As the strings vibrate they transmit pressure differences to the bridge attached to the sound-board (the bridge at the other end being as rigid as possible). The large area of the sound-board is thus set in motion, and this in turn agitates the air in its immediate vicinity, the aim being to provide a sen sitive and extremely elastic board, capable of reproducing the most complicated wave-forms, with as little loss of energy as possible. In addition, the board must be made of material of low density and of uniform texture. The timber of the spruce family (Picea excelsa), where cut "on the quarter," answers all these requirements, the best qualities transmitting sound-waves along the grain at the rate of 16,00o feet per second and upwards. The transmission across the grain is about one-fourth of this speed, a difficulty which is surmounted by gluing bars of the same material across the back of the board at right angles to the grain. The Grotrian-Steinweg sound-boards are called "homogeneous," which means that the material selected has as far as possible the same closeness of grain all over its surface. This timber became known as "Swiss Pine," but is not botanically a pine. In America sound boards are made of another timber of the same family, Picea alba, and a darker coloured wood of similar texture, Abies pectinata, is also used.
Most sound-boards are made with the grain running diagonally so that the long bridge runs almost parallel to the grain. The thickness is usually about
all over, some manufacturers insist ing, however, that the treble end should be thicker. There is some theoretical evidence in favour of this, but in practice it is difficult to discover any difference. Sound-boards are always fitted so that they are slightly "bucked" or high in the middle, as though part of the surface of a sphere of about sixty feet radius. This shape is secured in several ways, perhaps the best being to subject the board to a dry temperature of IIo degrees Fahrenheit for some hours, and then to glue on the bars which have been planed to a slight curve. The subsequent expansion as the board takes up its usual ten per cent of moisture from the air produces the desired curvature. Since the board is always under pressure from the strings it follows that it behaves as an arch, which shape is re tained through the rigidity of the rim to which it is fixed.
Good contact is maintained between strings and bridge by leaving the bridge a little higher than the general level of the two extremities. The resulting pressure of the strings on the sound-board is known as the "down-bearing," the regulation of which is one of the most critical processes in piano forte manufacture. It is agreed that about one-fortieth of the ten sion is a suitable amount for each string to press upon the bridge. Translated into degrees this means that the bridge must stand high enough to allow the string to dip one and a half degrees after it passes over the bridge on its way to the hitch-pin on the iron frame. Allowance has to be made for the fact that as the tension
is applied the whole board is pressed down somewhat, and the amount of such sinking depends upon its dimensions, barring and support round the edge. The extra height is not measured at the bridge, but by means of a straightedge laid across where the string will pass, the amount of clearance at the hitch-pin is allowed for. (See fig. 2.) It does not matter so much what the exact pressure is, but inequalities of pressure cause very serious results, affecting those strings which lie between places of ex cessive pressure, and robbing them of their adequate contact with the bridge. Good contact between strings and bridge is also maintained by means of bridge-pins which lie in the line of the strings and are inserted at an angle of 7o degrees with the plane of the bridge. By passing the string to the left of the front pin and to the right of the back pin a frictional grip is secured. This practice is almost universal, the only outstanding departure being Messrs. Broadwood's method of screwing studs to the bridge and passing the strings through these. Previous efforts to provide a studded bridge have been discarded owing to the studs working loose and causing falseness of tone, but Messrs. Broadwood claim to have overcome this difficulty.
Pianoforte tone is compound, each note consisting of a fundamental tone and its overtones, which, sounding together, form the "harmonic series," with their vibra tion rates in the ratio of : 2 :3 :4, etc. For instance, the tone of "middle C" has for harmonics the following notes, the frequencies of which are inserted in brackets: C (261), C (522), G (783), C (1,024), E (1,305), G (1,566), Bb (1,827), C (2,088), D (2,349) and so on. If these notes be played together as a chord it will be at once detected that the harmonics blend together pleasingly, with the exception of the seventh and ninth which are dissonant. By striking the strings at one-eighth of their length from the fixed end, it is believed that the best tone quality is produced, whether by discouraging the 7th and 9th harmonics or by encouraging the 4th is not yet scientifically proved. This strike proportion is used for all notes from the bass up to "pitch C," after which other considerations make it necessary to strike the string gradually nearer the end, the fraction at the extreme treble being empirically decided upon at about Since the
cordant harmonics of these notes are very weak and even beyond the limits of human audition it does not matter if they are pro duced, the striking point being arranged to allow of a quicker recoil of the hammer from the string.