A piano is a complex machine that translates a small physical input from a player’s finger into a rich, resonant sound. This transformation involves an intricate chain of mechanical levers, the physics of vibrating strings, and the acoustic amplification provided by a large wooden structure. The process begins when the player depresses a key, setting off a high-speed sequence of events that converts kinetic energy into audible sound waves. Understanding this mechanism requires looking closely at the internal system that strikes the strings and the physical properties that govern the resulting tone.
From Key Press to Hammer Strike
Pressing a piano key initiates a mechanical sequence known as the action, a sophisticated leverage system designed to launch a felt-covered hammer toward the strings. The key acts as a lever, lifting an assembly of wooden and felt parts. This assembly, particularly in a grand piano, uses compound leverage to translate the relatively slow, short movement of the key into the rapid, forceful arc of the hammer.
A mechanism called the jack pushes the hammer upward, but it must disengage immediately before impact to allow the hammer to fall away from the string. This separation is achieved by the escapement mechanism, which ensures the hammer flies freely for the last fraction of its journey. The escapement allows the hammer to strike the string and immediately rebound, leaving the string free to vibrate fully.
The Physics of String Vibration
When the hammer strikes, it sets the tightly stretched wire strings into motion, the direct source of the sound wave. The pitch, or frequency, of the note produced is governed by three primary factors: the string’s length, its tension, and its mass per unit length, or gauge. Shorter, tighter, and thinner strings vibrate faster, producing higher notes, while longer, looser, and thicker strings produce lower notes.
Piano builders use a combination of these factors to manage the immense range of the instrument. For the lowest bass notes, strings are made thicker by wrapping a copper wire around a steel core, which increases the mass without excessive length or tension. A vibrating string produces the fundamental frequency (pitch) and a series of higher frequencies called overtones or partials. These overtones, which are not always exact integer multiples of the fundamental, contribute to the piano’s distinctive timbre and acoustic richness.
How the Soundboard Amplifies Tone
The vibrations created by the strings alone are relatively soft because the strings have a small surface area and cannot efficiently move a large volume of air. The soundboard, often made from resonant woods like spruce, acts as the acoustic diaphragm of the instrument. The strings’ vibrations are transferred to the soundboard through a wooden component called the bridge, which runs across the soundboard’s surface.
The soundboard is a large, thin panel capable of displacing a far greater volume of air than the strings themselves. This mass movement of air amplifies the tiny string vibrations into a sound loud enough to fill a room. The material and design of the soundboard also act as a filter, favoring the transmission of lower-frequency vibrations and helping to shape the overall tone.
Modifying and Stopping the Sound
Once a note is played, a felt pad called a damper rests against the string, silencing the note until the key is depressed again. When a key is pressed, its corresponding damper lifts away from the string, allowing the string to vibrate freely. Releasing the key returns the damper to the string, terminating the vibration and stopping the sound.
The pedals offer additional control over the sound’s duration and character. The rightmost pedal, known as the sustain or damper pedal, lifts every damper in the piano simultaneously. This action allows all strings to vibrate, permitting notes to ring on after the keys are released and creating harmonic resonance. The leftmost pedal, or soft pedal, often called the una corda pedal in grand pianos, shifts the entire hammer mechanism slightly sideways. In many grand pianos, this causes the hammers to strike only two of the three strings typically assigned to a note, resulting in a softer sound and a change in the tone’s quality.