The concept of a light driven speaker was proposed as a new category of light driven actuator, and its feasibility was
experimentally demonstrated. If a diaphragm of a speaker is made up of a light-driven actuator, sound would be produced
upon irradiation of a modulated light. In this scheme, the light doubles as the signal and the energy source. With a
conversion efficiency of 1 %, an audible sound is supposed to be generated from a 100 mW light source, therefore,
energy-wise light actuated speaker should be possible. The limitation on the energy conversion efficiency of this scheme is
discussed taking into account the energy loss due to the presence of a carrier wave that accompanies transmission.
Unfortunately, a material that satisfies the requirements for this scheme is not found at present. Yet, since the purpose of
this work is to show the feasibility of this concept and evaluate its possibility, it is an option to employ a combination of
materials. In this preliminary study, we chose to combine a piezoelectric actuator and a solar cell with an appropriate
interface for matching impedance. In the experiment, an 808 nm diode laser beam was modulated at audio frequency, and
shined a solar cell. The audio signal was reproduced from the connected piezoelectric speaker as an audible sound. The
maximum sound output of 3.1 mW was obtained. In a different scheme the maximum energy conversion efficiency of 4.4
% was obtained. These results show that the light actuated speaker is indeed feasible.
As one category of light-driven actuators, we propose a light-actuated device which outputs sound. To verify the
feasibility of the concept of the light-actuated speaker, we performed experiments using a scheme where the light energy
is first converted into an electrical current which then drives the connected speaker. The method seems rather indirect,
but given the efficiency of the solar cell, which can reach around 25% with the use of an appropriate wavelength, and
with the adoption of impedance matching between the solar cell and the speaker, a high overall conversion efficiency is
expected. With its realization, it would be possible to create a sound system that is wireless and can be controlled
remotely without the need of a power supply which will provide advantages over the conventional electronic
transmission such as wireless operation with no external power source, and precise control of listening area compared to
radio transmission. Possible applications include a sound guidance system for the visually impaired.