Doppler ultrasound devices are essentially specialized audio frequency converters. The frequency shift of the returning sound wave indicates the velocity of blood flow. Biologists use AFCs to monitor bat echolocation or rodent communication, converting ultrasonic squeaks into audible hisses and clicks.
By filtering out the original signal and one of these products, you are left with a frequency-shifted version of your sound. Crucially, unlike a pitch shifter, this process does not preserve harmonic ratios. A musical interval of a perfect fifth (a 3:2 ratio) fed into a frequency converter will emerge as a dissonant, unrelated interval. This is why the device is often called a frequency translator rather than a pitch shifter . audio frequency converter
One of the most valuable uses is removing hums and buzzes. Suppose a recording captures a 50Hz electrical hum from a faulty ground wire. Using a narrow-band audio frequency converter, an engineer can shift only that 50Hz band deeper into infrasound (below 20Hz) where the human ear cannot detect it, or shift it onto a frequency that can be filtered out cleanly. Similarly, forensic analysts often use AFCs to shift ultrasonic sounds (like a bat click or a silenced weapon discharge) down into human hearing range. By filtering out the original signal and one