A method for processing a reflected echo signal which is received from an object and supplied to an ultrasonic binaural sensory aid comprising the step of stretching said reflected signal in time axis, and the step of converting the stretched signal into a signal of audible frequency, and a blind aid...http://www.google.com.tw/patents/US4761770?utm_source=gb-gplus-share專利 US4761770 - Ultrasonic binaural sensory aid for a blind person
