In the bathtub, submerge your head and listen to the sound you make when you click your fingernails together or tap the tub beneath the water surface. Compare the sound you make when both the source and your ears are above the water. At the risk of getting the floor wet, slide back and forth in the tub at different frequencies and see how the amplitude of the sloshing waves quickly builds up when you slide in rhythm with the waves. (The latter project is most effective when you are alone in the tub.)