this is a good jdk simulation of the Cathode Ray Oscilloscope (CRO).

will there be an Ejs version soon? i don't think anyone has made one in Ejs yet.
for your consideration  ;D

btw i found some java applet here by by daniel hoag and erwin d'souza copyirght NC state university by Peter Siegel California State Polytechnic University

the learning objective is
Determine the frequency of sound using a calibrated C.R.O.

Determination of frequency of sound using a calibrated C.R.O.

Sound are longitudinal waves of compressions and rarefactions through a physical medium, for example air.

The frequency of the sound waves from a given source could be determined using a calibrated cathode-ray oscilloscope (C.R.O.) as follows:

- the source of sound e.g. a loud speaker is placed a short distance from a microphone connected to a calibrated C.R.O. as shown.
- The time base of the C.R.O. is adjusted until a stationary trace is obtained.
- The period of the sound wave is given by the product of the length of one wave cycle and the time base setting.
- The frequency of the wave is the reciprocal of the period.