Use of ultrasound in automotive interior occupancy sensing: optimum frequency, beamwidth, and SNR from empirical data

Ultrasonic spectra from a multitude of natural and artificial sounds in the interior of an automobile environment under various driving conditions are measured. In addition, over 60000 ultrasonic echoes representing distinct passenger-side vehicle occupancy scenarios were collected with a nominal signal-to-noise ratio (SNR). This information, coupled with animal hearing ranges, is used to determine an optimum operating frequency and SNR for ultrasound-based automotive occupancy sensor (AOS) systems. Ultrasound sensor beamwidth considerations based on vehicle geometry and seat coverage requirements are also presented. Current results indicate that individual transducers operating in the range of 50-120 kHz are immune to most interfering sounds. Geometrical results indicate that transducers with beamwidths between 15-22 degrees provide adequate vehicle interior coverage with minimum interference from doors, cupholders, etc. Finally, AOS performance is not adversely affected over a wide range of SNR´s, thus relaxing sensor and electronic design requirements.