Every time you strike a tuning fork, you're setting off a tiny, invisible hurricane. Thrashing back and forth at tremendous speeds, the two prongs of the fork, known as "tines," are smashing against nearby air molecules, kicking off a chain of impacts that echo through the air. When these violent, microscopic collisions hit your eardrum, your brain processes them as a gentle hum.
By hitting a tuning fork, you're causing its tines to vibrate back and forth several hundred times per second. Often, the vibrations are so fast that they're not visible to the human eye. If you need proof, simply dip a humming tuning fork into a cup of water -- it'll kick up a surprisingly large jet of water. In scientific terms, the speed of a tuning fork's vibrations is known as its frequency, a quantity measured in hertz (Hz), or vibrations per second.
The way a tuning fork's vibrations interact with the surrounding air is what causes sound to form. When a tuning fork's tines are moving away from one another, it pushes surrounding air molecules together, forming small, high-pressure areas known as compressions. When the tines snap back toward each other, they suck surrounding air molecules apart, forming small, low-pressure areas known as rarefactions. The result is a steady collection of rarefactions and compressions that, together, form a sound wave.
The faster a tuning fork's frequency, the higher the pitch of the note it plays. For instance, for a tuning fork to mimic the top key on a piano, it needs to vibrate at 4,000 Hz. To mimic the lowest key, on the other hand, it would only need to vibrate at 28 Hz.
But how do you adjust the speed at which a tuning fork vibrates? Well, first, you could adjust the length of your tuning fork. The smaller a tine, the less distance it has to move, and the faster it will be able to vibrate. It's the same principle as strings on a guitar. Without much room to wobble, a tight string vibrates quickly. A loose string, on the other hand, takes longer to shudder back and forth, resulting in a lower tone. The largest tuning fork in the world, by the way, is a 45-foot (13.7-meter) sculpture in Berkeley, Calif. [source: City of Berkeley]. If someone ever finds a hammer big enough to hit it, the sound would most likely be too low to be heard by human ears.
You can also adjust the pitch of a tuning fork by making it out of different materials. Dense metals like copper and steel vibrate with a crisp, high pitch. Soft metals like brass have a low, dull pitch. Really soft metals like tin, gold and lead, meanwhile, won't make any noise at all. Due to cost considerations, however, most modern tuning forks are made out of stainless steel.
Keep reading to find out whether a tuning fork can make your teeth explode.