There is nine steel balls that demonstrate phases, beats, and frequencies. The link below is a demonstration of how the pendulum wave is used.
https://animoto.com/play/rX0606TZIUohMfb0p73d6A
SIUE STEM Resource Center
http://www.siuestemcenter.org/
Southern Illinois University Edwardsvillehttp://www.siuestemcenter.org/
Science East, Room 1276
Edwardsville, IL, 62026
USA
618-650-3065