A Study on Propagation Characteristics of Spread Spectrum Sound Waves Using a Band-Limited Ultrasonic Transducer

Propagation characteristics and range experiments of spread spectrum 40kHz ultrasonic waves have been explored under band-limited frequency. Comparative experiments were conducted using two arrangements combining a broadband speaker and a microphone for the broadband case, and an all-purpose receiving transducer and the broadband speaker for the band-limited case. Under the broadband condition, range accuracy measurements and evaluation of signal discrimination in a code division multiple transmission were made demonstrating 0.034% (corresponding to about 7mm in range distance) of range accuracy for a full-scale distance of 21.6m, demonstrating high signal discrimination. The severely band-limited experiment improved accuracy from 0.084% for a distance of 8.4m to 0.063% for a greater distance of 14.4m using a longer M-sequence length by extending shift registers from 10-bit to 15-bit. These experimental results show promise in the intended application of ultrasonic spread spectra to local positioning systems.