The tuning fork, explained.

What is a tuning fork?

A tuning fork is two parallel prongs joined by a curved bow and a stem. Strike one prong and both vibrate at a single frequency, measured in hertz (Hz): the number of vibrations per second. That frequency is fixed by the shape and the material. Temperature changes it only very slightly, unlike the wooden pitch pipes of the 17th century, which were affected by heat and humidity. That reliability is why musicians adopted it.

John Shore made the first fork in 1711. By the second half of the 19th century the tuning fork had become a precision instrument of physics, used by researchers such as Hermann von Helmholtz and the Paris instrument maker Rudolph Koenig to study sound.

How does a tuning fork work?

The two prongs always swing in opposite directions, like a pair of hands clapping. Their movements cancel out where they meet, so the stem barely moves. That is why you can hold a fork by its stem without stopping the tone, and why it rings for so long.

The fork also produces an unusually pure tone. Its first overtone lies about six and a quarter times above the fundamental, far away and quickly fading, so what you hear is almost a single sine wave. A string, by comparison, sounds a strong overtone just one octave up.

On its own a fork is quiet, because the thin prongs move very little air. Press the stem onto a table or a wooden box and the surface acts as a soundboard: the tone becomes clearly louder.

What decides the pitch?

The length and the thickness of the prongs. Longer prongs sound lower, and the effect is strong: the frequency falls with the square of the prong length, so a fork with prongs twice as long sounds about two octaves lower. Thicker prongs, measured in the direction they swing, sound higher. Material matters too, through its stiffness and density. Steel is the classic choice because it is stiff and stable.

How to use a tuning fork

• Hold it by the stem. Touching the prongs stops the tone immediately.
• Strike one prong against something firm but soft: a rubber mallet, your knee, the heel of your hand. A hard edge can dent the prong and adds a metallic clang.
• Listen, or press the end of the stem onto a table or wooden surface to hear it louder.
• To stop the sound, touch the prongs lightly with your fingers.

That is all a fork needs. The ritual around it, a breath and a moment of attention while the tone fades, is up to you.

What are tuning forks used for?

• Music: tuning instruments and voices to a reference pitch, most often A = 440 Hz.
• Medicine: clinicians use forks as diagnostic tools, commonly 512 Hz for the Rinne and Weber hearing tests and 128 Hz for checking vibration sense in the examination of the nerves.
• Science and teaching: demonstrating frequency, resonance and sound waves.
• Ritual and focus: a single, clean tone as a moment of pause. This is how Cosmic Peace Council uses it.