How Tuning Forks Work

How does a simple "pitch fork" get everyone in tune?
How does a simple "pitch fork" get everyone in tune?

The choir comes to a hush as the choirmaster steps into the room. "All right, let's tune," she declares to the assembled singers. Producing a two-pronged metal tool from her pocket, she strikes it against her knee. After a shrill whine, it begins to emanate a mysterious, angelic hum. Resting the base of the device against a wooden desk, the hum becomes louder. Listening intently to the small, singing piece of metal, the singers adjust their voices accordingly.

Pianos lose their tuning, guitars fall out of key -- even church organs need to be tuned every now and then. For centuries, the only sure-fire way to tell if an instrument was in tune was to use a tuning fork: A two-pronged, all-metal device the length of a butter knife. Simple to make and easy to use, tuning forks guaranteed 100 percent pitch accuracy. In antique shops today, you can find 300-year-old tuning forks that emit notes just as clear and crisp as the day they were made.

The device was first invented in 1711 by John Shore, a trumpet player working for the British Crown. In concerts, Shore jokingly referred to his invention as a "pitch fork." Shore's device quickly became the must-have tool of musicians around the world. In portraits, choirmasters posed while brandishing a tuning fork. Young musicians were given gold-plated tuning forks upon successful completion of a conservatory exam. Today, the $3.5-billion Yamaha Corporation has a trio of tuning forks as their logo. Tuning forks were even behind some of telephone inventor Alexander Graham Bell's earliest experiments. In 1876, Bell was able to use a battery, an electrified tuning fork and a cup of acidic water to transmit sound over an electric wire for the first time.

"Playing" a tuning fork is relatively easy: Taking care not to hold it by the prongs, you slap it against something hard like a tabletop -- or even the bottom of your shoe -- and then listen as it rings for about 5 seconds. Hit it hard or hit it soft, the note is always the same. It seems almost magical.

Read on to find out how a hunk of metal makes music.

How Tuning Forks Hum

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.

How to Use a Tuning Fork

Despite the ease of technology, many piano tuners still use tuning forks.
Despite the ease of technology, many piano tuners still use tuning forks.

Ever heard an out-of-tune piano? The piano's strings have been allowed to fall out of tune and as a result, the keys are no longer synchronized. Normally, the keys on a piano represent the different notes of a musical scale. But without proper tuning, they're nothing more than random notes cobbled together. To hear them played together, it just sounds chaotic. A band, choir or orchestra works the same way. If the instruments or voices haven't been adjusted to play in the same tone, they'll sound no better than an out-of-tune piano. A tuning fork's job is to establish a single note that everybody can tune to.

Most tuning forks are made to vibrate at 440 Hz, a tone known to musicians as "concert A." To tune a piano, you would start by playing the piano's "A" key while ringing an "A" tuning fork. If the piano is out of tune, you'll hear a distinct warble between the note you're playing and the note played by the tuning fork; the further apart the warbles, the more out-of-tune the piano. By either tightening or loosening the piano's strings, you reduce the warble until it's in line with the tuning fork. Once the "A" key is in tune, you would then adjust all of the instrument's 87 other keys to match. The method is much the same for most other instruments. Whether you're tuning a clarinet or guitar, simply play a concert A and adjust your instrument accordingly.

It can be a bit tricky to hold a tuning fork while manipulating an instrument, which is why some musicians decide to clench the base of a ringing tuning fork in their teeth. This has the unique effect of transmitting sound through your bones, allowing your brain to "hear" the tone through your jaw. According to some urban legends, touching your teeth with a vibrating tuning fork is enough to make them explode. It's a myth, obviously, but if you have a cavity or a chipped tooth, you'll quickly find this method to be unbelievably painful.

Luckily, you can also buy tuning forks that come mounted on top of a resonator, a hollow wooden box designed to amplify a tuning fork's vibrations. In 1860, a pair of German inventors even devised a battery-powered tuning fork that musicians didn't need to ring again and again [source: Case Western Reserve University].

Of course, even the most elaborate tuning fork has little use for most modern musicians. Like most things, the humble tuning fork has been made obsolete by computers. Most musicians now carry $20 electronic tuners the size of a pack of cards. Play any note, and the tuner will automatically detect which note it is, telling you whether it's sharp or flat. A Spanish company also recently launched an app allowing musicians to tune up with nothing more than their iPhone [source: Lewin]. But whether out of caution or sentimentality, it's not uncommon for most serious musicians to keep at least one tuning fork around the house.

Read on to find why a tuning fork is responsible for your last speeding ticket.

Non-musical Uses for Tuning Forks

Some doctors might use tuning forks to test hearing loss.
Some doctors might use tuning forks to test hearing loss.

While keeping orchestras and concert bands in check, tuning forks have also found plenty of work in hospitals, research labs and police stations around the world.

Among some audiologists, tuning forks remain a preferred method of testing for certain types of hearing loss. In a method known as a Rinne test, a doctor first holds a humming tuning fork to your skull and, using a stopwatch, times how long you can hear it. The doctor then strikes the tuning fork again and times how long you can hear it when it's held next to your ear. If you can hear the tuning fork through your jaw longer than you can hear it against your ear, you have a problem conducting sound waves through your ear canal. In a similar test (known as the Weber test), a vibrating tuning fork is held in the middle of a patient's forehead. By figuring out which ear hears the tuning fork the loudest, the doctor can zero in on which ear is damaged.

When X-rays are in short supply, tuning forks can also be a makeshift way to identify whether a bone is fractured. Simply hold a ringing tuning fork close to the site of a suspected fracture. If you feel a sudden surge of pain, it's time to go to the hospital.

Over the last 15 years, tuning forks have also been getting a fair bit of attention in the world of alternative medicine. In a practice known as tuning fork therapy, a patient lies motionless on a table while a healer passes vibrating tuning forks over their body -- the idea being that the vibrations improve mental clarity and physical energy. There's no scientific evidence for this, but hanging out in a room filled with gently-humming tuning forks could put anyone in a good mood.

Ever gotten a speeding ticket? A tuning fork is partly responsible. Police radar guns measure speed by bounding a radar signal off an approaching car. Depending on how quickly the radar signal bounces back determines how fast the car is going. To calibrate a radar gun, police will aim it at a tuning fork specially designed to vibrate at 50 miles per hour (80.5 kilometers per hour). If their radar gun doesn't register 50 miles an hour, they know it's time to take it in for adjustment.

For lots more information on music and technology, give your brain a tune-up with the links on the next page.

Related Articles


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