Tuning forks

Sept. 17, 1957

2,806,400

B. F. GRIB

TUNING FORKS Filed March 10, 1954

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INVENTOR.

B. F, GRIB

BY

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ATTORNEYS

United States Patent Office

2,806,400

Patented Sept. 17, 1957

Figure 2 is a perspective view of my improved tunin» fork.

Referring to the drawing, Figure 1 shows a tuning fork 10 and one form of associated circuit. The tuning fork 10 has a pair of tines 11 joined to a base 15 by resoective narrowed necks 30. As is well known, the cross sectional area of each of these necks 30 determines the stiffness of the respective tine 11 and thereby influences both the frequency and amplitude of vibration of the tmes. Since this circuit forms no part of the present invention, it will not be described further here, reference being made to the above-identified parent application Serial No. 171,480 for a fuller description. It is sufficient to state here that the tines 11 are caused to vibrate by driving and pickup coils 12α, 126, suitably connected in the. circuit. As shown in Fig. 1, the base 15 of fork 10 is apertured at 20 for mounting. For the purpose of temperature compensation, affixed at the end of each tine 11 is a bi-metallic element 32 bent in a right angle to form two legs 33 and 34. One leg 33 is affixed at the end of the tine 11 as an extension thereof, as shown most clearly in Figure 2, so that the other leg 34 extends transversely of the length of the tine 11. Each leg 34 carries a. weight 36 near the end thereof. This structure provides temperature compensation in the manner more fully described in the said parent application Serial No. 171,480.

. For. the. purpose of improving the isolation of the vibrating tines from the mount, recourse is had to the slots 41 placed between the mounting holes 20 of the base 15 and the vibrating tines 11. These slots 41 are formed most simply by drilling holes symmetrically about the center line or longitudinal axis of the fork, and then sawing out the outer faces of these holes as indicated. Alternatively, they may be sawn, ground, or milled from the edges of the fork body, or in any other way, forming slots or holes as may be desired. The size of these holes or slots and their spacing determines the reed frequency mentioned above. The size of the holes or slots is determined experimentally for a given size of a tuning 40 fork, and fine adjustments may be made by filing the edges or bottoms of these slots 41 to provide a reed frequency between 60 and 85% of the fork frequency, which has been determined as the value necessary to isolate the fork tine vibrations from the base or mount.

It will be seen that the slots 41 are separated from the narrowed. tine necks 30 by a distance at least as great as the width of each of the necks 30. In this way, the slots 41 do not influence the vibration of the individual tines 11 and have effect only in determining the desired relationship of 60 to 85% oMhe fork frequency, the thickness of the base material remaining between the slots 41 should be approximately the same percentage of the sum of the thicknesses of the tine necks 30, in a case such as the present where the height of the fork is uniform, thereby giving the indicated relationsnip between the cross sectional areas at these portions of the fork.

It will be understood that values at the center of the range of reed frequencies given are the best, since if the reed, frequency approaches too closely the fork frequency, intercoupling appears to take place between these two frequencies which reduces the isolation desired. In addition, if the reed frequency approaches a sub-harmonic (for example, half) of the fork frequency, similar interaction appears to take place. It is therefore necessary that the reed frequency avoid both the fork frequency and sub-harmonics thereof. A value of substantially 75% has been found optimum.

While an illustrated form of the invention has been shown, it is to be understood that the invention is not to be considered limited thereto, because many ap2,806,400

TUNING FORKS

Boris F. Grib, Brooklyn, N. Y., assignor to Philamon Laboratories, Inc., Westbury, N. Y., a corporation of New York

Application March 10,1954, Serial No. 415,318

Claims. (Ci. 84—457)

The present invention is a continuation-in-part of my copending application Serial No. 171,480, filed June 30, 1950, now Patent No. 2,732,748, dated January 31, 1956 for Tuning Forks.

The present invention relates to improvements in tuning forks and more particularly to improvements in the construction of tuning forks adapted to be electrically vibrated, such as tuning forks which serve as frequencydetermining elements for electrical oscillation generators.

As is well known, a tuning fork constitutes a resonant system and its vibrations can be utilized to determine the frequency of electrical oscillations produced by an electrical or electronic circuit suitably coupled to the tuning fork. In producing oscillations in this manner, an important problem has been that of isolating the vibration of the fork tines from the mount which supports the fork. If the fork tines are unbalanced or improperly mounted, vibration is transmitted to the mount. This results in a decrease in the energy available for vibrating the tines themselves and constitutes a form of damping for the system which reduces the effective Q of the system in an undesired manner. In addition, when vibrations are transmitted to the mount, often some portion of the mount or other elements affixed to it are also set into vibration which is transmitted back to the tines through the medium of the atmosphere, in the cases where the tuning fork is not vacuum mounted. This creates a type of feedback which also alters the frequency of oscillation of the tines undesirably.

According to the present invention, special arrangements. are provided for minimizing this transmission of vibration to the mount. I have discovered that when initially struck a tuning fork emits a tone of a frequency different from that produced during steady vibration. This tone I have termed the “reed” frequency as dis- _Kn , „---------------------/ - me tinguished from the fork frequency during steady vibra- reed frequency. To make the reed frequencies have the tion. I have further discovered that the undesirable ef- ^J fects of the transmission of vibration to the mount can be avoided by proper design of the fork so as to produce a reed frequency less than the fork frequency but more than half the fork frequency. As a practical matter, I have found that the reed frequency should be between 60 and 85% of the fork frequency.

I have also discovered that the reed frequency can be varied without substantial change in the fork frequency fio by reducing the cross-section of the tuning fork at the base of the tines. By the present invention simple ways of. restricting the cross-section of the fork base are utilized, specifically designed according to this invention to produce a reed frequency in the range specified so as to 65 provide improved results.

These and other features and objects and advantages of the present invention will become more apparent from consideration of the following specification and the appended drawings, wherein

Figure 1 is a schematic representation of my improved tuning fork and an associated electronic circuit; and

2,806,400 parently divergent embodiments of the invention can be easily conceived within the. spirit thereof. Therefore the invention is to be limited only as defined in the appended claims.