Sound Waves: Voices Swimming in the Air

Music and everything else we hear is made up of sound waves. Learn how pressure in the air transforms into sound with Groupon's explanation. Like the varying ripples in a pond after you skip a snapping turtle, sound travels in waves. When a guitarist plucks a note, for instance, the string causes the air molecules around it to vibrate, which in turn causes more molecules to vibrate, and on and on until the wave reaches your ears. The differences in the way we perceive these waves—that is, the variations of sound—are largely because of a few key characteristics:

• Frequency: Defined as the speed at which each crest of the wave passes any given point, the frequency determines a sound's pitch. Lower pitches have lower frequencies and spaced-out crests, whereas high frequencies appear as tight zig-zags.
• Amplitude: How loud or intense a given sound is entirely depends on its amplitude, which is easily visualized as the height of the wave from crest to valley.
• Medium: As a pressure wave, sound can travel through almost any medium, from water to solid rock, though each medium affects the speed (and distance) at which the wave can travel. A major exception is in a vacuum, where no air means no molecules to vibrate and propagate the wave.
• Uniformity: For the most part, the distinction between our perception of a noise and a musical tone is based on how consistent the wave is: noise is unpredictable and jagged, like a choppy sea, while a tone flows steadily, like a tide lapping on the sand.

Though humans only evolved ears to make wearing funny glasses possible, the organs happen to bear a key side effect: they are delicately, exquisitely attuned to receive—and translate—sound waves. The dish-like shape of the ear helps direct the waves into the ear canal, where they eventually strike the eardrum—a thin, tiny membrane that vibrates at the same amplitude and frequency as the waves themselves. Next, a trio of tiny bones in the middle ear—in fact, the tiniest bones in the entire body—move in tandem with the vibrations, thereby transferring the pressure waves into mechanical energy. From there, the wave travels through the fluid contained in a coiled tube known as the cochlea. This tube is lined with microscopic hair cells, each tuned to a specific frequency. When the frequency of the wave matches the right hair cell, a nerve impulse sends a message to the brain to interpret the sound. Through this instantaneous process, the transformation of air pressure to sound is complete, and we finally perceive the invisible miracle of a siren blaring at 3 a.m.