One Day in 1932 Josef Hofmann, who had already spoken of the Steinway piano's 'extraordinary perfection of action,' came to Steinway Hall and said, 'It isn't quick enough: can't you make it still more sensitive, still more responsive? — “Commentary on the New Steinway Accelerated Action.” Presto-Times, Nov. — Dec. 1934
Hofmann's challenge was the impetus to Steinway becoming the most responsive and sensitive of any piano made. Frederick Vietor, grandnephew to C. F. Theodore Steinway, fulfilled Hofmann’s request by creating the Steinway Accelerated Action®; enhancing the Steinway action to respond to the touch instead of being forced into action. Today, the Accelerated Action is found on all American-made Steinway pianos.
Laboratory tests have proven that the keys on a Steinway piano can repeat 13% more quickly than any other piano. The same features that allow for this faster repeat also provide a much more sensitive, responsive keyboard, an aspect that can be appreciated even by beginning pianists.
1) Balanced Rail Bearing
The balance rail bearing, as you can see in the illustration taken from the 1931 patent, is a rounded felt-covered piece of maple, which serves as the fulcrum on which the key pivots. Only Steinway incorporates a rounded surface; other brands have flat rail bearings.
Why is a Rounded Surface Important?
Scenario 1: Imagine a long plank balanced on a flat piano bench. It's easy to place the plank so that it balances. The plank can be moved slightly one way or the other without either end touching the ground.
If you place your hand on one end and press very lightly, the plank might bend a little but the other end will not move. As you gradually press harder, the other end of the plank will eventually move, but only after you have applied considerable pressure.
Scenario 2: Now imagine the same situation but with a round surface on top of the flat bench. In this case the plank moves easily (friction free), and it is somewhat difficult to balance; once balanced, pressure on either end will cause the opposite end to move.
This principle underlies every Steinway action. The benefit is that the Steinway keys move friction free on the rounded balance rails making for the most responsive action possible.
2) Weighted Keys
The second distinguishing factor in the touch is the weighting of the keys: Larger weights are placed closer to the balance rail bearing, causing the keys to return faster.
So why doesn't every piano manufacturer incorporate these features? In one word: Time. Every key found on Steinway pianos is individually weighed off — a remarkably time consuming process.
Because the Steinway action has a much more sensitive fulcrum than actions of other pianos, all other action-related regulation is also more sensitive. The sensitive fulcrum of a Steinway action increases the complexity of all parts of the action.
Steinway, of course, is happy to put in the extra work, especially when the result is the most responsive piano action in the world. As the founding credo states "Build the best piano possible."