Yes, that is normal. Our creator decided to limit our hearing range to around 20 kHz. The ultrasonic works over 1 MHz. If the switch is in the "up" position the input frequency is divided by 256. If it is down it is divided by 16. In most instances, the audio sound will be way above our hearing threshold. For example, if you are running the BX frequency of 1604000 Hz, the ultrasonic transmitter will be producing 6.266 kHz in the up position and 100.25 kHz in the down position. You can see why you can only hear the low-speed audio signal.
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