Cavity resonators designed such that a nozzle with an annular slit is located at the exit of the nozzle near a cavity a short distance away. Air blown through the slit passes over a circular knife edge at the near end of the cavity. A micrometer is used to adjust the depth of the cavity and another adjusts the distance between the slit and knife edge. The device makes it possible to generate ultrasound in air over a wide range of frequencies. Named for Sir Francis Galton who invented it in the mid 1800s to test human and animal hearing.
