From Web Physics

Simple beats applet with SOUND

Link – http://webphysics.davidson.edu/faculty/dmb/JavaSounddemos/beats.htm

From Surendranath Reddy

Simple beats applet showing wave traces of two sources and the combined wave.

Can change the source frequencies

Link – http://www.surendranath.org/Applets/Waves/Beats/BeatsApplet.html