Piano hammers are made of highly compressed sheep's wool to create their distinct sound.

The use of felted sheep's wool for piano hammers became the industry standard in the mid-19th century. In 1826, Henri Pape patented the use of felted hair or wool, replacing the leather coverings used on earlier fortepianos. This transition allowed for the increased string tension and heavier frames developed by manufacturers like Steinway & Sons and Yamaha. Modern hammers are produced by compressing high-quality wool at pressures exceeding 20 tons per square inch.The physics of the hammer strike involves a complex interaction called 'nonlinear elasticity.' When a hammer hits a string, the felt compresses and then rebounds. The density of the wool determines the contact time between the hammer and the string, which directly influences which harmonics are excited. A harder hammer produces a brighter, more percussive sound, while a softer hammer creates a warmer, mellow tone.Technicians perform a process called 'voicing' to manage this density. They use specialized needles to prick the felt, which breaks up some of the compressed fibers and softens the strike. Conversely, they may apply chemical hardeners to brighten a dull-sounding hammer. Because wool is a natural protein fiber, it is also highly sensitive to humidity, which can cause the felt to swell or shrink and alter the piano's tuning and touch.