What is a Surface Acoustic Wave (SAW) touch screen?

A SAW touch screen uses a solid glass display as the touch sensor. Across the surface of the
glass, two surface acoustic sound waves are transmitted – one for vertical detection and the other
for horizontal detection. Each sound wave spreads across the screen by bouncing off reflector
arrays along the edges of the display, and is detected by two receivers for each axis. The time
at which the sound waves arrive at each receiver is known because the velocity of the acoustic
wave through the glass is known and the size of the display is fixed. When the user touches the
glass surface of the display, some of the energy of the acoustic wave is absorbed by the water
content of the user’s finger. The controller measures the time at which the received amplitude
dips to determine the X and Y coordinates of the touch location. In addition to the X and Y
coordinates, SAW technology can also provide Z axis (depth) information. The harder the user
presses, the more energy the finger will absorb.