At the left end of the three-pedal group is the soft pedal. The proper term for this in an acoustic grand piano — una corda, or "one string"—relates to its function. In an acoustic grand, this pedal, when depressed, laterally shifts the entire action — from keys to hammers — slightly to the right. Recall (from "Tone Production," above) that, on an acoustic piano, most notes have two or three strings associated with them. When the action is shifted to the right by the soft pedal, the hammer strikes only two of the three strings in each three-string unison. This has two effects: it reduces the volume of the sound, and it slightly alters the tonal quality.
As with the sustain pedal, the digital version of this pedal is simply an electronic switch that activates an equivalent effect. Since the digital piano action can play at much lower volumes than the acoustic piano, the practical importance of this pedal for reducing sound volume is considerably lessened. However, its ability to alter tonal quality remains relevant — assuming it actually does so. Most do not.
The mysterious center pedal is the sostenuto. The easiest way to think of the sostenuto's function is as a selective sustain pedal. Play one or more keys anywhere on the keyboard and, while holding these keys down, press and hold the sostenuto pedal. The sostenuto mechanism will hold the dampers for these keys away from the strings, sustaining them even after you release the keys, but any subsequent keys played will not sustain when released (unless you also use the sustain pedal). Clear? The bottom line is that all three-pedal digital pianos incorporate this feature exactly as it works on an acoustic piano. In written music, the sostenuto pedal is called for in only a few pieces of classical music. If you need it, it's there, but chances are you never will. In digital pianos, the middle pedal is often assigned another function, discussed in Part 2 of this article.
The Audio System
The final component of most digital pianos is the audio system — its amplifiers and speakers — which perform the same job as an acoustic piano's soundboard: making the piano's sound audible at useful volume levels. I say most digital pianos because some instruments designed specifically for stage use lack an onboard audio system, as they will always be connected to a sound-reinforcement, or public address (PA), system.
The digital pianos currently on the market offer anywhere from 12 to 360 watts (W) of output power, channeled through from two to twelve speakers. To understand why there is such a wide range of options, we need to look at how the system's power-output capability (and the type, number, and placement of speakers) relates to what we hear.
The smallest change in volume that most people can detect is 3 decibels (dB), and to achieve a 3dB increase in volume requires a doubling of the output power in watts. With these relationships in mind, let's look at some numbers.
Based on measurements of three of the most frequently encountered concert grand pianos — Bösendorfer model 290, Steinway & Sons model D, and Yamaha model CFIIIS—I arrived at a model dynamic range. This range extends from the softest note possible, at 64dB, to the loudest chord I could produce, at 103dB. Assigning a modest 0.015W — we're assuming a very efficient audio system — to produce the softest (64dB) note, the chart below traces the progression of amplifier power required to keep up with the increasing volume to the top of the piano's dynamic range. Different audio systems will have different starting points, depending on the size and number of speakers being powered, the efficiency of those speakers' use of power, and the notes played (bass requires more power to match the treble volume). Dynamic markings have been added to bring some musical perspective to the numbers.
If you've not seen this sort of table before, the results are startling. It's the last three or four steps of volume that really demand power from the amplifiers.