24. Soft Pedal: Hammer Voicing, Physics of the Piano Sound

Grand pianos: The soft pedal is used to change the mood of the sound from percussive to more serene and gentle when the soft pedal is depressed. It should not be used solely for producing a softer sound because it will also change the timbre. In order to play pianissimo, you must simply learn how to play more softly. You can produce very loud sounds with the soft pedal depressed. One difficulty with the soft pedal is that it (una corda, or more correctly due corda for the modern grand) is often not indicated, so the decision to use it is often left to the pianist. For uprights, it mostly decreases the volume of the sound. The soft pedal on most uprights has a negligible effect on timbre. Unlike the grands, the uprights cannot produce loud sounds with the soft pedal depressed.

Many pianists do not understand the importance of proper hammer voicing for the soft pedal to be effective. If you tend to need the soft pedal to play softly, or if it is distinctly easier to play pianissimo with the grand lid closed, the hammer almost certainly needs voicing. See “Voicing” in 7. Making Minor Repairs (Voicing and Polishing Capstans). With properly voiced hammers, you should be able to control soft playing to any desired degree without the soft pedal. With worn, compacted, hammers, playing softly is impossible and the soft pedal has less effect in changing the tone. In most cases, the original properties of the hammer can be restored by voicing (re-shaping, needling, etc.). The action must also be well regulated, with the let-off properly minimized, in order to enable PPP.

The use of the soft pedal is controversial because too many pianists are unfamiliar with how it works. For example, many use it to play pianissimo, which is incorrect usage. As shown in 7. Making Minor Repairs (Voicing and Polishing Capstans), energy transfer from the hammer to the string is most efficient at impact, before the string starts to move. A compacted hammer transfers its energy during an extremely short time interval at impact and the hammer immediately bounces off the strings. This high efficiency of energy transfer gives the impression of a very light action. That is why there are old grands that feel feather light. Soft hammers on the same piano (with nothing else changed), would make the action feel heavier. This is because, with the softer impact point of the hammer, it stays on the string much longer, and the string is pushed out of its original position before all the hammer energy is transferred to the string. In this position, the energy transfer is less efficient (see 7. Making Minor Repairs (Voicing and Polishing Capstans)) and the pianist must push harder to produce the same volume of sound. Thus voicing can be more effective in changing the apparent key weight than lead weights. Clearly, the effective key weight is only partly controlled by the force required to depress the key, since it also depends on the force required to produce a given amount of sound. The pianist does not know which factor (lead weights or soft hammer) is affecting the effective key weight. The piano technician must strike a compromise between voicing a hammer sufficiently soft so as to produce a pleasant tone but sufficiently hard so as to produce adequate sound. For all except the highest quality pianos, the hammer needs to be on the hard side in order to produce enough sound and to make the action feel nimble, which makes such pianos difficult to play softly. This in turn can “justify” use of the soft pedal where it should not be used. Piano owners who neglect voicing can make the piano tuner’s job difficult because, after the hammers are properly voiced, the owner will complain that the action is too heavy to play. In reality, the owner had gotten used to playing a feather light action and never learned how to play with real power to generate that gorgeous piano sound.

In most uprights, the soft pedal causes all the hammers to move closer to the strings, thus restricting hammer motion and decreasing the volume. Unlike the grands, loud sounds cannot be produced in an upright when the soft pedal is depressed. One advantage of uprights is that a partial soft pedal works; partial soft pedaling is a complex subject for grands and will be treated in detail below. There are a few upscale uprights in which the soft pedal works similarly to that of the grands.

In modern grands, the soft pedal causes the entire action (including the hammers) to shift to the right so that the hammers miss one string in the 3-string section. Thus the hammers hit only two strings, causing a serendipitous transformation in the character of the sound, as explained below. The shift is exactly half the distance between adjacent strings in the 3-string section; thus the two active strings will hit the less used portions of the hammer between string grooves, creating an even gentler sound. The horizontal motion must not shift one string distance because then the strings will fall into the grooves made by adjacent strings. Since string distances and the amount of shift cannot be controlled sufficiently accurately, this would cause some strings to fall exactly into the grooves while others will miss, creating uneven sound.

Why does timbre change when two strings are struck instead of three? Here, timbre is controlled by at least four factors:

  1. Existence of the unstruck string
  2. The prompt-/after-sound ratio
  3. The harmonic content
  4. Vibrational polarization of the strings

Let’s examine these in more detail. The unstruck string acts as a reservoir into which the other two strings can dump their energy and produces many new effects. Since the vibration of the 3rd string is in anti-phase with the struck strings (a driven string is always in anti-phase with the driver), it takes the edge off the initial prompt sound (see below) and at the same time, excites vibrational modes that are different from those that result when all three are struck in unison. This is why the soft pedals in uprights don’t work as well – all the strings are struck even when the soft pedal is depressed, and the timbre cannot change.

The piano produces an initial prompt-sound and a sustaining after-sound; see the “Scientific American” and “Five Lectures” articles reviewed in the Reference Section for more details on topics discussed in this paragraph. Unlike the simplified picture of fundamental and harmonic frequencies that we use when tuning a piano, the actual string vibrations consist of a complex time dependent series of events that are still incompletely understood. In such situations, the actual data from existing pianos are of more practical value, but those data are closely held trade secrets of piano manufacturers. Therefore, I summarize here some general knowledge based on the physics of the piano sound. The string vibrations can be polarized, either parallel to the soundboard, or perpendicular to it. When the strings are struck, vertically polarized traveling waves are generated that move away from the hammer in both directions, towards the agraffes (capo bar) and towards the bridge. These waves travel so rapidly that they reflect back from both ends of the strings and pass the hammer several hundred times before the hammer bounces off the strings; in fact it is these waves that throw the hammer back. Horizontally polarized waves are generated from the vertical waves because the piano is not symmetric. These traveling waves decay into standing waves consisting of harmonics (including the fundamental) because the standing waves are “normal vibration modes” (see a mechanics text book) that transfer energy slowly to the soundboards and are therefore long-lived. However, from the very beginning, the concept of fundamentals and harmonics remains valid because the Fourier coefficients (see a math or physics textbook) of the fundamental and harmonic frequencies are always large, even for the traveling waves. This is easily understood because the ends of the strings do not move, especially for well-constructed, large, heavy pianos. In other words, mostly wavelengths that have nodes (points of zero motion) at both ends are generated when the ends are fixed. This explains why, in spite of the traveling waves, tuners can tune accurately using only the fundamental and harmonic frequencies. The vertically polarized waves transfer energy more efficiently to the soundboard than the horizontally polarized waves and therefore produce a louder sound but decay faster, and create the prompt sound. The horizontally polarized standing waves produce the after-sound which gives the piano its long sustain. When the soft pedal is depressed, only 2 strings can produce the prompt sound but eventually, all 3 strings contribute to the after-sound. Therefore, the prompt-/after-sound ratio is smaller than for 3 strings and the sound is less percussive with the soft pedal.

The harmonic content is also different because the energy of the hammer is transferred to only 2 strings instead of 3, which is like hitting the string with a heavier hammer, and it is known that heavier hammers produce stronger fundamentals. The polarizations of the strings also change with the soft pedal because the third string will be polarized more horizontally, which contributes to the gentler sound.

This type of understanding helps us to use the damper pedal correctly. If the damper pedal is depressed before a note is played, the initial time dependent traveling waves will excite all strings, creating a soft background roar. That is, in the prompt sound, the non-harmonic Fourier coefficients are not zero. If you place your finger on any string, you can feel it vibrate. However, octave and harmonic strings will vibrate with higher amplitudes than the dissonant strings, which is a consequence of the larger Fourier coefficients for the harmonics. Thus the piano not only selectively traps the harmonics, but also selectively generates them. Now if the pedal is depressed after the note is struck, there will be sympathetic vibration in octave and harmonic strings, but all the other strings will be almost totally quiet because the standing waves contain only pure harmonics. This produces a clear sustained note. The lesson here is that, in general, the pedal should be depressed immediately after striking the note, not before, in order to avoid dissonances. This is a good habit to cultivate.

A partial soft pedal works on an upright; but can you use a half soft pedal on a grand? This should not be controversial but is, because even some advanced pianists think that if a full soft pedal gives a certain effect, a partial soft pedal will give a partial effect, which is false. If you use a partial soft pedal, you will of course get a new sound. There is no reason why a pianist shouldn’t be allowed to do that, and if it produces an interesting new effect that the pianist likes, there is nothing wrong with that. However, this mode of play was not intentionally designed into the piano and I know of no composer who composed for partial soft pedal on a grand, especially because it is not reproducible from piano to piano, and from note to note on the same piano. Extensive use of partial soft pedals on the grand will cause some strings to shave off one side of the hammer, thus throwing the system out of regulation. Also, it is impossible for the piano technician to align all hammers and strings so accurately that the third string will miss the hammer at the same pedal travel for all 3-string notes. Thus the partial soft pedal effect will be uneven, and different from piano to piano. Therefore, unless you have experimented and are trying to produce some strange and irreproducible new effect, half-pedaling is not recommended for the soft pedal on a grand. Nonetheless, anecdotal accounts indicate that use of partial soft pedal on a grand does occur, almost always because of ignorance on the part of the pianist about how it works. The only way to use a partial soft pedal with reproducible results is a very slight soft pedal, in which case all the strings will hit the sides of the grooves in the hammer. Even this scheme will not really work, because it will affect only the 3-string section, resulting in a jarring transition from 2-string to 3-string sections.

In the double and single string sections, the strings have much larger diameters, so when the action moves sideways, the strings hit the side walls of the grooves, thus giving them a horizontal motion and increasing the after-sound by increasing the horizontally polarized string vibrations. Thus the change in timbre is similar to that in the 3 string section. This mechanism is fiendishly ingenious!

In summary, the name soft pedal is a misnomer for a grand. Its main effect is to change the timbre of the sound. If you play a loud sound with the soft pedal depressed, it will be almost as loud as without the soft pedal. This is because you have put roughly the same amount of energy into making the sound. On the other hand, it is easier to play softly using the soft pedal on most grands because the strings hit the less used, softer parts of the hammers. Provided that the piano is well regulated and the hammers are properly voiced, you should be able to play just as softly without the soft pedal. A partial soft pedal will produce unpredictable, uneven effects and should not be used for an acoustic grand. A partial soft pedal works on most uprights and all electronic pianos.