Fechner's double sound pendulum for testing aural discrimination, by E. Zimmerman, Leipzig and Berlin

Psychologists used Fechner’s double sound pendulum in the laboratory during the late 1800s. It tested a person’s ability to identify sounds. The double sound pendulum was adjusted so one mallet started slightly higher than the other. The psychologist then manually or electromagnetically released the two mallets at the same time. One struck slightly sooner than the other. The difference between the two was gradually reduced until the listener could no longer tell the pendulums were hitting at different times - it sounded like one click. The listener did not see the psychologist making the changes because this could influence the result. The sound pendulum was manufactured in Germany by E. Zimmerman.