US3562027A - Piano plate of low loss factor - Google Patents

Abstract

A PIANO PLATE MADE OF A CAST FERROUS METAL HAVING A LOSS FACTOR (Q-1) OF FROM 1.5X10**-3 TO 0.2X10**-3, WHEREBY THE SOUND DAMPING EFFECT OF A PIANO IN WHICH THIS PLATE IN INSTALLED BECOMES EXTREMELY SMALL, AND THE ACOUSTICAL CHARACTERISTICS OF THE PIANO ARE GREATLY IMPROVED. AT THE SAME TIME, A GREAT REDUCTION IN WEIGHT OF THE PLATE CAN BE EFFECTED WITHOUT SACRIFICING THE MECHANICAL STRENGTH AND THE IMPROVED ACOUSTICAL CHARACTERISTICS. SUITABLE CAST FERROUS METALS ARE CAST STEELS, SPHEROIDAL GRAPHITE CAST IRONS, MALLEABLE CAST IRONS, AND MEEHANITE CAST IRONS.

D R A W I N G

Description

I Feb. 9, 1971 MASAYUKI TAKAMURA ET AL 3,562,027

PIANO PLATE OF LOW LOSS FACTOR Filed March 13, 1968 5 Sheets-Sheet 1 IO 8 *1 ATTENUATOR l2 y F GATING DEVICE f L J 9 B E O W I N VEN TOR. W Wk mm 1 BY 9" M ,Qw W

Feb. 9, 1971 MASAYUKl TAKAMURA ET AL I 3,562,027

PIANO PLATE OF LOW LOSS FACTOR Filed March 13, 1968 3 Sheets-Sheet 2 Feb; 9, 1971 Filed March 13, 1968 souwo PRESSURE LEVEL (db) MASAYUKI TAKAMURA ET AL PIANO PLATE OF LOW LOSS FACTOR FIG. 4

3 Sheets-Sheet 5 T V 3 E ,0 2 3 (m 9 n ,(0) A 16 I6 I53 I6 16 STRAIN 100- FIG. 5

o 5 IO I5 20 United States Patent 3,562,027 PIANO PLATE 0F LOW LOSS FACTOR Masayuki Takamura, Hironobu Hayashi, and Kinya Nozaki, Shizuoka-ken, Japan, assignors to Nippon Gakki Seizo Kabushiki Kaisha, Hamamatsu-shi, Shizuoka-ken, Japan Filed Mar. 13, 1968, Ser. No. 712,652 Claims priority, application Japan, June 15, 1967, 42/ 38,361 Int. Cl. C22c 37 00; GlOc 3/06 US. Cl. 148-35 5 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates generally to the field of musical instruments, more particularly to pianos, and still more particularly to new and improved piano plates (or string frames) contributing excellent acoustical characteristics to the pianos in which they are installed.

The plate used in a piano constitutes a support base or structure on which a large number of piano strings are strung, the strings being strung under high tension between respective tuning pins and hitch pins (or frame pins). Since the total stress of the tensions in these strings reaches a value as high as 20 tons, the plate must have sufiicient strength to withstand this total stress.

However, because of the necessity of avoiding excessive weights and of making possible the use of acoustically necessary string lengths and dispositions, the plates inevitably assume complicated shapes with suitable distributions of projections and depressions and flat portions as indicated in FIGS. 1 and 2 of the accompanying drawings briefly described hereinafter. Accordingly, piano plates must be formed by metal casting, at least at the present time.

For this reason, at the present time piano plates are generally made of ordinary cast iron, i.e., gray cast iron, since ordinary casting iron flows readily in molten state and is suited to casting operations. Ordinary cast iron, however, cannot be said to be completely satisfactory for piano plates in all respects, being accompanied by the following problems.

The first problem is that of acoustics. As stated hereinabove, the original function of a piano plate is primarily to constitute a support structure for holding, in tensioned state, piano strings which are to emit sounds through a sound board. Nevertheless, it is conjectured or understood in a vague way in the present art that a piano plate, which is in direct contact with the strings in this manner, probably has some acoustical contribution or influence with respect to the piano.

For example, it is said that a piano plate must have great capability of suppressing vibration and must not vibrate in resonance with the strings; that the vibration characteristics will be impaired if the thicknesses of the projections and depressions and flat portions of the plate are made excessively large; and that if the plate is provided wtih an amply large mass, its sound quality will 3,562,027 Patented Feb. 9, 1971 be greatly improved. However, it may be considered that, in the present state of the art, there is yet much to be fully clarified regarding the contribution or influence of the plate with respect to the piano in which the plate is installed.

The second problem, although of a relatively secondary nature, is the extremely heavy weight of the plate. Piano plates have been conventionally made of ordinary cast iron as mentioned hereinabove, but since this material has inadequate strength, the thicknesses of the various parts of the plates must be made large. Consequently, the

weight of the plate in a conventional piano is as much as approximately /3 of the total weight of the piano.

In view of this weight problem, the substitution of another material of lower density would naturally be thought to be a solution. Such a change in the material, however, cannot be easily accomplished when it is considered that the plate is a component member in the composition of a piano which is a musical instrument and that the plate, moreover, has an acoustical effect to some degree.

For example, the fabrication of the plate by casting an aluminum alloy for the purpose of reducing the weight of the plate has been proposed. The incorporation of such a plate in a piano, however, results in poor tone quality and has not been widely used. In addition, in changing the material of the plate, it is necessary to consider the following points in view of the fact that the member is a plate of a piano.

A piano plate has bearing ridges which constitute supports for the piano strings and which tend to be worn progressively by the abrasive rubbing action of the strings which is caused when the strings are struck by their respective hammers. Accordingly, the material of a piano plate must have ample Wear resistance. Furthermore, the

bearing ridges must have ample lubricity so as not to interfere with the shifting of the strings under high tension during tuning thereof.

. SUMMARY OF THE INVENTION A principal object of the present invention is to provide piano plates of highly desirable acoustical characteristics. Another object of the invention is to provide a piano plate which can be reduced in weight without sacrificing the acoustical characteristics or mechanical strength.

Still another object of the invention is to provide a piano plate having string bearing ridges of high wear resistance and good lubricity with respect to rubbing action by the strings.

For achieving the foregoing objects, a basic object of the invention is to study the nature of the effect of the material of a plate and the properties thereof on the acoustical characteristics of the piano in which the plate is installed with a view to establishing a definite basis 5 for optimum design and fabrication of piano plates.

The foregoing objects as well as other objects and advantages have been achieved by the present invention according to which, briefly summarised, there is provided a piano plate made of a cast ferrous metal wherein the internal damping factor or loss factor Q is from In preferred embodiments of the invention, there are respectively provided piano plates made of cast steel, spheroidal graphite cast iron (sometimes referred to as spherulitic graphite cast iron), malleable cast iron, and Meehanite cast iron. The invention, however, is not limited to the use of only these cast ferrous metals.

BRIEF DESCRIPTION OF DRAWINGS The nature, principle, details, and utility of the invention will be more clearly apparent from the following 3 detailed description when read in conjunction with the accompanying drawings.

In the drawings:

FIGS. 1 and 2 are planar views showing the plates of an upright piano and a grand piano, respectively;

FIG. 3 is a block diagram showing the essential organisation of one example of apparatus for measuring loss factor in a piano plate;

FIG. 4 is a graphical representation indicating relationships between loss factor and strain of various cast ferrous metals; and

FIG. 5 is a graphical representation indicating damping curves of a piano tone (440 Hz.) in pianos provided respectively with a plate of the invention and a conventional plate.

DETAILED DESCRIPTION OF THE INVENTlON As conducive to a full understanding of the nature and utility of the invention, the following consideration of the theory and principle thereof is first presented. The detailed description then concludes with specific examples of preferred embodiment of the invention.

Since a piano plate, in combination with the piano strings, constitutes a vibratory structure, the natural result of vibration of one or more of the strings upon being struck by their respective hammers should be that the plate is also vibrating. Directing our attention to this point, we have carried out measurements relating to the sounds of pianos and, as a result, have found that suitably reducing the absorption of vibratory energy by the plate produces the desirable result of decreasing the damping of sound waves of the piano as a whole.

That is, while the manner in which the material of a plate affects the acoustical characteristics of the piano in which the plate is installed has not been definitely determined in the prior art as mentioned hereinbefore, the

present invention considers for the first time the loss factor Q- of the plate material for clarification of this point. We have verified the fact that the acoustical characteristics of a piano vary considerably with the value of this loss factor Q- and have succeeded in making possible the manufacture of pianos having desirable acoustical characteristics by establishing a specific range for this value Q- that is, from 1.5 to 0.2 l0

Basically, the internal friction loss of a metal is a physical phenomenon within the metal and, in general, differs with the nature of the vibration imparted to the metal material. Furthermore, the loss factor Q which is a measure of this internal friction loss, varies in various ways, for example, varying with the amplitude of an applied vibration, and exhibiting a historical phenomenon (that is, an effect whereby a sample, upon once being caused to deform, assumes a state wherein internal friction loss, or damping, easily occurs and gradually returns to the initial state with time).

Consequently, the methods of measuring the loss factor are accompanied by considerable difiiculties. However, in view of the fact that the acoustical influence of the loss factor of a piano plate on the piano in which the plate is installed is great, we have carried out a study of plates made of cast ferrous metals and, as a result, have found that it is possible to measure this loss factor with considerable accuracy.

We have found that an apparatus as illustrated by block diagram in FIG. 3 is suitable for making this measurement of loss factor. Briefly, the apparatus comprises a driving coil 3; a detection coil 4, the coils 3 and 4 being electromagnetic coils and being disposed to confront the lateral sides of respective ends of a sample 1 suspended by suspension wires 2; amplifiers 6 and 7 with output and input sides respectively connected to coils 3 and 4; a switch 5 inserted between amplifier 6 and coil 3; an attenuator 8 connected between the input and output sides, respectively, of amplifiers 6 and 7; an amplitude comparator 9, the input side of which is connected by way a switch 12 to a junction between attenuator 8 and amplifier 7, the switch 12 being intercoupled to switch 5 as indicated by intermittent line 13; a gating device 10 receiving the output of amplitude comparator 9 and connected to the junction between attenuator 8 and amplifier 7; and a frequency counter 11 connected to device 10.

An example of the manner in which this apparatus is used to measure the loss factor of a sample 1 will now be described.

First, a sample 1 measuring 1 x 10 x mm. is prepared by cutting it out of a piano plate which has previously been made. The sample is cut out of the central part of the cross section in the thickness direction of the plate. The above stated dimensions are selected because the vibration frequency of this sample will then be from 800 to 1,000 Hz. and will correspond to the vibration region near the middle part of piano vibration frequencies of from 27.5 to 4,186 Hz.

The sample 1 thus prepared is suspended horizontally by very thin wires 2 respectively secured at two points positioned 15.7 mm. inboard from the two ends of the sample. The electromagnetic coils 3 and 4 are positioned to confront the two ends of the sample 1, which is then driven to undergo forced vibration. The driving power applied to the same is then shut off, and the amplitudes during the natural damping of the sample are measured in a vacuum of at least 10 mm. Hg at room temperature.

The loss factor Q- is determined from the following equation.

where A is the amplitude at the measuring point at the start of measurement; and A is the aplitude of the nth wave thereafter.

Since the loss factor Q- has an amplitude dependency, it is necessary to carry out measurements with various values of the amplitude at the start of measurement and to determine the effect of this variation. Accordingly, the relationship with respect to the strain occurring at maximum amplitude is investigated. The maximum amplitude is measured by means of a microscope up to 0101 mm., and for values therebelow, the amplitudes are calculated from the db values. Then the strain is determined from the following equation.

where e is the maximum strain at the sample surface; 1 is the vibration amplitude;

h is the sample thickness;

1 is the sample length.

Examples of values obtained by measurement in the above described manner with respect to samples of various cast ferrous metals are indicated in FIG. 4 which represents their respective relationships between loss factor and strain. Curve (A) represents ordinary cast iron of a conventional product; curve (B), a Meehanite cast iron; (C), a spheroidal graphite cast iron (Sample 1); curve (D), a malleable cast iron; curve (E), spheroidal graplhite cast iron (Sample 2); and curve (F), a cast stee As indicated by these curves in FIG. 4, with the exception of curve (A) of ordinary cast iron, the values of the loss factor become substantially constant for values of strain below 10-*, particularly below 10- Therefore, the values of the loss factor corresponding to a strain of 10* is taken. While the upper limit of each of these values is l.5 l0 the lower limit should be set at approximately 0.2 l0 The reason for this is that, if the internal friction loss is excessively small,

the tone quality of the resulting piano Will be excessively metallic.

As mentioned hereinbefore, the piano plate according to the present invention is made of a cast ferrous metal having specific properties. The term cast ferrous metal is intended to include, but not be limited to, cast steel, Meehanite cast iron, spheroidal graphite cast iron, and malleable cast iron. The loss factor of a cast ferrous metal of this character differs Widely depending on factors such as the composition of the metal, the structure thereof, and the graphite configuration, as indicated in FIG. 4. This is true also in the case of spheroidal graphite cast iron, which constitutes a representative piano plate material of this invention, and in this case, the effect of the graphite configuration is particularly great.

The reason for the extremely large absorptive power of ordinary cast iron with respect to vibration energy may be explained by the fact that when this cast iron is considered metallographically, graphite is crystallised in the form of flakes in dispersed state, and this flake graphite imparts a notch effect whereby the viscosity of the regions around the graphite is increased.

The loss factor decreases with the progress of spheroidisation of the graphite in a cast ferrous metal and has a close relationship to the spheroidisation rate of the graphite. The spheroidisation of the graphite within a cost ferrous metal is related also to the tensile strength of the metal, which tensile strength increases with progress of the spheroidisation. Accordingly, from the effect of the internal friction loss of the metal as a piano plate materi-ay, it is necessary that the spheroidisation rate of the graphite within this cast ferrous metal be so selected as to correspond to a tensile strength of this metal of at least 37 kg./mm.

This tensile strength is measured in the ordinary manner. In the tests relating to the present invention, each test specimen was prepared by cutting out a sample of S-mm. thickness and -mm. width from any part of a piano plate and forming therein a test part of parallel sides of 15-cm. width.

Since the loss factor of the plate according to the invention is made small in this manner, the sound damping of a piano in which this plate is installed becomes extremely small. Therefore, the tone colour becomes bright, and, moreover, the sound volume becomes great.

Since a piano is a well established musical instrument which has already been developed to a high degree of perfection, various constitutional parts of a piano which have been considered to contribute acoustically to the piano have already been provided with various improvements. Therefore, it would appear unlikely that any further improvements in these parts would be of further great contribution to the acoustical improvement of the piano.

We have found, however, that the improvement effect afforded by the change in plate material according to the invention is substantially greater than the effect which could possibly be afforded by improvements in the other parts of the piano. That is, we have found that, for example, it is possible even for those who have not received special musical training to distinguish clearly the acoustical difference between a piano in which the plate of the invention has been installed and a conventional piano.

Another highly advantageous feature of the present invention is the low weight of the plate afforded thereby for the following reason. A cast ferrous metal in which graphite has been spheroidised has mechanical and physical properties which both approach those of steel. That is, the tensile strength of such a metal is from 37 to 100 kg./mm. which is substantially high in comparison with the tensile strength of ordinary cast iron which is of the order of from 15 to 35 kg./mm. Accordingly the thicknesses of various parts of the plate can be reduced, whereby a great reduction in weight of the plate can be made.

Still another advantage of the plate material of the invention is its high degree of toughness in comparison with that of ordinary cast iron, whereby the plate of the invention is stronger than a plate of ordinary cast iron in withstanding impact loads. Furthermore, since the wear resistance of the plate material is improved, the bearing ridges of the plate are highly durable. Moreover, if the existing graphite is spheroidal, the bearing ridges will not easily chip or undergo exfoliation, and, at the same time, good lubricity will be afforded between the bearing ridges and the strings at their contact parts.

In order to indicate still more fully the nature and utility of the invention; the following example of preferred embodiment of the invention is set forth, it being understood that this example is represented merely for the purpose of illustration and is not intended to limit the scope of the invention.

EXAMPLE For the purpose of comparison, grand piano plates were fabricated from ordinary cast iron and spheroidal graphite cast iron.

More specifically, base metal for ordinary cast iron of suitable composition for high yield was melted in a cupola. The resulting molten metal, or melt, was scooped in a ladle and poured into a sand mould prepared beforehand to fabricate a piano plate.

On the other hand, spheroidal graphite cast iron frames Were fabricated by adding approximately 0.2 percent of calcium carbide to molten metal within a cupola to accomplish, disulphurisation, scooping the melt is a ladle, inoculating the melt with a Mg-Si-Fe alloy (the amount being 0.3 percent in terms of the Mg quantity) to cause spheroidisation of graphite, and pouring the melt into a sand mould.

The chemical compositions and mechanical properties of the cast ferrous metal plates thus fabricated were measured and found to be as indicated in Table 1.

The plates thus fabricated were installed in respective pianos and strung, and measurements of the sound waves emitted by these pianos were made. More specifically, each of the plates was installed in a grand piano, and a ZOO-gram piece of lead was caused to drop freely from a height of 20 mm. onto the key corresponding to a tone of 440 Hz. The sound thus emitted was recorded on a sound-recording tape and then repeatedly reproduced, and the damping curves for various frequency components were recorded by means of a high-speed level recorder.

As a result, it was found that the piano in which the plate according to the invention was used had less damping effect than the piano in which the conventional plate was used. In FIG. 5, curve I represents the damping curve with respect to a tone of 440 Hz. in the case of the piano in which the plate of the invention was installed, while curve K represents that in the case of the piano in which the conventional plate was installed. These curves clearly indicate that the damping time is extended in the former case relative to that of the latter case.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

What We claim is:

1. A piano plate of improved acoustical characteristics, said plate being made of a cast iron having a loss factor ((2- as defined between lines B and E in FIG. 4.

2. The piano plate as claimed in claim 1 in which the cast iron is selected from the group consisting of spheroidal graphite cast irons, malleable cast irons, and Meehanite cast irons.

3. The piano plate as claimed in claim 1 in which the cast iron has a tensile strength of at least 37 kg./mm.

4. The piano plate as claimed in claim 2 in which the cast iron has a tensile strength of at least 37 k g./mm.

5. The piano plate as claimed in claim 1 wherein the cast iron is spheroidal graphite cast iron, the loss factor (Q is from O.6 10- to 1.0 1()- and said plate has a tensile strength of at least 37 kg/mmfl.

References Cited UNITED STATES PATENTS OTHER REFERENCES Physical and Engineering Properties of Cast Irons, The British Cast Iron Research Association, Birmingham, England, 1960, pp. 188-492.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

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