CHAPTER VIII.RESONANCE AND THE RESONANCE-APPARATUS OF THE PIANOFORTE.We have now made a somewhat lengthy and thorough investigation into the nature and behavior of the various materials and substances that are employed in the construction of pianoforte strings. From this inquiry we have been able to deduce a set of rules which, when practically applied, will furnish us with a guide to the solution of many perplexing problems which take their root in the conditions imposed upon the designer by the limitations of space and the other mechanical conditions of pianoforte construction. It would not be proper, however, to proceed forthwith to the practical questions of support for the strings. For we must still find the correct solutions of another series of problems that spring, not from the strings themselves, but from their important and necessary accessories, the sound-board and belly bridges.The belly-bridge is the medium of connection between the strings and the sound-board. Through it the vibrations excited in the strings are conveyed to the freely vibrating surface of the sound-board, and the sonority of the generated sounds is thereby enormously increased. This is the process in bare outline, but, in order to obtain a proper view of the matter under discussion it will be necessary to examine the phenomena to which the juxtaposition of strings, bridge and sound-board give rise. We must, in fact, make another brief excursion into the realms of acoustics.The property which the sound-board possesses of reinforcing and emphasizing the sounds generated by the strings is called “resonance.” Important as this property of sonorous bodies is to musicians and the makers of musical instruments, the fact remains that it is a matter very little understood by the mass of them. This is the more remarkable when one considers that, without resonant properties, no musical instruments would be possible. For it is not difficult to perceive that music, as we know it, could not exist were themeans of expressing it limited to the actual and immediate bodies that perform the motions which are the direct causes of musical sounds. This fact is most clearly illustrated in the case of the pianoforte. The unaided sound of a pianoforte string is ridiculously feeble; in fact, it is quite inaudible at the distance of a few feet. Yet we are all familiar with the wonderfully harmonious and powerful sounds that the same string will be the means of producing when aided by the sound-board.Resonance may be defined as the property which one sonorous body possesses of impressing its vibrations upon another sonorous body. The existence of this power may be demonstrated in a variety of ways. The most simple proofs are afforded by the pianoforte itself. For example, if we strike any key upon the instrument and at the same time gently press down the corresponding key one octave higher, so as to raise the damper without at the same time raising the hammer, we shall find that if the first key be released while the other is held open, the string corresponding to the latter will continue to give its proper sound. In this case the vibrations excited in the first string travel along the belly-bridge until they reach the nearest open string whose vibration rate is synchronous with that of the original sounding string. When such a string is reached it is immediately impressed with the motions excited in the former string, with the results above described. This is a case of resonance of two attached bodies. Peculiar as it may seem, however, it is notessentialas a preliminary condition to the existence of resonance between them that two sonorous bodies be tangibly connected. For instance, the foregoing experiment may be varied by employing two pianofortes and choosing one of the sounds from each. The result will be precisely the same. It will, however, be noted that only such sounds as have either synchronous or nearly synchronous rates of vibration will exhibit the phenomena of resonance when separated from each other. Where they are connected, however, especially when the connecting body is a sound-board prepared for the purpose, synchronism is not necessary. In fact, it is a matter of common observation that the sound-board of the pianoforte, in conjunction with the belly-bridge, operates to set up more or less intense vibration in every string within the compass of the instrument when the dampers are raised, even if only one string be struck. When the damper pedal is raised in playing, every string throughout the instrument is immediately thrown into a state of vibration, and begins to sound. The result is a large augmentation of the total volumeof sound produced. Of course, the sound of any one string thus sympathetically excited is relatively feeble, but the total volume is considerable, with especial strength in the particular partials of each string that are more or less synchronous, as to their vibration rates, with the sounds originally produced by striking the keys. When the dampers are permitted to rest in their normal positions, on the other hand, the sound-board exercises its resonant powers in a different manner. Whenever a string or group of strings are struck, the board is thrown into a state of vibration which affects only itself and not the strings that remain damped. The result of this excitement is to expose a relatively great vibrating surface to the atmosphere, with the immediate consequence that the quantity of air impelled into a state of periodic motion is multiplied many times. Thus the size of the impelled layers of air, and the resultant sonorous waves is augmented until we obtain sounds of the intensity and richness which we are accustomed to associate with the pianoforte.Now, from what we have already learned of the laws of tone-quality, it is obvious that the resonant medium must be capable of reinforcing not only the fundamental but the partials of all the tones which it influences. To this end we must provide a substance that combines elasticity with the freedom of vibration that is, of course, essential. It is not possible to employ metal on account of its excessive stiffness and consequent resistance to the influence of impressed vibrations; while on the other hand a wooden body will not be sufficiently stiff and rigid unless artificially strengthened. For this reason it is customary to construct sound-boards of a freely-vibrating wood (the spruce-fir is generally employed for this purpose) and to strengthen them by fastening to one side bars of hard wood called “ribs.” In this manner the requisite stiffness is imparted to the board, which at the same time is sufficiently susceptible to the impressed vibrations from the strings.It is a fact that this accepted and universal form of resonance table is essentially similar to that which was used in the ancient harpsichord and spinet. While there has been much experimentation along these lines, it does not appear that any lasting improvements have been devised as yet, at least in the governing principles of sound-board construction. We may then confine ourselves to a description of the accepted styles.The wood used in the construction of sound-boards is the spruce-fir, which, as stated above, has been found to be the best possible for the purpose. It is prepared in a sheet of suitable size, and isarranged so that the grain runs approximately at right angles to the plane of the belly-bridge.It can easily be understood that the thickness of the board must vary according to the dimensions of the strings that act upon it. In other words, we can perceive that more resonating power is required for the relatively weaker treble strings than for the relatively stronger bass strings. The actual thicknesses vary with individual makers. From3⁄8inch in the treble to1⁄4of an inch in the bass may be regarded as a fair approximation. Nevertheless, it is necessary to bear in mind that these dimensions are subject to modification according to the variations in the total amount of tension that the instrument is made to bear. Other things being equal, an increased tension load implies a thicker board, and vice-versa.After the dimensions and material of the board are thus determined, it remains to consider the bridging, the reinforcement, and the adjustment of the board. We shall consider these in their natural order, as given above.The belly-bridges are placed upon the surface of the board, as we know, for the purpose of conveying to the latter the minute blows that are inflicted by the vibrating strings, in order that the vibrations may be impressed upon the board and there amplified and intensified as described at the beginning of this chapter. A secondary duty is that of delimiting the lower boundaries of the speaking length of the strings. The bridges must naturally be constructed with a curved outline that is determined during the draughting of the scale. The actual shape of this curve has no effectper se, upon the activities of the bridge, but has to do entirely with the string lengths. The bridge which carries the overstrung portion of the scale may be considered as being similarly affected, as to outline, by the exigencies of the bass string dimensions. The bridges are made of hard wood, and their sizes are usually from one inch and one-quarter to one inch and one-half high, and in width about one-eighth of an inch less all round. The variations occur principally on account of the necessity which arises of giving a bearing to the strings as they cross over the bridges.It is necessary that the strings be raised at the bridges in order that they may be firmly held at the points of contact by means of the strain imposed by them on the surface of the bridge when they are stretched at proper tension. Of course it is most essential that this bearing be not too high, as in that case the strain becomes too much for the board to bear with facility and its durability is therebyimpaired. The necessary immobility of the portions of the strings that lie upon the bridge is secured by diverting the line of travel, and causing them to bear against pins placed on either edge of the bridge, so as to slant the line of the string as it passes over. The waste ends should run parallel to the speaking-length after the bridge has been crossed.It may be further properly remarked that there are interesting and complicated problems to be overcome in choosing the material and the exact method of building the belly-bridges. It is desired to combine extreme facility of vibration with the requisite resisting power. In other words, the bridge must allow the fullest possible scope to the impressed vibrations from the strings and, at the same time, must possess such strength that it can successfully resist the torsions imposed upon it by the pull of the strings. The only method that appears to be thoroughly practical and, at the same time, acoustically correct is one which most manufacturers have already had the acuteness to adopt. The bridge, according to this method, is built up of a number of layers of hard wood (generally maple) which are glued together in such a way that the grain of each layer crosses that of the other. In this way both the requisite strength and more or less facility of vibration are obtained. But it has remained for one distinguished piano maker to go a step further, and to apply thoroughly scientific methods to the design of the belly-bridges. In the instruments made by him, he has built the bridges in such a way that the impressed vibrations will travelin the line ofthe grain instead of across it. The bridges, in fact, are built of end-grain and not, as is general, of cross-grain wood. This ingenious and simple device facilitates the passage of the impressed vibrations and, in consequence, tends to impart a greater clarity to the various partials of the compound tones. Some existing pianofortes might be greatly improved as to their clarity of speech if a similar device for increasing the power of resonance were fitted to them.There is another point to be emphasized in reference to the bridges. In some makes of pianos the line of the bridge construction is permitted to be broken wherever there is a corresponding break in the hammer line caused by the interposition of the various braces of the iron frame. The obvious result of such a method of construction is that the resonance of the board is much interfered with and the consequent tonal efficiency of the instrument lessened. For it is easy to see that if the bridge line be broken at any point, the vibrations that are carried from any sounding string along the bridgeto the surface of the sound-board will be stopped at the break and will be unable to reach those parts of the board that are remote from its path, with rapidity and ease. Incontestably, therefore, the bridge line should, if possible, be continuous. Many manufacturers, however, while apparently recognizing the force of this proposition, seem to be afraid to follow it out to its logical conclusion. They are willing to make the line of bridge continuous until the end of the plain wire strings is reached. After that point they seem to think that it is no longer necessary that continuity of communication between the various sounding members of the scale should subsist. This idea is, of course, quite fallacious. The bass strings are simply a continuation of the higher ones, and are, in fact, precisely similar except in regard to the details of thickness and length. Moreover, it is quite as important that the portions of the board over which the bass bridge exercises control should be made freely resonant, as it is that this process should be applied to the others. The bass bridge ought invariably, therefore, to be connected with the bridges that serve the rest of the strings.The reinforcement of the board is accomplished by gluing ribs of wood across its back surface in a direction crossing the grain of the board. These ribs are usually made about one inch square in the middle portions. This size is continued until near the edge of the board on each side, when they are gradually pared down in a graceful curve until at the actual edge the thickness is no more than about one-thirty-second of an inch. According to the most approved modern practice it is found advisable to pocket these ribs into the wooden framing of the instrument, by continuing them past the edge of the sound board and making suitable apertures in the framing, into which the extensions are adjusted and fastened. This has the effect of holding the board more firmly in its fastenings and also of preventing the early loosening of the ribs from their places; an occurrence which causes much rattling, and complete impairment of tonal quality. It is usual to have twelve ribs upon the surface of the board, but the number may be varied whenever it is considered necessary. If it is required to give specially ample support to the board on account of unusually great strain, or for any other reason, the number may be increased, but such procedure must be taken with caution, as too many ribs weight the board to such an extent as to deaden its power of molecular and undulatory vibration. This must at all costs be avoided.It is usual to glue the ribs upon the surface of the board first—thatis before the bridges—and good practice dictates that the surface of the board be dried out in a hot-box for at least 24 hours before either of these processes take place. If this be carried out properly, the resultant shrinking of the wood will be taken up after the board has become thoroughly cooled, and if the process is repeated when the board is glued into the framing of the instrument, the result will be to endow it with a natural “crown,” or arch, caused by the re-active swelling that takes place after the artificially induced shrinking.Although the above methods of ribbing are to be considered the best and as representing the most advanced practice, yet it will be found that some makers dispose the ribs in a fan-like manner, having the diverging points of the fan at the upper end of the board, while others adopt an oblique disposition and arrange them as before described. Also, we find a straight up-and-down arrangement whereby the ribs are glued parallel to the plane of the treble strings. We term these three styles the fan form, the oblique form, and the vertical form respectively.As for the comparative advantages of the three types of construction thus described, it may be said that they all represent individual features that are more or less beneficial. For example, the fan-like disposition gives a greater number of long ribs, while the oblique form provides more of separate units. The vertical system may be considered as a mean between the other two.In general, we shall be well advised in remembering that the prime function of ribbing is to increase the tension of the board and its elasticity, and thus to promote the power of resonance. A secondary function is that of providing extra resisting power. Now it is obvious that both of these duties can be better performed by a multiplicity of ribs, and consequently a system is to be recommended that permits the employment of the largest total area of ribbing. At the same time unduly long ribs are not good, for they have a greater tendency to become loose and to spring up from the surface of the board, with dire results to tone and durability. It would therefore seem that the oblique disposition has more to recommend it than the others, since it provides enough total ribbing area without imposing inconveniently long ribbing units upon the surface of the board.When the ribbing of the sound-board and the fixing of the bridges has been accomplished, it remains to adjust the completed structure within the wooden back-framing of the instrument. It isnecessary that the board be so secured that it shall acquire a position analogous to that of a stretched membrane—at least as far as concerns the rigidity with which its edges are fixed to the framing. There are several methods for obtaining the required rigidity of the edges of the board. The natural or artificial crowning of the board’s surface is best attained through the medium of particularly rigid edge fastening; and the adoption of a continuous closing rim for the board, as in certain grand pianofortes, together with the use of a system of screw compression, alike indicate the various directions in which the ideas of experimenters have led them. The underlying notion in all these devices is to endow the vibrating surface with both elasticity and durability to an extent that could not be attained with the unaided wood.The gluing of the sound-board to the framing is a process that demands the greatest skill and care. It is essential that the board be warmed, and that the glue which is used be in just the proper condition; neither too thick nor too thin, and, above all, boiling hot. If the fastening be done when the board is in the shrunken condition described above, and with the required skill and care, it will be found that the fibres of the wood have been squeezed together so as to raise the center part of the board somewhat above the level of the edges. This gives what we have denominated the “crown,” and is important as affecting the durability and resisting power of the entire board. It must be remembered that by relieving the sound-board of as much as possible of the strain imposed by the strings, we are able to increase its durability and to preserve its tone-producing quality more surely than is otherwise possible. Boards that are not so protected must inevitably become entirely flattened out in the course of a few years. When this happens the level of the belly-bridge sinks and the bearing of the strings upon the latter is destroyed. Hence an immediate and inevitable deterioration of tone quality. For the altering of the level in this manner affects the impression of the vibrations of the strings upon the bridges and hence upon the board itself. If the height of the bridges be too great, the bearing of the strings upon them will likewise be excessive, and the board will be crushed down in the same manner. If, however, the directions as to bridging, ribbing and adjustment that have been given are followed with discretion, the troubles outlined here are likely at least to be minimized.Of course, the later care of the pianoforte after it is sold has much to do with the manifold troubles that occur within the entireresonance apparatus. These things cannot be foreseen, and it is, therefore, most essential to guard against them as much as possible by careful attention to the details of construction and adjustment.Lastly, we may observe that the practice of screwing the bridges down on to the board by screws driven in from the rear is to be condemned. While it is undoubtedly advantageous to take some measure to increase the permanency of the fastening, it will be found that it is far better, acoustically, to provide the bridge with wooden dowels and glue these into suitable holes in the board. Thus the conducting power of the bridge is increased and the vibrating surface of the sound-board is not broken up by the insertion of foreign metallic substances. Another and concomitant advantage is the absence of the wooden washers under the heads of these bridge-screws. Such devices are too often, as they become loose, a source of rattling and jingling.It is well to be rid of them, as of all possible things that are likely to be similarly affected by wear or atmospheric conditions.
We have now made a somewhat lengthy and thorough investigation into the nature and behavior of the various materials and substances that are employed in the construction of pianoforte strings. From this inquiry we have been able to deduce a set of rules which, when practically applied, will furnish us with a guide to the solution of many perplexing problems which take their root in the conditions imposed upon the designer by the limitations of space and the other mechanical conditions of pianoforte construction. It would not be proper, however, to proceed forthwith to the practical questions of support for the strings. For we must still find the correct solutions of another series of problems that spring, not from the strings themselves, but from their important and necessary accessories, the sound-board and belly bridges.
The belly-bridge is the medium of connection between the strings and the sound-board. Through it the vibrations excited in the strings are conveyed to the freely vibrating surface of the sound-board, and the sonority of the generated sounds is thereby enormously increased. This is the process in bare outline, but, in order to obtain a proper view of the matter under discussion it will be necessary to examine the phenomena to which the juxtaposition of strings, bridge and sound-board give rise. We must, in fact, make another brief excursion into the realms of acoustics.
The property which the sound-board possesses of reinforcing and emphasizing the sounds generated by the strings is called “resonance.” Important as this property of sonorous bodies is to musicians and the makers of musical instruments, the fact remains that it is a matter very little understood by the mass of them. This is the more remarkable when one considers that, without resonant properties, no musical instruments would be possible. For it is not difficult to perceive that music, as we know it, could not exist were themeans of expressing it limited to the actual and immediate bodies that perform the motions which are the direct causes of musical sounds. This fact is most clearly illustrated in the case of the pianoforte. The unaided sound of a pianoforte string is ridiculously feeble; in fact, it is quite inaudible at the distance of a few feet. Yet we are all familiar with the wonderfully harmonious and powerful sounds that the same string will be the means of producing when aided by the sound-board.
Resonance may be defined as the property which one sonorous body possesses of impressing its vibrations upon another sonorous body. The existence of this power may be demonstrated in a variety of ways. The most simple proofs are afforded by the pianoforte itself. For example, if we strike any key upon the instrument and at the same time gently press down the corresponding key one octave higher, so as to raise the damper without at the same time raising the hammer, we shall find that if the first key be released while the other is held open, the string corresponding to the latter will continue to give its proper sound. In this case the vibrations excited in the first string travel along the belly-bridge until they reach the nearest open string whose vibration rate is synchronous with that of the original sounding string. When such a string is reached it is immediately impressed with the motions excited in the former string, with the results above described. This is a case of resonance of two attached bodies. Peculiar as it may seem, however, it is notessentialas a preliminary condition to the existence of resonance between them that two sonorous bodies be tangibly connected. For instance, the foregoing experiment may be varied by employing two pianofortes and choosing one of the sounds from each. The result will be precisely the same. It will, however, be noted that only such sounds as have either synchronous or nearly synchronous rates of vibration will exhibit the phenomena of resonance when separated from each other. Where they are connected, however, especially when the connecting body is a sound-board prepared for the purpose, synchronism is not necessary. In fact, it is a matter of common observation that the sound-board of the pianoforte, in conjunction with the belly-bridge, operates to set up more or less intense vibration in every string within the compass of the instrument when the dampers are raised, even if only one string be struck. When the damper pedal is raised in playing, every string throughout the instrument is immediately thrown into a state of vibration, and begins to sound. The result is a large augmentation of the total volumeof sound produced. Of course, the sound of any one string thus sympathetically excited is relatively feeble, but the total volume is considerable, with especial strength in the particular partials of each string that are more or less synchronous, as to their vibration rates, with the sounds originally produced by striking the keys. When the dampers are permitted to rest in their normal positions, on the other hand, the sound-board exercises its resonant powers in a different manner. Whenever a string or group of strings are struck, the board is thrown into a state of vibration which affects only itself and not the strings that remain damped. The result of this excitement is to expose a relatively great vibrating surface to the atmosphere, with the immediate consequence that the quantity of air impelled into a state of periodic motion is multiplied many times. Thus the size of the impelled layers of air, and the resultant sonorous waves is augmented until we obtain sounds of the intensity and richness which we are accustomed to associate with the pianoforte.
Now, from what we have already learned of the laws of tone-quality, it is obvious that the resonant medium must be capable of reinforcing not only the fundamental but the partials of all the tones which it influences. To this end we must provide a substance that combines elasticity with the freedom of vibration that is, of course, essential. It is not possible to employ metal on account of its excessive stiffness and consequent resistance to the influence of impressed vibrations; while on the other hand a wooden body will not be sufficiently stiff and rigid unless artificially strengthened. For this reason it is customary to construct sound-boards of a freely-vibrating wood (the spruce-fir is generally employed for this purpose) and to strengthen them by fastening to one side bars of hard wood called “ribs.” In this manner the requisite stiffness is imparted to the board, which at the same time is sufficiently susceptible to the impressed vibrations from the strings.
It is a fact that this accepted and universal form of resonance table is essentially similar to that which was used in the ancient harpsichord and spinet. While there has been much experimentation along these lines, it does not appear that any lasting improvements have been devised as yet, at least in the governing principles of sound-board construction. We may then confine ourselves to a description of the accepted styles.
The wood used in the construction of sound-boards is the spruce-fir, which, as stated above, has been found to be the best possible for the purpose. It is prepared in a sheet of suitable size, and isarranged so that the grain runs approximately at right angles to the plane of the belly-bridge.
It can easily be understood that the thickness of the board must vary according to the dimensions of the strings that act upon it. In other words, we can perceive that more resonating power is required for the relatively weaker treble strings than for the relatively stronger bass strings. The actual thicknesses vary with individual makers. From3⁄8inch in the treble to1⁄4of an inch in the bass may be regarded as a fair approximation. Nevertheless, it is necessary to bear in mind that these dimensions are subject to modification according to the variations in the total amount of tension that the instrument is made to bear. Other things being equal, an increased tension load implies a thicker board, and vice-versa.
After the dimensions and material of the board are thus determined, it remains to consider the bridging, the reinforcement, and the adjustment of the board. We shall consider these in their natural order, as given above.
The belly-bridges are placed upon the surface of the board, as we know, for the purpose of conveying to the latter the minute blows that are inflicted by the vibrating strings, in order that the vibrations may be impressed upon the board and there amplified and intensified as described at the beginning of this chapter. A secondary duty is that of delimiting the lower boundaries of the speaking length of the strings. The bridges must naturally be constructed with a curved outline that is determined during the draughting of the scale. The actual shape of this curve has no effectper se, upon the activities of the bridge, but has to do entirely with the string lengths. The bridge which carries the overstrung portion of the scale may be considered as being similarly affected, as to outline, by the exigencies of the bass string dimensions. The bridges are made of hard wood, and their sizes are usually from one inch and one-quarter to one inch and one-half high, and in width about one-eighth of an inch less all round. The variations occur principally on account of the necessity which arises of giving a bearing to the strings as they cross over the bridges.
It is necessary that the strings be raised at the bridges in order that they may be firmly held at the points of contact by means of the strain imposed by them on the surface of the bridge when they are stretched at proper tension. Of course it is most essential that this bearing be not too high, as in that case the strain becomes too much for the board to bear with facility and its durability is therebyimpaired. The necessary immobility of the portions of the strings that lie upon the bridge is secured by diverting the line of travel, and causing them to bear against pins placed on either edge of the bridge, so as to slant the line of the string as it passes over. The waste ends should run parallel to the speaking-length after the bridge has been crossed.
It may be further properly remarked that there are interesting and complicated problems to be overcome in choosing the material and the exact method of building the belly-bridges. It is desired to combine extreme facility of vibration with the requisite resisting power. In other words, the bridge must allow the fullest possible scope to the impressed vibrations from the strings and, at the same time, must possess such strength that it can successfully resist the torsions imposed upon it by the pull of the strings. The only method that appears to be thoroughly practical and, at the same time, acoustically correct is one which most manufacturers have already had the acuteness to adopt. The bridge, according to this method, is built up of a number of layers of hard wood (generally maple) which are glued together in such a way that the grain of each layer crosses that of the other. In this way both the requisite strength and more or less facility of vibration are obtained. But it has remained for one distinguished piano maker to go a step further, and to apply thoroughly scientific methods to the design of the belly-bridges. In the instruments made by him, he has built the bridges in such a way that the impressed vibrations will travelin the line ofthe grain instead of across it. The bridges, in fact, are built of end-grain and not, as is general, of cross-grain wood. This ingenious and simple device facilitates the passage of the impressed vibrations and, in consequence, tends to impart a greater clarity to the various partials of the compound tones. Some existing pianofortes might be greatly improved as to their clarity of speech if a similar device for increasing the power of resonance were fitted to them.
There is another point to be emphasized in reference to the bridges. In some makes of pianos the line of the bridge construction is permitted to be broken wherever there is a corresponding break in the hammer line caused by the interposition of the various braces of the iron frame. The obvious result of such a method of construction is that the resonance of the board is much interfered with and the consequent tonal efficiency of the instrument lessened. For it is easy to see that if the bridge line be broken at any point, the vibrations that are carried from any sounding string along the bridgeto the surface of the sound-board will be stopped at the break and will be unable to reach those parts of the board that are remote from its path, with rapidity and ease. Incontestably, therefore, the bridge line should, if possible, be continuous. Many manufacturers, however, while apparently recognizing the force of this proposition, seem to be afraid to follow it out to its logical conclusion. They are willing to make the line of bridge continuous until the end of the plain wire strings is reached. After that point they seem to think that it is no longer necessary that continuity of communication between the various sounding members of the scale should subsist. This idea is, of course, quite fallacious. The bass strings are simply a continuation of the higher ones, and are, in fact, precisely similar except in regard to the details of thickness and length. Moreover, it is quite as important that the portions of the board over which the bass bridge exercises control should be made freely resonant, as it is that this process should be applied to the others. The bass bridge ought invariably, therefore, to be connected with the bridges that serve the rest of the strings.
The reinforcement of the board is accomplished by gluing ribs of wood across its back surface in a direction crossing the grain of the board. These ribs are usually made about one inch square in the middle portions. This size is continued until near the edge of the board on each side, when they are gradually pared down in a graceful curve until at the actual edge the thickness is no more than about one-thirty-second of an inch. According to the most approved modern practice it is found advisable to pocket these ribs into the wooden framing of the instrument, by continuing them past the edge of the sound board and making suitable apertures in the framing, into which the extensions are adjusted and fastened. This has the effect of holding the board more firmly in its fastenings and also of preventing the early loosening of the ribs from their places; an occurrence which causes much rattling, and complete impairment of tonal quality. It is usual to have twelve ribs upon the surface of the board, but the number may be varied whenever it is considered necessary. If it is required to give specially ample support to the board on account of unusually great strain, or for any other reason, the number may be increased, but such procedure must be taken with caution, as too many ribs weight the board to such an extent as to deaden its power of molecular and undulatory vibration. This must at all costs be avoided.
It is usual to glue the ribs upon the surface of the board first—thatis before the bridges—and good practice dictates that the surface of the board be dried out in a hot-box for at least 24 hours before either of these processes take place. If this be carried out properly, the resultant shrinking of the wood will be taken up after the board has become thoroughly cooled, and if the process is repeated when the board is glued into the framing of the instrument, the result will be to endow it with a natural “crown,” or arch, caused by the re-active swelling that takes place after the artificially induced shrinking.
Although the above methods of ribbing are to be considered the best and as representing the most advanced practice, yet it will be found that some makers dispose the ribs in a fan-like manner, having the diverging points of the fan at the upper end of the board, while others adopt an oblique disposition and arrange them as before described. Also, we find a straight up-and-down arrangement whereby the ribs are glued parallel to the plane of the treble strings. We term these three styles the fan form, the oblique form, and the vertical form respectively.
As for the comparative advantages of the three types of construction thus described, it may be said that they all represent individual features that are more or less beneficial. For example, the fan-like disposition gives a greater number of long ribs, while the oblique form provides more of separate units. The vertical system may be considered as a mean between the other two.
In general, we shall be well advised in remembering that the prime function of ribbing is to increase the tension of the board and its elasticity, and thus to promote the power of resonance. A secondary function is that of providing extra resisting power. Now it is obvious that both of these duties can be better performed by a multiplicity of ribs, and consequently a system is to be recommended that permits the employment of the largest total area of ribbing. At the same time unduly long ribs are not good, for they have a greater tendency to become loose and to spring up from the surface of the board, with dire results to tone and durability. It would therefore seem that the oblique disposition has more to recommend it than the others, since it provides enough total ribbing area without imposing inconveniently long ribbing units upon the surface of the board.
When the ribbing of the sound-board and the fixing of the bridges has been accomplished, it remains to adjust the completed structure within the wooden back-framing of the instrument. It isnecessary that the board be so secured that it shall acquire a position analogous to that of a stretched membrane—at least as far as concerns the rigidity with which its edges are fixed to the framing. There are several methods for obtaining the required rigidity of the edges of the board. The natural or artificial crowning of the board’s surface is best attained through the medium of particularly rigid edge fastening; and the adoption of a continuous closing rim for the board, as in certain grand pianofortes, together with the use of a system of screw compression, alike indicate the various directions in which the ideas of experimenters have led them. The underlying notion in all these devices is to endow the vibrating surface with both elasticity and durability to an extent that could not be attained with the unaided wood.
The gluing of the sound-board to the framing is a process that demands the greatest skill and care. It is essential that the board be warmed, and that the glue which is used be in just the proper condition; neither too thick nor too thin, and, above all, boiling hot. If the fastening be done when the board is in the shrunken condition described above, and with the required skill and care, it will be found that the fibres of the wood have been squeezed together so as to raise the center part of the board somewhat above the level of the edges. This gives what we have denominated the “crown,” and is important as affecting the durability and resisting power of the entire board. It must be remembered that by relieving the sound-board of as much as possible of the strain imposed by the strings, we are able to increase its durability and to preserve its tone-producing quality more surely than is otherwise possible. Boards that are not so protected must inevitably become entirely flattened out in the course of a few years. When this happens the level of the belly-bridge sinks and the bearing of the strings upon the latter is destroyed. Hence an immediate and inevitable deterioration of tone quality. For the altering of the level in this manner affects the impression of the vibrations of the strings upon the bridges and hence upon the board itself. If the height of the bridges be too great, the bearing of the strings upon them will likewise be excessive, and the board will be crushed down in the same manner. If, however, the directions as to bridging, ribbing and adjustment that have been given are followed with discretion, the troubles outlined here are likely at least to be minimized.
Of course, the later care of the pianoforte after it is sold has much to do with the manifold troubles that occur within the entireresonance apparatus. These things cannot be foreseen, and it is, therefore, most essential to guard against them as much as possible by careful attention to the details of construction and adjustment.
Lastly, we may observe that the practice of screwing the bridges down on to the board by screws driven in from the rear is to be condemned. While it is undoubtedly advantageous to take some measure to increase the permanency of the fastening, it will be found that it is far better, acoustically, to provide the bridge with wooden dowels and glue these into suitable holes in the board. Thus the conducting power of the bridge is increased and the vibrating surface of the sound-board is not broken up by the insertion of foreign metallic substances. Another and concomitant advantage is the absence of the wooden washers under the heads of these bridge-screws. Such devices are too often, as they become loose, a source of rattling and jingling.
It is well to be rid of them, as of all possible things that are likely to be similarly affected by wear or atmospheric conditions.