An ultrasonic range-finder with haptic feedback called The Bat Hat by Jeff Buswell, Clifford Chou and Andrew Knauss from Cornell University, built as a final project:
For our ECE 4760 final project, we designed and implemented an ultrasonic range-finding hat that uses haptic feedback to alert its wearer about obstacles in his or her path. The hat is equipped with an ultrasonic transmitter/receiver circuit, which is capable of emitting short pulses of ultrasonic-frequency (approximately 40 kHz) sound, at a level of about 120 dB. These pulses then echo off the closest object in the line of sight of the hat and are picked up by the receiver. The time delay between sending the initial pulse and receiving the echo gives a sense of how far away the obstacle is from the ultrasonic sensor, which can be conveyed to the person by vibrating the hat at a level proportional to that distance.
Via Hacked Gadgets.