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Claims  |
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I claim:
1. A tine-type electromechanical musical instrument, which comprises:
a. a tine or reed one end portion of which is fixed and the remaining
portion of which is free to vibrate in a predetermined direction,
b. magnetic means including a protuberance to generate in the air in the
region of said protuberance a magnetic field which is asymmetrical with
respect to said protuberance,
said means being such that the lines of force in said magnetic field loop
back instead of following a substantially closed low reluctance magnetic
circuit including a tine or reed,
said means causing said asymmetry of said magnetic field to be such that
the magnetic field strength along a hypothetical straight line which
extends through the air past said protuberance is relatively high adjacent
one side of said protuberance and relatively low adjacent the other side
thereof,
c. means to mount said magnetic means in such relationship to said tine
that said hypothetical line is parallel to said predetermined direction of
vibration, and also in such relationship to said tine that a part of said
free tine portion is disposed in said magnetic field,
d. means to set said free tine portion into vibration in said predetermined
direction, and
e. means associated with said magnetic means to generate an electrical
signal in response to said vibration of said part in said field.
2. The invention as claimed in claim 1, in which said magnetic means is
such that said magnetic field is stationary, and in which said mounting
means of clause (c) is independent of said tine.
3. The invention as claimed in claim 2, in which said magnetic means
comprises a permanent magnet and a low reluctance pole piece disposed
adjacent said magnet, said pole piece having said protuberance formed
thereon, and in which said means recited in clause (e) is a coil of wire.
4. The invention as claimed in claim 3, in which the axis of said coil of
wire is offset a substantial distance from said protuberance.
5. The invention as claimed in claim 1, in which said means recited in
clause (d) comprises a piano action, and in which amplifier and
loudspeaker means are connected to said means recited in clause (e).
6. The invention as claimed in claim 1, in which said tine part recited in
clause (c) is formed of a material having a low reluctance.
7. The invention as claimed in claim 1, in which said tine is formed of
steel and is so mounted that the tip of said tine is said part recited in
clause (c).
8. A tine-type electromagnetic piano, which compirses:
a. a tine or reed formed of steel,
said tine having a fixed end portion and a distal end portion,
said distal end portion being free to vibrate in a predetermined direction,
b. magnetic means including a protuberance to generate in the air in the
region of said protuberance a steady-state magnetic field which is
asymmetrical with respect to said protuberance,
said means being such that the lines of force in said magnetic field loop
back instead of following a substantially closed low reluctance magnetic
circuit incorporating said tine or reed,
said means causing said asymmetry of said magnetic field to be such that
the magnetic field strength along a hypothetical straight line which
extends through the air past said protuberance is relatively high adjacent
one side of said protuberance and relatively low adjacent the other side
thereof,
c. means independent of said tine to mount said magnetic means in such
relationship to said tine that said hypothetical line is parallel to said
predetermined direction of tine vibration, and said distal end portion of
said tine is disposed in said magnetic field adjacent said protuberance,
d. piano action means to set said distal end portion of said tine into
vibration in said predetermined direction,
said piano action means, said mounting means and said tine being such that
said distal end portion when vibrating at a normal amplitude will move
from said one side of said protuberance to said other side thereof, and
e. a coil of wire associated with said magnetic means in such manner that
an electrical signal is generated in said coil in response to said
vibration of said distal end portion.
9. The invention as claimed in claim 8, in which said protuberance has a
peak portion, and in which said distal end portion of said tine, when at
rest, is in a position generally opposed to said peak portion.
10. The invention as claimed in claim 9, in which said magnetic means
comprises a permanent magnet and a low reluctance pole piece adjacent
thereto, in which said coil of wire is mounted generally coaxially around
said magnet, in which said peak portion of said protuberance is offset a
substantial distance from the axis of said coil and of said magnet, and in
which said tine is generally parallel to said axis.
11. The invention as claimed in claim 8, in which said protuberance has a
peak portion, in which a second protuberance is incorporated in said
magnetic means, said second protuberance being spaced from said peak
portion of said first-mentioned protuberance in the direction of said
hypothetical line, said first-mentioned and second protuberances
cooperating with each other in maximizing said asymmetry of said magnetic
field with respect to said first-mentioned protuberance.
12. In an electromagnetic piano of the type wherein a tine or reed having a
relatively low reluctance is fixed at one end and is free at the other end
to vibrate in a predetermined direction, wherein a piano action is
provided to effect striking of said tine to set said other end into
vibration, and wherein an electromagnetic pickup having a sensing coil is
provided adjacent said other end to sense the resulting vibrations, the
combination with said sensing coil of a magnetic means having a
protuberance located adjacent the path of movement of said vibrating other
end, said protuberance being such that the magnetic field strength at one
side thereof is relatively high and the magnetic field strength at the
other side thereof is relatively low, said one side and said other side
being in line with said path of movement whereby said other tine end when
vibrating with sufficient amplitude will be first in said high-strength
field and then in said low-strength field, thus causing generation in said
coil of an electrical signal characterized by desired higher harmonics
without an excess of the second harmonic.
13. The invention as claimed in claim 12, in which the magnetic field
adjacent said protuberance is characterized by a steep magnetic
field-strength gradient which does not reverse from said one side to said
other side.
14. The invention as claimed in claim 13, in which said protuberance is
located adjacent said other tine end when the latter is at its rest
position.
15. The invention as claimed in claim 12, in which said protuberance has a
peak which is less than about . inch from said path of movement, measured
along the shortest distance between said peak and said path.
16. The invention as claimed in claim 12, in which said protuberance has a
peak, and said combination further comprises a second protuberance spaced
from said peak of said first-mentioned protuberance in the direction of
said path of movement.
17. The invention as claimed in claim 16, in which the depth of the region
between said protuberance is at least about 1/8 inch.
18. The invention as claimed in claim 17, in which said protuberances have
peaks which are spaced at least about y5 inch from each other.
19. The invention as claimed in claim 12, in which said protuberance has a
relatively sharp peak the shape of which is selected from a class
consisting of (a) a substantial point, and (b) a short edge oriented
transversely to said path of movement.
20. An electromagnetic musical instrument in the nature of a piano, which
comprises:
a. a substantially cylindrical elongated steel tine,
b. means combining with said tine to fixedly mount only one end thereof and
to form an asymmetrical fork such that said tine will vibrate in a single
predetermined plane and in substantially the fundamental mode only,
c. means to set said tine into vibration in said plane, and
d. electromagnetic pickup means mounted adjacent the free end portion of
said tine to sense said fundamental-mode vibrations and to create an
electrical signal characterized by the presence of the fundamental and of
desired higher harmonics but without an excess of the second harmonic,
said pickup means comprising a coil having an elongated permanent magnet
mounted generally coaxially therein,
said pickup means further comprising a low reluctance pole piece disposed
adjacent said magnet and having a protuberance which is offset
substantially from the common axis of said coil and magnet,
said protuberance having a peak which is adjacent said tine when it is
vibrating in said plane,
said protuberance and said magnet being so constructed and related to each
other that the magnetic flux emanates from said protuberance into the air
and then loops back to the end of said magnet remote from said
protuberance,
said protuberance and said magnet also being so constructed and related
that there is a steep unidirectional gradient of magnetic field strength
at said protuberance,
the field strength a predetermined short distance on one side of said
protuberance being high and the field strength the same predetermined
short distance on the other side of said protuberance being low, said
short distances being generally in said plane of vibration whereby said
free end portion of said tine vibrates through regions of rapidly varying
magnetic field strength, and
e. amplifier and loudspeaker means connected to said coil to convert said
signal into sound.
21. The invention as claimed in claim 20, in which said pole piece also has
a second protuberance the axis of which is offset substantially from the
common axis of said coil and magnet, the axes of said first-mentioned and
second protuberances being on opposite sides of said common axis, the
direction of offsetting of said axes of said protuberances from said
common axis being generally in said plane of vibration and transverse to
said tine.
22. An electromechanical musical instrument in the nature of a piano, which
comprises:
a. a tine or reed which is fixed at one end,
the other end portion of said tine being adapted to vibrate in a
predetermined direction,
b. a protuberance disposed generally opposite said other end portion,
c. electromagnetic pickup means including said protuberance to generate at
said other end portion a magnetic field such that vibration of said other
end portion in said predetermined direction from one side of said
protuberance to the other side thereof will generate in said pickup means
an electrical signal characterized by the absence of an excessive second
harmonic,
said pickup means being such that said magnetic field has a steep
field-strength gradient in said predetermined direction from a first point
opposite said one side of said protuberance to a second point opposite
said other side thereof,
said first and second points being along the path of vibration of said
other end portion,
said field-strength gradient being unidirectional between said points,
d. means to cause vibration of said other end portion in said predetermined
direction, and
e. amplifier and loudspeaker means to convert into sound the electrical
signal generated in said pickup means.
23. The invention as claimed in claim 22, in which the rest position of
said other end portion is between said points.
24. The invention as claimed in claim 23, in which said means recited in
clause (c), and the characteristics of said tine, are such that said other
end portion normally vibrates, when in steady-state vibration, from said
one side of said protuberance to said other side thereof.
25. An electromechanical piano, comprising:
a. a low reluctance tine or reed which is fixed at one end and adapted at
the other end to vibrate in a predetermined direction,
b. electromagnetic pickup means to generate an electrical signal in
response to said vibration in said predetermined direction,
said pickup means including means to generate two magnetic field regions
along a straight line which (1) is parallel to said predetermined
direction, and (2) is near said other end of said tine, each of said
magnetic field regions being such that a graph of field strength versus
distance along said line is convex,
one slope of such graph of one of said magnetic field regions being
relatively adjacent one slope of such graph of the other of said magnetic
field regions,
at least the first of said two relatively adjacent slopes being steep,
c. means to set said other end into vibration in said predetermined
direction, and
d. means to so locate said pickup means relative to said tine that (1) the
rest position of said other tine end is on said first slope, and (2) said
other tine end when vibrating at its normal steady-state amplitude will
spend little, if any, time on the second of said two relatively adjacent
slopes.
26. The invention as claimed in claim 25, in which said normal steady-state
amplitude is at least 1/64 inch.
27. An electromagnetic pickup for a tine-type electromechanical piano,
which comprises:
a. a coil of wire, and
b. means including at least one protuberance to create magnetic flux lines
through said coil of wire,
said protuberance being so shaped and so related to said coil that said
flux lines emanate in large numbers from one side of said protuberance but
only in small numbers from the other side thereof,
said means and said protuberance being such that said lines after emanating
from said protuberance loop back around said coil substantially
independently at all but a small portion of the tine with which the pickup
is adapted to be associated,
said protuberance being adapted to be disposed adjacent said small tine
portion in such relation thereto that when the tine is vibrating said
small portion will vibrate back and forth between said one side of said
protuberance, at a region where there are many of said flux lines, and
said other side thereof at a region where there are small numbers of said
flux lines,
whereby there is induced in said coil a non-sinusoidal voltage wave having
desirable harmonic characteristics.
28. The invention as claimed in claim 27, in which there are two of said
protuberances, one on one side of the axis of said coil, the other on the
opposite side of said axis, each of said protuberances having large
numbers of flux lines emanating therefrom from the side thereof remote
from said axis, and small numbers of flux lines emanating therefrom from
the side thereof relatively adjacent said axis.
29. The invention as claimed in claim 27, in which said protuberance is a
substantial point.
30. The invention as claimed in claim 27, in which said protuberance is a
sharp edge.
31. A method of manufacturing an electromechanical piano or the like, which
method comprises:
a. providing a protuberance,
b. employing said protuberance to generate adjacent one side of said
protuberance a relatively strong magnetic field region,
c. causing the magnetic field region adjacent the other side of said
protuberance to be relatively weak,
d. providing a fixed-free tine or reed having a vibrating portion,
e. providing means to effect vibrations of said tine in such relationship
to said protuberance that said vibrating portion moves suddenly between
said strong and weak magnetic field regions, and
f. so positioning a coil relative to said protuberance that a voltage wave
is generated in said coil in response to said vibrations through said
field regions as recited in clause (e).
32. The invention as claimed in claim 31, in which said method further
comprises causing the magnetic field-strength transition between said
relatively strong magnetic field region and said relatively weak magnetic
field region to be sharp and unidirectional.
33. The invention as claimed in claim 31, in which said method further
comprises causing said protuberance to have a peak offset a substantial
distance from the axis of said coil.
34. The invention as claimed in claim 31, in which said method further
comprises causing the great majority of the magnetic field-strength lines
adjacent said protuberance to loop through said coil without passing along
the length of said tine.
35. A method of creating, by use of a fixed-free vibrating tine and an
associated electromagnetic transducer which is connected to amplifier and
loudspeaker means, a musical tone which has bright characteristics, is
relatively simulative of a tone generated by an acoustic piano, and does
not have an excessive second harmonic, which method comprises:
a. employing as said electromagnetic transducer one wherein the lines of
magnetic flux emanate from a protuberance, and then loop back through the
air and through a coil which is connected to said amplifier and
loudspeaker means, and also wherein there is a much greater concentration
of said flux adjacent one side of said protuberance than adjacent the
opposite side thereof, and
b. causing a low reluctance portion of said tine to vibrate adjacent said
protuberance in such direction as to be first relatively near said one
side and then relatively near said opposite side.
36. The invention as claimed in claim 35, in which said method further
comprises causing the steady-state amplitude of said vibration recited in
clause (b) to be sufficiently great that said low reluctance tine part is
alternately on said one side and said other side of said protuberance.
37. The invention as claimed in claim 35, in which said method further
comprises employing as said transducer one wherein there is a steep and
unidirectional magnetic field-strength gradient along the path of
vibration of said tine portion, said gradient continuing across said
protuberance between said one side and said other side thereof.
38. The invention as claimed in claim 35, in which said method further
comprises employing as said transducer one wherein there is a second
protuberance spaced from said first-mentioned protuberance generally in
the direction of vibration of said tine portion, said second protuberance
also having magnetic flux lines emanating therefrom and looping back
through said coil, and causing said vibration of said tine portion to be
substantially entirely in the vicinity of said first-mentioned
protuberance and not said second protuberance.
39. The invention as claimed in claim 35, in which said method further
comprises employing as said transducer one wherein said protuberance is
offset a substantial distance from the axis of said coil.
40. The invention as claimed in claim 39, in which said method further
comprises causing said tine portion to pass less than 1/8 inch from said
protuberance.
41. The invention as claimed in claim 20, in which the end of said pole
piece adjacent said tine has a generally cylindrical exterior surface.
42. The invention as claimed in claim 41, in which said pole piece end has
a generally V-shaped groove formed transversely thereacross, whereby to
form two protuberances offset on opposite sides of said common axis, the
width of said groove being sufficient that said peak of said protuberance
adjacent said tine has a relatively small dimension in the direction of
vibration of said tine.
43. The invention as claimed in claim 42, in which said peak of said
protuberance adjacent said tine has a substantial dimension in a direction
transverse to said plane of tine vibration. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of electric pianos incorporating
electromagnetic mechanical-electrical transducers (commonly referred to as
electromagnetic pickups).
2. Description of the Prior Art
Because of their portability, their ability to remain in tune over long
periods of time, their ability to be heard either by large numbers of
people or (with earphones) only by the pianist himself, and their ability
to generate numerous vibrato and tonal effects, electric pianos are
becoming increasingly popular. Of electric pianos, the best are the
electromechanical type in which the strings of conventional pianos are
replaced by vibrating tines or reeds. The vibrations of the tines or reeds
are sensed by pickups, thus generating voltage signals which are
electronically amplified and fed to a loudspeaker.
A major problem relating to such electromechanical pianos resulted from the
fact that vibrating tines or reeds normally have dissonant overtones which
are not at all pleasing to the ear. These overtones are not harmonics
(integral multiples of the fundamental frequency). In this respect, tines
or reeds are to be contrasted with conventional piano strings, wherein the
overtones are substantial harmonics and are pleasing to the ear.
The problem stated in the preceding paragraph, and other important
problems, were solved as set forth in U.S. Pat. No. 2,972,922, inventor H.
B. Rhodes. The invention stated in such patent, together with later Rhodes
inventions as described (for example) in U.S. Pat. Nos. 3,384,699,
3,418,417, 3,644,656, produced a highly successful piano which is popular
with large numbers of professional and other musicians.
There has, however, long existed a desire for an electromechanical
tine-type piano which sounds more in the nature of a conventional
string-type acoustic piano, particularly in the mid-range of the piano. To
accomplish this, it is necessary not only to eliminate the dissonant reed
overtones (as was done by Rhodes) but also to cause introduction at the
pickup of harmonics having a different and more acoustic piano-like "mix"
or character than in the prior art. The present invention does this
economically, simply and practically, and also achieves excellent apparent
loudness and brightness of tone, with consequent minimized requirements
for amplification. The present invention also permits generation of tones
and chords, even in octaves below middle C, which are relatively acoustic
piano-like in character.
With relation to certain prior art other than the stated Rhodes inventions,
U.S. Pat. No. 3,038,363, inventor B. F. Miessner, discusses theories
relating to the creation of piano-like tones by means of an
electromechanical reed-type piano. Harmonic content, and the generation of
asymmetrical peaked voltage waves, are discussed at length. However, the
electromagnetic pickup structures taught by U.S. Pat. No. 3,038,363 (FIGS.
49 through 54), and by other Miessner U.S. patents such as No. 3,215,765,
are expensive, inefficient, large, and (it is believed) incapable of
accomplishing important results achieved by the present pickup. Another
prior-art U.S. Pat. No. 2,581,963, inventor R. J. Langloys, shows in FIG.
22 an electromagnetic pickup of the general type used by Rhodes, and
discusses a frequency-doubling effect which will be referred to later in
this specification. Langloys also mentions that the shape of the pole
piece extremity permits to grade the closing of the harmonics, but gives
no indication of what shape to use or how or why to create any such
"grading". A further U.S. Pat. No. 2,510,094, inventor E. O. Fleury,
discloses an electromechanical piano of the type wherein the lines of
magnetic force extend for the full length of the reed, and follow a
substantially closed low-reluctance magnetic circuit, instead of looping
back through the air around the coil. Fleury's object is to achieve
sinusoidal variations of the magnetic flux, which variations (it is
believed) produce dull tones very unlike those of an acoustic piano.
In summary, the prior art does not teach or suggest how to create, in a
tine-type piano, acoustic piano-like sounds in a practical, economical,
commercially feasible way -- with excellent brightness and apparent
loudness and with the ability to generate acoustic piano-like tones and
chords both below and above middle C.
SUMMARY OF THE INVENTION
In the present apparatus and method, there is disposed adjacent a vibrating
low-reluctance tine or reed a protuberant magnetic pole piece portion from
which the lines of magnetic force (magnetic flux) loop back through the
air in an asymmetrical manner -- the concentration of the lines being
sharply more dense on one side of such portion than on the other side
thereof. There is thus generated in the associated sensing coil a voltage
signal which, when amplified and fed to a loudspeaker, creates a bright,
live sound simulative of an acoustic piano.
The tine in its rest (nonvibrating) position is preferably positioned
opposite the protuberant portion, not spaced a substantial distance to one
side thereof. Despite such positioning, the resulting voltage signal is
not characterized by an excess of the second harmonic, having instead the
desired pianistic harmonic "mix". At the stated rest position of the tine,
the magnetic field strength gradient is substantially the maximum, so that
even a small change in tine position will cause the tine to shift from a
region of one magnetic field strength to a region where the field strength
is very different.
To summarize certain aspects of the invention in a somewhat different
manner, the magnetic field is caused to be such that:
a. The rest position of the tine may be directly opposite the peak of the
protuberance, yet vibration of the tine will not cause an excessive second
harmonic.
b. There are two regions of relatively high magnetic field strength
separated by a region of relatively low field strength, the two
high-strength regions being spaced sufficiently far from each other that
the tine may have its rest position at one of such high-strength regions
and where there is substantially maximum magnetic field-strength gradient,
yet (when vibrating at its normal amplitude) will spend a minimum (if any)
amount of time at or near the other region of high field strength.
c. The tine may move a substantial distance from one side of the
protuberance to the other side thereof, in a single direction, without at
any time entering a region where the magnetic field-strength gradient
reverses.
The protuberant magnetic pole piece portion is preferably a substantial
point or edge, and is offset from the axis of the sensing coil.
Preferably, two protuberant portions are provided, one on each side of the
coil axis.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a transverse sectional view of a piano incorporating the present
electromagnetic pickup, showing the various components associated with
each key of the piano;
FIG. 2 is an enlarged isometric view of the pickup assembly;
FIG. 3 is a view, partially in side elevation and partially in vertical
section, showing the preferred relationship between the pickup assembly
and an associated tine of the piano;
FIG. 4 is an enlarged side elevational view showing the pole piece and the
adjacent tine tip;
FIG. 5 is a top plan view of the showing of FIG. 4;
FIG. 6 is an end elevational view of the face of the pole piece;
FIG. 7 is a set of curves showing magnetic field strength in regions
adjacent the present pole piece;
FIg. 8 is a set of curves showing the field strength adjacent a prior-art
pole piece;
FIG. 9 is a view corresponding to FIG. 4, but showing a pole piece wherein
the second protuberance has been cut off;
FIG. 10 is a view corresponding to FIG. 5, but showing one type of pole
piece wherein each peak is a short edge instead of a substantial point;
FIG. 11 is a side elevational view of the form of pole piece which is
preferred as of the time of filing of the present application;
FIG. 12 is a view of the face of such pole piece of FIG. 11, as seen from
the right in FIG. 11;
FIG. 13 is a side elevational view of the pole piece of FIG. 11, as seen
from above and at right angles to the showing of FIG. 11; and
FIG. 14 is a set of curves corresponding to that of FIG. 7, but made with a
pole piece substantially identical of that of FIGS. 11-13.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
For information concerning the background and construction of the present
piano, specific reference is hereby made to the above-cited U.S. Pat. Nos.
2,972,922; 3,384,699 (specifically, to FIGS. 14 and 15 of such patent);
3,418,417; and 3,644,656. Said patents are hereby incorporated by
reference herein as though set forth in full.
The piano may have numbers of keys, for example, 73 or 88. The
constructions of the tone generators, etc., change from key to key as
indicated in the cited patents. In the present drawings, only one tone
generator, and the associated hammer means, key, etc., are shown. The
illustrated tone generator is one adapted to generate a tone in the
midrange of the piano, this being where the present invention has its
greatest importance. The "midrange" is hereby defined to extend from the C
one octave below middle C (having a vibrational frequency about 130 Hz.)
to the C two octaves above middle C (about 1046 Hz.).
Referring to FIG. 1, the piano incorporates numerous elongated support
elements, including wooden members 10 through 19, inclusive. Two of such
members, numbers 18 and 19, are respectively associated with metal angle
bars 20 and 21 in order to increase the rigidity thereof. The support
means incorporated in the piano further comprises an extrusion 22 which is
mounted on wooden member 12 and supports the hammers and dampers of the
piano action.
Each piano action comprises a hammer 23, damper spring 24, and key 25, the
latter being pivotally mounted on support 14. The illustrated piano action
is of the type described in U.S. Pat. No. 3,270,608, inventor H. B.
Rhodes, which patent is hereby incorporated by reference herein as though
set forth in full. It is to be understood, however, that numerous other
types of piano actions may be employed, although with less satisfaction
and with greater cost.
The preferred piano action includes a solid rubber tip 27 on the hammer,
and which is adapted to strike the tone generator as described below. The
preferred action is also one wherein each spring 24 is integrally
connected at its base to a number of adjacent springs (for other piano
actions) in modular relationship. Each damper spring 24 is associated with
hammer 23 by a strap 28, so that when the outer (left) end of key 25 is
depressed by the finger of the pianist, causing the hammer 23 to pivot
upwardly, the damper spring 24 will be pulled down. This causes a felt pad
29 at the end of spring 24 to move away from the tone generator, so that
vibrations may continue. A suitable sustaining pedal, not shown, is
incorporated in the instrument.
The hardness of the tips 27 varies from key to key, the tips being softest
in the bass regions of the piano and hardest in the highest-pitched
regions. The place where the tip 27 strikes the tone generator is adjusted
in such manner as to maximize the relationship whereby the tone generator
will vibrate substantially entirely in its fundamental mode.
The tone generator is indicated generally at 31, being of the type
described in U.S. Pat. Nos. 2,972,922 and 3,644,656. It is mounted to
support 18 in the general manner described relative to FIGS. 14 and 15 of
U.S. Pat. No. 3,384,699.
As described in the cited patents, the tone generator 31 is an asymmetrical
tuning fork, having a high-mass leg (or tone bar) 32 and a low-mass leg
(or tine) 33. Tine 33 is composed of hard steel, and is formed by rotary
swaging as set forth in U.S. patent application Ser. No. 179,763, now
abandoned, which is hereby incorporated by reference as though set forth
in full (said application Ser. No. 179,763 also describes the hammer tips
in detail). The base region (left in FIG. 1) of the tine 33 is enlarged at
34 and very rigidly associated with a bar 35 which is in turn rigidly
associated with tone bar 32. A spring 36 is slidably mounted at the end
region of tine 33 in order to permit tuning thereof, as generally
described in U.S. Pat. No. 2,972,922.
Tine 33 thus has one end portion 34 which is fixed, and another end portion
(the distal end portion) which is free to vibrate. The steel tine may be
referred to as having a low reluctance, since it is much more magnetically
permeable than (for example) air or various nonferrous metals.
As soon as it is struck by the rubber tip 27 of hammer 23, the tine begins
to vibrate in the vertical plane. The normal amplitude of vibration
(measured from the rest position to one extreme position) of the tine tip
ranges (with typical striking force on the key) from a substantial
fraction of an inch in the extreme bass regions of an 88-key piano to a
minute fraction of an inch in the highest-pitched regions thereof. At the
lower end of the above-defined midrange of the piano, namely one octave
below middle C, the normal amplitude of steady-state vibration of the tip
of tine 23 is (with typical striking force on the key 25) about 1/8 inch
(measured a short time after striking, after termination of the initial
percussive action). At the upper end of such midrange, two octaves above
middle C, such normal amplitude of steady-state vibration is about 1/64
inch.
Because of the tuning-fork relationship between elements 32 and 33, and
because the tone generator 31 is only loosely associated with the wooden
support 18, the tine 33 (although cylindrical instead of flat) continues
to vibrate in the vertical plane instead of orbiting.
At the instant of striking of tine 33 by the hammer tip 27, such tine
vibrates not only in its fundamental mode but also (particularly in the
middle and lower regions of the piano) with dissonant overtones. However,
because of the tuning-fork relationship between elements 32-33, which are
in substantial resonance with each other, and because of other factors
described in the cited patents, the overtones very quickly disappear so
that the tine 33 continues to vibrate substantially entirely in its
fundamental mode. The undesired overtones having thus been removed, it is
of great importance that desired overtones, namely harmonics in the right
proportion and the right amounts for the desired acoustic piano-like
sound, be introduced by the pickup means and method described below.
GENERAL DESCRIPTION OF THE PICKUP MEANS
The pickup assembly is designated generally by the reference numeral 37,
and comprises a headed pole piece 38 the shank portion of which is
introduced into a bobbin 39, there being a large number of turns of fine
wire around the bobbin 39 to thus form a coil 41. Mounted in axial
alignment with the shank of pole piece 38 in the bobbin is a
longitudinally-polarized elongated permanent magnet 42 (FIG. 3). Bobbin 39
has a base or support portion 43 which is shaped as a recess into which is
introduced the distal end of a nonmagnetic support arm 44. Arm 44 extends
away from the pole piece 38, and rests on the upper surface of wooden
support 19 (FIG. 1), there being a slot 46 (FIG. 2) in arm 44 to receive a
mounting screw 47 (FIG. 1). Loosening of screw 47 permits the entire
pickup assembly 37 to be adjusted toward and away from the tip of tine 33.
The pickup is thus suitably mounted, independently of the tine, so that
adjustments can be made relative to pickup position.
The bobbin 39 is preferably made of plastic. Accordingly, and since the
support arm 44 is made of aluminum or other nonmagnetic substance, the
only magnetic or magnetizable substances in the pickup assembly 37 are the
permanent magnet 42 and the pole piece 38. Pole piece 38 is made of some
substance, such as mild steel, having a low reluctance (high magnetic
permeability).
The magnet 42 and pole piece 38 comprise a magnetic means to create a
steady-state magnetic field. By "steady-state" it is meant that the
generated field is stationary, namely fixed in space. The coil 48
constitutes a means, responsive to such magnetic means and to vibration of
the low reluctance tine in the generated magnetic field, to create an
electrical signal. Such signal is fed to an amplifier and loudspeaker
indicated at A in FIG. 1.
The strength of permanent magnet 42 is not particularly great, being
sufficiently low that there will not be created an excessive magnetic
"drag" on the vibrating tine 33. If the magnetic strength were great,
there would be strong force tending to cause the tine 33 to cease
vibrating, this being undesirable in many of the pitch regions of the
electric piano.
Pole piece 38 has a shank 48 which is preferably cylindrical and coaxial
with the coil 41, a portion of such shank 48 being knurled at 49 whereby
to lock the shank in the plastic bobbin 39 when the pole piece is
press-fit therein. The inner end of the shank fits closely adjacent and
engages one end of the permanent magnet 42.
DESCRIPTION OF THE POLE PIECE HEAD, AND OF THE METHOD
In accordance with one aspect of the present apparatus and method, there is
provided a protuberance (protuberant portion) 51 from which the lines of
magnetic force (magnetic flux) loop back, for example as shown in FIG. 3,
and so related that the magnetic field on opposite sides of such
protuberance is highly asymmetrical. The tine, preferably the tip of the
tine, is then caused to vibrate in such asymmetrical magnetic field --
moving rapidly between positions at which the magnetic field strengths are
drastically different. Preferably, the tine tip moves back and forth
between opposite sides of the apex or "peak" of the protuberance 51, as
indicated by the two-headed arrow in FIG. 3. The tine tip is caused to
pass relatively close to such peak, preferably as close as is permitted by
reasonable manufacturing tolerances. (The "peak" of the protuberance 51,
and of protuberance 52 described below, is that portion closest to the
tine. In the present drawings, the lead lines for the numbers 51, 52,
etc., extend to such peak.)
The result of the described tine movement, and of the described
asymmetrical field, is the generation in coil 41 of a voltage signal which
has the desired harmonic "mix" for achieving bright and acoustic
piano-like sounds. Such sounds result from the transmission of the
indicated voltage signal to the amplifier-loudspeaker A shown in FIG. 1.
The sounds have excellent apparent loudness, thus minimizing the amount of
amplification which is required.
There is generated or induced in the tip of tine 33 a magnetic pole the
polarity of which is the opposite of that at the protuberance. For
example, in the apparatus illustrated in FIG. 3 there is generated in such
tip a south pole S since the north pole N of magnet 42 is adjacent the
pole piece 38. Thus, some of the flux which emanates from protuberance 51
is present in the tine tip. It is emphasized, however, that no substantial
part of the magnetic flux passes along the full length of tine 33, the
flux instead looping upwardly and back through the air, and through
substances which are desirably (for various reasons, including economy)
nonmagnetic, generally as shown by the upper dashed lines 53 in FIG. 3.
The peak of the protuberance 51 is offset (spaced) from the extended axis
of coil 41, which axis is preferably coincident with that of magnet 42 and
shank 48. Furthermore, such peak is a substantial point or a short edge in
order that flux concentration will be substantially maximized. By
employing an edge instead of a point, as described below relative to FIGS.
10 and 11-13, manufacturing is facilitated in that it is then not
necessary to line up with a point the plane of tine vibration. Whether it
is a short edge or point, the peak should be relatively "sharp", that is
to say should have a relatively small dimension in the direction of tine
vibration.
To provide a high degree of asymmetry in the generated magnetic field, and
for other reasons, the present apparatus and method preferably employ a
second protuberance 52 the peak of which is spaced from the
first-mentioned protuberance and is on the opposite side of the extended
axis of coil 41. Thus, the head of the pole piece 38 is provided with a
central groove or valley G (FIG. 3) defined between the two protuberances
51-52.
Because, in the preferred form, there are two protuberances 51-52 the peaks
of which are spaced apart, the lines of flux extending axially from
permanent magnet 42 tend to divide at the groove or valley G. Some of such
lines pass generally upwardly (FIG. 3) through protuberance 51 and then
loop back above coil 41 as illustrated (the upper dashed lines 53). Others
of such lines pass generally downwardly through protuberance 52 and then
loop back below the coil 41 (the lower dashed lines 53).
Since the mild steel forming pole piece 38 has a reluctance lower than that
of air, and since the lines of magnetic flux tend to follow relatively
low-reluctance paths and also relatively short paths, there are only a few
lines in the valley or groove G between the protuberances | | |
