Navigation aid is mportant for the visually impaired person to travel safely. A white cane is normally used to detect obstacles in the person's path but it is limited in its detecting ability as it can only detect obstacles around the length of the cane and is unable to detect moving or overhanging objects. Many types of navigational aids are being developed using ultrasonic sensors to assist the visually impaired to detect obstacles and they are known as Electronic Travel Aids. This paper is focused on studying the influence of acoustical absorbing material on the obstacle detection angle of an ultrasonic sensor. Results showed that a tube lined with acoustical absorbing material attached to the sensor is able to narrow the detection angle for more precise obstacle detection.