For a visual picture of waves, you may want to browse through some of these
simulations: they show animation of various concepts from waves and sound.
https://www.acs.psu.edu/drussell/demos.html
What is a wave?
https://www.acs.psu.edu/drussell/Demos/waves-intro/waves-intro.html
https://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
Wave motion in space and time
https://www.acs.psu.edu/drussell/Demos/wave-x-t/wave-x-t.html
Reflection of waves on a string, with fixed or free boundaries.
Also includes transmission through an interface.
https://www.acs.psu.edu/drussell/Demos/reflect/reflect.html
Reflection from an Impedance Discontinuity
to form standing waves
https://www.acs.psu.edu/drussell/Demos/SWR/SWR.html
Superposition
https://www.acs.psu.edu/drussell/Demos/superposition/superposition.html
Superposition of waves for several cases.
https://www.acs.psu.edu/drussell/Demos/superposition/superposition.html
Beats (show this with the two vibe tubes)
http://www.walter-fendt.de/html5/phen/beats_en.htm
Demos from Walter Fendt:
http://www.walter-fendt.de/html5/phen/
Fourier synthesizer with sound.
http://www.walter-fendt.de/html5/phen/
Simple Pendulum
http://www.walter-fendt.de/html5/phen/pendulum_en.htm
Coupled Pendula
http://www.walter-fendt.de/html5/phen/coupledpendula_en.htm
Spring Pendulum
http://www.walter-fendt.de/html5/phen/springpendulum_en.htm
Forced oscillations (resonance)
http://www.walter-fendt.de/html5/phen/resonance_en.htm
Coupled Pendula
http://www.walter-fendt.de/html5/phen/coupledpendula_en.htm
If more and more of these are coupled together, we can get waves:
Waves
Standing waves on string
http://www.walter-fendt.de/html5/phen/standingwavereflection_en.htm
https://www.acs.psu.edu/drussell/Demos/doppler/doppler.html
Standing Longitudinal Waves
http://www.walter-fendt.de/html5/phen/standinglongitudinalwaves_en.htm
Doppler Effect
http://www.walter-fendt.de/html5/phen/dopplereffect_en.htm
https://www.acs.psu.edu/drussell/Demos/doppler/doppler.html
Ripple Tank Simulation
http://www.falstad.com/ripple/
Fourier simulation (may need IE)
http://www.falstad.com/fourier/