A device composed of two pairs of infrared headphones and a table. On the tabletop, you hear the sound above the sea and when you crawl under the table, you hear the sound underwater.
Sound vibration has different acoustic resonance with different media, therefore it will be heard totally differently when heard in the air and in the water. The surface of water - the border that separates the air and the water - is a plane without thickness, and the table-board, likewise, separates its above and below.