A "piezoelectric speaker" is a type of speaker that produces sound by using a special material that can generate sound waves when it vibrates. Full definition
The proximity sensor has been replaced by ultrasound, the earpiece has been replaced with a piezoelectric speaker that uses the metal frame to generate sound, and the front - facing camera is relocated to the bottom (though the phone can thankfully be rotated upside down for more flattering selfies). (theverge.com)
That device uses ultrasound for proximity sensing, repositions the selfie camera, and has a piezoelectric speaker built into the screen. (theverge.com)
In order to ditch the bezel, the proximity sensor is replaced by ultrasound, the earpiece is replaced by a piezoelectric speaker that uses the metal frame to generate sound and the front - facing camera takes its place at the bottom of the device (which can be flipped for easier selfie use). (mobilesyrup.com)