Propagating sound waves are basically compression and rarefaction of molecules in a gas, liquid, or solid, and the character of those waves is related to the various features of amplitude, frequency, wavelength, phase, speed, and intensity. Propagating sound waves may be created accidentally, as when our tree falls, or purposefully by sound-producing organs or technology. For healthy, younger humans, any sound waves with frequencies between 20 and 20,000 hertz can be received and processed by the structures of the ear and perceived as sound by the brain, although that optimal range usually diminishes with age. The snapping shrimp uses a specialized claw to make sound, which it snaps to create a “cavitation” bubble containing a sound wave. Entering the Industrial Age, humans began to create a multitude of technologies to produce, reproduce, preserve, and amplify sound, but the first 50 or so years focused largely on amplifying, recording, and reproducing human voices.