Use Audio Techniques|Speaker Test to check your speakers or for an in class demo to teach polarity and phase.

Included sounds: Pink noise, Sine Wave Sweep 20 to 20k, Clicks and 1k Sine Wave.
Flip all 4 included sounds polarity with a touch of a button for easy demo or speaker test.

When testing the polarity flip on the mono speaker of the iOS device the sound will seem to disappear. Normal left +/- right +/- , polarity flipped left +/- right -/+ (or inverted).