The soft pedal on a grand piano makes the notes played "softer", that is, quieter, with not such a sharp attack.

The way the pedal works is very simple, but before I can explain it I have to explain something about the strings in a piano. For the bass notes, the lowest notes on the piano, there is one string for each note. For the middle notes, there are two strings for each note, tuned identically. When you strike one of the middle notes two strings sound. For the upper notes, there are three strings per note, all three tuned identically for each note. The reason for this is to even out the volume levels so that the treble notes and middle notes and bass notes will all sound at about the same volume when struck with the same force.

So back to the soft pedal. The soft pedal on a grand piano works by physically shifting the entire action, that is, the entire keyboard, and all those crazy felt hammers and everything sideways just a little bit. With the keyboard and hammers shifted over, when a note is played in the middle of the keyboard, only one of the two strings is struck. In the upper register, only two of the three strings are struck. And in the bass, the hammers don't hit the strings dead-on, but glance off the side just a little bit. The effect of all this is to make the piano sound not just quieter, but also a little bit different than it normally does.

The soft pedal on some upright pianos does not seem to work the same way.

On an upright piano, the action of the soft pedal is quite different: pushing the pedal down advances the hammers in the cabinet by about one third of the way towards the strings.

This has the effect of diminishing the kinetic energy of the hammer throw--which is the characteristic action of any piano--thus, making the tone produced softer, or quieter.

For myself, I do not usually use this, because the effect is marginal at the best of times. For students, this can just be an additional distraction from the notes and all the other things that go into a good performance.

The reason for the difference in the tone, the less sharp attack ferrouslepidoptera mentions, has to do with the felt on the hammer tip--actually a rounded end. When used, the felt will grove and become hard, thus giving a brighter tone to the struck string. When the entire action is moved, when the soft pedal is pushed, then a lessor used portion of the felt strikes the string. This gives a softer, darker tone to the note.

This is why in some composers--I think of Claude Debussy's Serenade of the Doll, in the Children's Corner Suite in paticular--specify forte even when the soft pedal is to be used, just to take advantage of this change of tone.

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