The process known as "toning" the hammers is an attempt to control the time of contact between hammer and string, a soft hammer staying long enough to damp out some of the partial tones, while a hard-nosed hammer ac centuates them. If a string vibrates a thousand times per second, its "periodic time" is one-thousandth of a second, and experi mental evidence proves that the best tone is emitted if the hammer is toned so that contact is maintained for about half this period. English felt is used wherever the highest quality is re quired, its resiliency being second to none. Great pressure is used in modern hammer-making and the density at the treble end where the hammer dimensions are small, is more than double that of the bass hammers. The toner, by means of pricking the felt, lessens the hardness beneath the nose of the hammer, without disturbing it at the actual point of contact. In this he is entirely guided by his ear, the aim being to eliminate undesirable tone components and secure an evenly graded scale, with no audible break between the treble and bass strings.
The hammers are actuated by a system of levers known as the "action," which is now made by firms specialising in this unit alone, rather than by the piano manufacturer himself. The noses of the hammers rest at a distance of two inches from the strings, but if a note is held down after having been played, the hammer is "checked" when it has rebounded five-eighths of an inch, so that the blow can be repeated rapidly. When the key is half-way down, the felt "damper" is lifted from the strings and remains lifted until the key is released.
The tone is modified by means of two pedals, the left-hand pedal taking the hammers nearer to the strings and thus decreasing the force of the blow. Sometimes the whole action is shifted to the right, so that the hammer only strikes two strings instead of three. The right-hand pedal lifts all the dampers from the strings so as to sustain harmonies as long as required, and not merely for increased power. In America a central pedal is often added, called the "Sostenuto," which, instead of lifting all the dampers, affects only those which happen to be already lifted, keeping them so until the pedal is released. Thus one is able to sustain a chord, leaving the hand free for other notes.
The ideal point of contact for the dampers is at the same place as the striking point, and in grand pianos this is approximately arranged, the dampers lying along the strike line above the strings, and operated by gravity only. In upright pianos a position just under the strike line is favoured, such actions being known as "under-damper," and the pressure obtained by springs. The
cheaper piano is sometimes provided with an older type wherein the damper is operated by gravity at a point just above the ham mers. This is known as the "over-damper" action, and although the vibrations of the strings are of less amplitude above the hammer-line than below it, a well-made over-damper action is still in favour by certain Continental makers of repute, owing to its more effective silencing of the harmonics.
The depth of touch for the keys has been standardised in all countries at 9 millimetres, with an additional millimetre to allow of the locking of the check and the free escapement of the "Jack" lever. The pivoting of the key has to be so placed as to give the correct amount of lift at the other end, usually about 6 millimetres, but varying a little with the different makes of action. The best "touch" is secured if the keys slope backwards a little when at rest, becoming exactly level when half depressed. At this point begins the extra load due to the lifting of the dampers, but the momentum already gained is sufficient to render it unnotice able. A "touch weight" is used during the regulating of the action, so that when it is placed upon the end of the key, and suddenly released, the force exerted is just sufficient to make the note sound faintly. This "dead weight" of touch is not standardised, but averages 90 grammes in the bass, down to 6o grammes in the treble.
The iron frame, which takes most of the load exerted by the pull of the 22o strings, is cast in one piece at the foundry, from an iron pattern which has been made 1 per cent larger than the finished frame, in order to allow for the contraction of the casting during cooling.
The tuning pins of mild steel, about 7 milli metres in diameter, are driven into a wrest-plank of quartered beech or maple, sometimes of ply construction to prevent splitting. The holes are bored slightly smaller than the pins, a ratio of 15:17 giving the correct amount of tightness. Since the plank is bolted to the iron frame and glued to the wooden back it follows that the load is shared between the frame and the back, the tendency in modern construction being to eliminate the wooden back alto gether. The wrest-plank is often covered with a web of iron into which maple plugs have been driven, the tuning pins passing through these plugs into the plank. This method has proved very efficient and is being increasingly adopted in both upright and grand pianos. Patent methods of securing the tuning-pins have been introduced from time to time, but the method above de scribed remains, and would seem likely to survive, being both efficient and free from complications.