The piezoelectric effect is a versatile and extremely useful phenomenon. Engineers have adopted this phenomenon in various transducer applications. Some of these applications involve transforming the applied voltage to mechanical strain output, for use as a basic source of sound. In a complementary mode, the application of mechanical stress to the Piezo material causes the rugged sensor to produce a voltage. Piezoelectric devices are low-cost, reliable, and rugged, and this allows engineers to exploit their unique properties.
Piezo-based speakers offer many attributes as sound sources. Unlike electrodynamic speakers, piezo-based speakers can be relatively thin, yet create very high sound pressure levels. However, mechanical and physical material issues can limit their audio quality. Now, a team at MIT is changing all this. They have developed a dense array of tiny dome speakers that they have based on Piezo technology. They have significantly transformed the classic analog function of loudspeakers. Their new loudspeakers are paper-thin, very flexible, and fully capable of turning any surface into an active audio source.
Although there are conventional thin-film loudspeakers, the basic requirement is the film must be free to bend to produce sounds. Firmly mounting such thin-film loudspeakers to a surface would attenuate their output and dampen the vibrations, while limiting their frequency response tremendously.
However, the ingenious approach of the MIT team has solved the problem in a rather unique way. Their new loudspeaker does not have to vibrate the entire material surface. Rather, they have fabricated tiny domes on a thin layer of piezoelectric material, such that each dome can vibrate independently. Each dome is about 15 µm in height, and they move up and down by only half a micron when vibrating. As each dome forms a single sound-generating unit, it requires thousands of these tiny domes to vibrate together to produce audible sounds. While the basic loudspeaker is only 120-µm thick, it weighs only 2 grams. Only standard processes are necessary to manufacture this loudspeaker at low costs.
Spacer layers surround the domes on the bottom and top of the film. This helps to protect the domes from the mounting surface, and at the same time allows them to freely vibrate. These spacer layers also protect the domes from impact and abrasion during daily handling, thereby enhancing the durability of the loudspeaker.
To make the film loudspeakers, the researchers used a thin sheet of PET or polyethylene terephthalate. This is a standard plastic used for a variety of applications. They used a laser to cut tiny holes in the sheet while laminating its underside with an 8-µm thick film of PVDF or polyvinylidene fluoride. This is a common industrial and commercial coating. Then they applied vacuum and heat to bond the two sheets.
As the PVDF layer is very thin, the pressure difference that the vacuum creates together with the heat causes it to bulge, but it cannot force its way through the PET layer. This makes the tiny domes protrude through the holes. The researchers laminated the free side of the PVDF layer with another layer of PET and this acts like a spacer between the bonding surface and the domes. Regardless of the rigid bonding surface, the film loudspeaker could generate a sound pressure level of 66 dB at 30 cm.