Giving Off Good Vibes

Since their introduction smartphones have received a steady stream of enhancements, from faster processors to longer battery life and thinner case designs. At this point, they have been upgraded and optimized to such a high degree that adding any meaningful additional functionality, or shrinking the size of the case, can seem like quite a tall order. But there are still a few components — like the speaker and haptic motor, for example — that are long overdue for an overhaul.

Present speaker technology still relies on an energized coil and a physically moving cone to displace air and produce the sound waves that we hear. These components can only be miniaturized just so much. If they are made too small, they will not be capable of moving enough air for acceptable performance. Not only does this limit how thin a phone can be, but it also introduces some design trade-offs. Generally, speakers must be on the side of a phone case, which means that sounds will not appear to come from the actions seen on the display, as would be expected. This design also requires that holes be present in the side of the case, which allows foreign objects and moisture to interact with the internal electronics.

The linear resonant actuators typically leveraged for haptic feedback operate on similar principles as speakers, but produce vibrations at different frequencies. As such, they contribute to many of the same problems.

One interesting approach that can solve these problems involves repurposing the display itself as the speaker and haptic generator. As a large, flat, and somewhat flexible surface, the screen can produce both sound and haptic feedback if vibrated at the proper frequencies. This has been done with televisions, but they have required the use of powerful transducers that draw a lot of power. That is not a viable option for mobile devices.

Through a series of innovations, a team at Synaptics has produced a method that appears to overcome these problems, making display-based audio and haptics possible on low-power, mobile devices. The technique leverages piezoelectrics, in which tiny crystals are deformed by applying a voltage to them. By applying an alternating voltage, a piezoelectric material can be made to vibrate at a specific frequency.

This is not a new idea, but to make it work on a smartphone, an efficient, low-noise amplifier is needed. Signal preprocessing must also be included in the equation for high-quality audio to be produced. Towards this goal, the team developed a low-noise, high-voltage boost amplifier and a digital signal processor on a chip that can drive a ceramic piezoelectric transducer. This transducer causes the device’s existing display to vibrate at a specified frequency.

This hardware takes up less space than existing technologies (only one millimeter of thickness is added by the components), and it also pulls double duty as the speaker and haptic generator. Accordingly, the device can be thinner. It also has the advantage that sound appears to be coming from the display itself, providing a more natural experience. And since the screen itself is vibrating, no holes in the casing are needed, as with a traditional speaker.

Quality is not sacrificed by using this technique. Both the quality and loudness of the sound produced rival the best miniature speakers on the market today. As this technology is incorporated into new devices, it could allow for the production of thinner, and more capable, devices in the future.