I. INTRODUCTION
Blind since old age have been using walking stick for their mobility purposes. In addition some of them are also using guide-dog for their mobility. Due to the development of modern technologies, many different types of devices are available to assist the blind [I]. They are commonly known as electronic travel aid [2]. Many of these travelling aids use either ultrasound or laser beam. One of such devices is the laser cane that is similar to a long cane with built-in laser ranging facilities [3]. But most of the commonly used electronic travelling aids use ultrasound for functioning [4]. These devices use the reflectance property of the high frequency ultrasound and are available in different models such as:
A. Sonic torch
It is battery operated and is about the size of a large electric torch. It is a hand held device and operates on the principle of transmitting ultrasound in the forward direction and receiving the reflected beam from a nearby object.
B. Mowat sensor
It is a light weight,'hand held, pocket size device and is quite similar to a common torch that is used by the sighted people. It can detect object by sending high frequency ultrasound in pulses and receiving the reflected beam from any nearby object.