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Ultrasonic investigations in shopping centres
Many public address systems emit a continuous near-ultrasonic sound that is used for system health monitoring.
It wasn't the metal box type, the walls were made of glass, so I thought the pilot tone should be at least somewhat audible inside. The frequency shift goes from positive to negative at the same moment that the scooter passes us, seen as the wideband wheel noise changing color. Automated speed trap in the car park Detect when the escalators stop working Modulate it with a positioning code to prevent people getting lost in the maze of commerce Use it to deliver ads somehow Use it to find your way to the quietest spots in a shopping centre
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