So far, speakers have been electromechanical devices, with a coil moving within a magnetic core, attached to a baffle or driver to move the air for producing the sound. With devices going down in size, manufacturers have been facing difficulties in producing electromechanical speakers in smaller sizes. Piezoelectric speakers are available, but they operate on a very narrow bandwidth.
Now USound GmBH, from Graz, Austria, has presented an audio speaker based on micro-electro-mechanical-system (MEMS) technology. This chip-sized speaker is suitable for small equipment such as Internet of Things (IoT) devices, wearables, smartphones, and earbuds.
By the end of the current year, USound expect to reveal Megaclite, a reference design using its MEMS speaker, Ganymede. So far, USound has fitted Ganymede to sunglasses at the high end. According to USound, Ganymede is suitable for mobiles, earbuds, and high fidelity, multidriver speakers playing above ear levels.
According to Mark Laich, senior adviser for business development at USound, making the diminutive MEMS drivers sound good across the audible spectrum was a huge challenge for the engineers. The major difficulty they faced was from the sound related physics, as it dictates the diaphragm size to push the air to be proportional to the wavelength of the sound emitted. That is why high-fidelity speaker systems use 12- to 15-inch drivers for producing low frequency bass sounds, 3- to 6-inch midrange drivers for the mid-frequency sounds, and 1 or less than 1-inch tweeter speakers for producing high-frequency sounds.
For the tiny speakers used in wearables, the size of the driver has to be some small portion of the wavelength of the sound it emits. Usually, some electronic or mechanical frequency equalization is necessary to make them sound high fidelity. Highest fidelity, as some headphones at the high end provide, is achievable only with multiple drivers. Typically, most of the reasonably priced earbuds have to sacrifice fidelity as they use a single driver, while adding electronic equalization to sound better.
As it is not possible to circumvent the sound related physics, MEMS speakers from USound are similar. Their low-end model has a single driver along with electronic equalization within a chip-scale package, and this bonds directly to the MEMs die. The MEMS frame is actually a longish actuator that moves a diaphragm using suspension beams made of piezoelectric material. The surrounding diaphragm also seals the entire chamber.
According to Laich, this arrangement achieves high-speed actuation, with a response time in microseconds. The company says this will help in noise cancellation in models to come, when they build them with a MEMS codec partner. At present, the air-pushing cone or diaphragm lies at the bottom side of a cavity, with thin piezoelectric drivers suspending it by the corners. The drivers supply the necessary energy to move the diaphragm in synchronization with the audio signal.
Listeners describe the sound from the MEMS speakers as digital, similar to the sound from a CD in comparison to that from a vinyl record. Of course, even when fortified with electronic equalization boosting the low frequencies, the sound from a single driver design does not match the high fidelity demonstrated by multidriver design.