MIT engineers develop paper-thin speakers that turn any surface into an active audio source. These membrane speakers produce very little distortion and use a fraction of the energy of conventional speakers. The palm-sized speaker demonstrated by the research team weighs about a dime and can produce high-quality sound no matter what surface it is glued to. To achieve these properties, the researchers pioneered a deceptively simple fabrication technique that requires only three basic steps and can be scaled up to produce ultra-thin speakers large enough to cover the interior of a car or wallpaper in a room. A typical speaker in a headphone or audio system uses a current input through a coil to generate a magnetic field that moves the speaker membrane, which pushes the air above it to produce the sound we hear. Instead, the new speaker simplifies speaker design by utilizing a shaped film of piezoelectric material that moves when a voltage is applied, pushing the air above it and producing sound. They tested their membrane speakers mounted on a wall 30 centimeters from the microphone, measuring and recording the sound pressure level in decibels. When a current of 25 volts is passed through the device at 1 kHz (1000 cycles per second), the speaker produces 66 decibels of high-quality conversational sound. At a current frequency of 10 kHz, the sound pressure level increases to 86 dB, roughly equivalent to the volume level of urban traffic. This energy-efficient device requires only about 100 milliwatts of power per square meter of speaker area. In comparison, an average home speaker might consume more than 1 watt of power to produce similar sound pressure at similar distances.
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