Low-Frequency Ultrasonic Communication for Speech Broadcasting in Public Transportation

Speech broadcasting via loudspeakers is widely used in public transportation to send broadcast notifications. However, listeners often fail to catch spoken context from speech broadcasts due to excessive environmental noise. We propose an ultrasonic communication method that can be applied to loudspeakerbased speech broadcasting to cope with this issue. In other words, text notifications are modulated and carried over lowfrequency ultrasonic waves through loudspeakers to the microphones of each potential listener’s mobile device. Then, the received ultrasonic stream is demodulated back into the text and the listener hears the notification context by a text-to-speech engine embedded in each mobile device. Such a transmission system is realized with a 20 kHz carrier frequency because it is inaudible to most listeners but capable of being used in communication between a loudspeaker and microphone. In addition, the performance of the proposed ultrasonic communication method is evaluated by measuring the success rate of transmitted words under various signal-to-noise ratio conditions.