Demonstrations for the Physics of Music
Monochord - some sort of simple stringed instrument is invaluable for demonstrating intervals and the dependencies of the pitch on length and tension.
Toy (or real!) xylophone, tuning fork - shows nonlinear scaling of pitch with length, tuning fork shows a practical application of the non-harmonic overtone series.
Chladni plates - simply a classic.
Helium - I used to do the demo where you breath in helium and your voice goes up. Apparently, this is not so good for your vocal chords and it is not very quantitative. I found a much better demo. If you understand Lab8 and Section 6.1 and have something equivalent working, you can introduce helium into the cavity and the pitch of the feedback shoots up. In some ways it is more dramatic than talking with helium. Also, you can introduce a piece of dry ice into the cavity and the pitch starts to drop. This emphasizes the point that you can start learning about the contents of a cavity through spectroscopy.
Polarizers - for discussing the fact that light is a transverse wave. Many things can be done with crossed polarizers.
Tube spectrometers - this is just a cardboard tube about 2 feet long. At one end you put a slit and at the other end a piece of plastic transmission grating (from Edmund Scientific). It makes a very inexpensive visible spectrometer. I then set up several different discharge lamps (hydrogen, neon, nitrogen, etc.). Each lamp shows a different discrete spectrum. I.e. each has its own "scale". By understanding these scales we can determine what distant objects (stars, galaxies) are made of.
Echocardiogram - I happen to have a video of an echocardiogram of my heart. It shows amazing detail through the use of sound. However, it also has Doppler imaging. I.e. it shows in color the direction of the blood flow. A good application of the Doppler effect.
Whip - I still do not know if this is true, but I have been told that a whip "cracks" because the tip exceeds the speed of sound. An example of a sonic boom. Anyway, the students really respond to this one!
Physlets - try out the different Physlets. See http://webphysics.davidson.edu/Applets/Applets.html for more info about Physlets.
Perception - the Harmonic Synthesizer Physlet can be used for many things, especially if you open it twice. The two different windows can work independently.
1) JND - it is easy to demonstrate the JND. At 440 Hz, the JND is 2.2 Hz. A 5 Hz difference everyone will hear. 2 Hz about a third of the students will hear and a 1 Hz difference no one will hear.
2) Consonance and dissonance of pure tones - just use the fundamental and start with a unison. Then start increasing one frequency and you will hear beats, then roughness (dissonance), then dissonance with two distinct pitches, and finally consonance. With a separation greater than about a minor third, all pairs of frequencies sound consonance (even if they correspond to a normally dissonant interval).
3) Consonance and dissonance of complex tones - repeat the above sequence, but add 2f, 3f, 4f, 5f harmonics. See how things change.
4) Missing fundamental I - Make 2f, 3f, 4f, 5f, louder than 1f, but pitch stays the same, even as the timbre changes.
5) Missing fundamental II - Go down by octaves with pure tones, and then with complex tones. With complex tones, you hear the pitch go down by octaves, even though you can no longer hear the fundamental.
6) Missing fundamental III - Play 2f and 4f, the pitch will seem to be 2f. Then add 3f and the pitch should appear to drop by an octave. Works best at lower frequencies. Some people find this demo a bit confusing - they can't say what they hear!
7) Play a major third with different tunings - Pythagorean, Just, Equal Temper. See if anyone can hear the difference. Try with pure tones and complex tones. Also play major triads with three windows open.