEMS processes. The upper and lower polymer layers were found to both enhance charge storage capacity significantly and to improve the machining property. It was identified that lower concentration of PS led to sphere-like morphology distributed uniformly within the COC/PS blends, which created better electret properties than that of pure COC or pure PS polymers. In addition, it was also found that these COC/PS blends have better adhesion to both metal and polymers. A series of processes developed to optimize this line of new electret polymers for actuator development are detailed. The recipe of this new material and the associated fabricating process to develop an electret loudspeaker are also detailed. In comparison, the pros and cons of this speaker system versus a typical electrostatic loudspeaker or a headphone, which require both bulky and expensive DC-to-DC converters, are detailed as well.

Tailoring the performance of flexible electret loudspeakers by varying cell actuator formation

We present a design of electret-based soft cell actuators which can improve the performance of flexible electret loudspeakers. Electret loudspeakers have advantages over traditional speakers such being thin, lightweight and flexible. They have potential application to 4C products (e.g. computers, communication, consumer electronics, and cars). Flexible electret loudspeakers are composed of an array of small cell actuators. The first mode of each electret cell actuator greatly affects the low frequency of an electret loudspeaker. To tailor the bandwidth of an electret loudspeaker for different applications, the shape and size of each cell actuator can be varied according to the specific requirements. A finite element analysis (FEA) model was used to simulate a vibration mode. An AVIDTM (advanced vibrometer/interferometer device) system and ESPI (electronic speckle pattern interferometry) system were two non-destructive optical detection tools used to detect vibration and the corresponding vibration mode shape of the electret cell actuators, respectively. Our experimental results verified the simulated results. We showed that finite element analysis (FEA) can be used to design flexible loudspeakers. We can tailor the sound characteristics of an electret loudspeaker by varying the size and shape of cell actuators such that flexible electret loudspeaker performance can be dramatically improved.

Study and application of free-form electret actuators

In this paper, electret based actuators were developed to create a novel free-form loudspeaker. The flexible electret actuator sheet can be easily mass produced in roll or sheet form for good cost efficiency. Compared to the rigid structure of traditional electret actuators, this novel loudspeaker structure is not only thin, flexible, and lightweight at 685 grams/m2, but also it can be cut into any shape to fit various applications and requirements. The free-form electret loudspeaker is composed of centimeter-scale cell electret actuators which possess a bandwidth from 550 Hz to 55 kHz. To demonstrate the benefits of this new technology, a large area loudspeaker was created by joining by 350 pieces of B4-sized electret actuator sheets to create a projection screen 9 m (L) × 3.5 m (W) × 1.1 mm (H). The frequency response of the loudspeaker was measured to have a nearly flat response between 500 Hz and 4 kHz with less than 6% total harmonic distortion (THD) between 400 Hz and 6 kHz. The capacitance value (1.1 μF) of the loudspeaker was limited only by the current capability of the amplifier in the high frequency range. Another creation was a loudspeaker made of over 200 leaf-shaped electret actuator sheets. This unique flexible loudspeaker design was displayed and demonstrated at a year-long public exhibition. The sound level generated by these free form electret speakers were measured and are discussed in this paper.

A blended polymer electret-based micro-electronic power generator

Recently, power harvesting technologies for low-power electronic devices have attracted much interest. In this paper, the design and fabrication methods of a micro-electrostatic power generator is presented. This power generator comprises a stator developed using an electret film for charge storage and a rotor covered by an interdigital electrode for electric power generation. The newly developed electret material is made from mixing two solutions. The first solution was made by blending polystyrene (PS) and cycloolefin copolymer (COC). The second solution was obtained by an additive process as polar molecule was added into COC. This unique two solution electret method can easily be integrated and adopted to the micro fabrication process. The charge storage capability of this new electret material was investigated and results showed that low concentration of polystyrene in the blended material will not only have more stable but also higher electrostatic charge than that of pure COC. In addition, the polar molecular additives also improve the electret properties of COC due to micro-cavities formation and the interactions between molecules and polymer. Our newly developed blended electret material has excellent mechanical properties and is easy to use when compared to using Teflon Fluorinated Ethylene Propylene (FEP) and polypropylene (PP). A feasibility study of a micro electrostatic power generator based on our blended electret material was performed. Experimental results demonstrate the feasibility and effectiveness of this new type of micro electrostatic power generator.