Noises generated when the finger strikes the key at rest, and a moment later when the key reaches the bottom, are transmitted through the keys, key frame, key bed, and rim to the soundboard, bridges, and strings. How prominent those noises are can be easily demonstrated: keeping the damper pedal depressed, hold a key loosely depressed at the bottom of its travel, then pound it with the other hand. The noise is amplified and sustained for several seconds.
The duplex portions of strings, where present, amplify the noises even when the damper pedal is not used.
The amount of time between the noises generated by the key and the hammer striking the strings depends on how loudly one plays the note. A piano touch can delay the bottom-of-key-travel noise by as much as 30 ms (0.03 second), whereas a fortissimo blow can make it precede the hammer strike by almost 10 ms. (See Alba, Ramon and Asami Inouye, “Piano Tone Color and Touch: A Controversy Compromised.” The Piano Quarterly, Fall 1979, p. 36.) This has a profound effect on the perception of tone. In soft playing, the key bed noise is a relatively soft part of the sound envelope, whereas loud thumps usurp the sound of the fortissimo strikes, especially in the treble.
Piano manufacturers have generally suppressed the noise of the key return in various ways. Many pianos have a felt strip under the backrail cloth, and both the felt and the cloth are glued onto the rail with a thin bead of glue in the front, leaving the area in the back where the key hits them loose and soft. At the turn of the 20th century, the German manufacturer Blüthner used to wrap multiple loose layers of cloth and felt with a thin sheet of felt to provide maximum cushioning.
But when it comes to the front punchings, manufacturers couldn't diverge more. These punchings delimit key travel, but also provide cushioning for the fingers. Without sufficient cushioning, the pianist would develop calluses and keys would thump on every strike. Yet this is exactly how some fine European pianos were shipped in the 1960s and 1970s, probably to provide a very precise sense of the bottom of key travel. In some pianos from that era the punchings were starched or treated with a hammer hardener. By contrast, in the early 20th century Blüthner and Pleyel used soft, double front punchings. Instead of paper and cardboard punchings, placed under the cloth punchings to control the key dip, Blüthner used punchings made of a softer, rag-like material.
Harder punchings are still preferred in highest quality grands. Whereas traditional punchings are cut cylindrically out of woven cloth, they can be made of felt too. Conical punchings made of highly resilient felt are the preference of many pianists because they combine the cushioning with precise delimiting and the desirable percussive quality.