 Further experiments were conducted in 1992 aboard United States Microgravity Laboratory 1 (USML-1), and in 1995 aboard USML-2.The most common levitator from at least the 1970s until 2017 was the Langevin Horn, consisting of a piezo-electric actuator, a metal transmitter and a reflector.However, this required precise tuning of the distance between the transmitter and the reflector as the distance between source and reflector needed to be an exact multiple of the wavelength.