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.
A testing system contains an advanced vibrometer/interferometer device (AVID) and a high-speed electronic
speckle pattern interferometer (ESPI) was developed. AVID is a laser Doppler vibrometer that can be used to detect
single-point linear and angular velocity with DC to 20 MHz bandwidth and with nanometer resolution. In swept
frequency mode, frequency response from mHz to MHz of the structure of interest can be measured. The ESPI
experimental setup can be used to measure full-field out-of-plane displacement. A 5-1 phase shifting method and a
correlation algorithm were used to analyze the phase difference between the reference signal and the speckle signal
scattered from the sample surface. In order to show the efficiency and effectiveness of AVID and ESPI, we designed a
micro-speaker composed of a plate with fixed boundaries and two piezo-actuators attached to the sides of the plate. The
AVID was used to measure the vibration of one of the piezo-actuators and the ESPI was adopted to measure the two-dimensional
out-of-plane displacement of the plate. A microphone was used to measure the acoustic response created by
the micro-speaker. Driving signal includes random signal, sinusoidal signal, amplitude modulated high-frequency carrier
signal, etc. Angular response induced by amplitude modulated high-frequency carrier signal was found to be
significantly narrower than the frequency responses created by other types of driving signals. The validity of our newly
developed NDE system are detailed by comparing the relationship between the vibration signal of the micro-speaker and
the acoustic field generated.
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.
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