[0001] The present invention relates to a stringed musical instrument and particularly to
a so-called electrical musical instrument.
[0002] It is known for stringed electrical musical instruments to employ at least one transducer
or pick-up sensitive to the vibrations of strings stretched over a bridge to generate
electrical signals which can be amplified, and in some cases further modified, for
reproduction. The majority of such pick-ups utilise electromagnetic transducers which
necessitate that the strings be ferromagnetic, although piezo-electric transducers
capable of producing electrical signals resulting solely from mechanical vibrations
have been known. Typical examples of known such piezo-electric transducers for stringed
musical instruments are discussed in British Patent No. 1 524 833 and British Patent
application No. 2 070 313. Various problems have been associated with known piezo-electric
transducers; one such problem has been that of faithfully reproducing in the electrical
signals the tonal qualities of the vibrating string of the musical instrument. United
States Patent 4 030 396 discloses one prior art attempt to so construct a pick-up
for a stringed musical instrument incorporating a piezo-electric transducer that the
electrical signal will reproduce faithfully, or more nearly faithfully, the tonal
qualities of the original vibrating string.
[0003] The present invention seeks to provide an electrical musical instrument in which
a conceptual shift is made away from such prior art attempts, and the electrical musical
instrument of the present invention has its own fundamental qualities.
[0004] In prior art stringed musical instruments the major factor influencing the necessary
length of the vibrating strings is the modulus of elasticity of the material of which
the string is made. This also influences the tension under which the string must be
placed in order to vibrate at a given frequency in order to be able to obtain a desired
range of pitch. In known musical instruments this tension, particularly when applied
to four or six strings, has been sufficiently high as to make it necessary, at least
in the case of instruments such as guitars, for the instrument neck, carrying the
finger board or fretboard, to be reinforced with a metal rod to prevent bowing.
[0005] Although the present invention will be described hereinbelow with specific reference
to its application to guitars and like such instruments it is to be understood that
no loss of generality is to be engendered thereby and that the present invention is
equally applicable to any instrument employing vibrating strings and includes such
instruments as the harp, bango, dulcimer and even the piano, harpsichord or like such
encased stringed instruments.
[0006] Although, by using rather thick and stiff strings, it has in the past been possible
to generate notes in the lower register with instruments such as, for example, an
electric bass guitar without a resonance chamber, instruments such as the double bass
have still required a resonance chamber and rather long strings in order to produce
the low notes required.
[0007] The present invention seeks to provide a stringed musical instrument in which it
is possible to generate such low notes as were previously only produced from a large
instrument such as the double bass, using an instrument of small dimensions, the vibrations
of the strings of which are detected by electro-acoustic transducers and suitably
amplified. The present invention also seeks to provide an instrument in which reinforcement
of the neck, such as by means of a steel rod, is not necessary, thereby making the
instrument both light in weight as well as small in scale.
[0008] According to one aspect of the present invention, therefore, a stringed musical instrument
having one or a plurality of strings stretched between a first point at one end of
the string and a second point at the other end thereof and passing over a bridge incorporating
one or more transducer elements for converting mechanical vibrations of the strings
into electrical signals is characterised in that the string or strings is or are composed
of an elastomeric material and the transducer element or elements are not in direct
mechanical contact therewith.
[0009] Preferably the string or strings have a substantially circular cross-section. This
latter term includes octagonal, hexagonal and elliptical sections but excludes flat
band or strip-like strings as used on children's instruments in place of proper strings.
[0010] By using an elastomeric material of the above-defined cross-sectional shape in place
of the conventional steel or nylon materials used for musical instrument strings (which
are only of circular section), it is possible to generate notes of the required register
without placing the strings under the very high tension which has previously been
unavoidable. The use of such low tension is also made possible by employing a highly
sensitive piezo-electric transducer fitted into the bridge to detect the vibrations
of such strings. Conversely, such a delicate and sensitive transducer can only be
used in an arrangement where it is not subjected to the high forces exerted due to
the high tension of a conventional stringed instrument and thus the present invention
favours this also. It is to be noted that typically the modulus of elasticity of steel
is in the region of 20 x 10
11 dynes /cm
2 whilst the modulus of elasticity of elastomeric materials may be a factor of 10
9 less than this and tests conducted on a practical instrument constructed in accordance
with the present invention have demonstrated that strings having a modulus of elasticity
in the region of between 5 and 15 x 10
2 dynes /cm
2 can produce very acceptable results comparable to a bass guitar or even to a double
bass.
[0011] In a preferred embodiment of the invention the length, diameter and material of the
strings are chosen such that the tension in the or each string does not exceed a few
kg when the string is tuned to the required pitch and typically, the tension exerted
by all four strings of an instrument tuned as a bass guitar would be in the region
of up to 10 kg.
[0012] In prior art instruments the strings intended for producing the lower notes are of
composite construction, incorporating a central elongate core around which is wound
a winding of steel or other metal the properties of which are such as to modify the
vibrating characteristics of the string, in particular to damp out the higher harmonics
and to stiffen the string such that it vibrates with a low fundamental and the lower
harmonics only. In the stringed instrument of the present invention, however, such
composite string structures are unnecessary and, even for the lowermost notes, it
is sufficient to use strings which are each composed of an homogeneous monolithic
element. This not only simplifies the string manufacturing techniques but also economises
on the cost of replacement strings should these become necessary after an extended
period of use.
[0013] Typically, the diameter of strings for the stringed instrument of the present invention
may lie in the range of 1.5 - 6 mm when unstressed. Because elastomeric material having
a very low Young's modulus is used the diameter of the string will, of course, be
substantially modified when placing it under tension, and a diameter reduction in
excess of 50 per cent has in some cases been observed using the elastomeric materials
described hereinbelow.
[0014] Another advantage accruing from the use of elastomeric strings lies in the fact that
a range of low notes can be produced using strings very much shorter than has hithertofore
been found necessary. Indeed, an electrical musical instrument capable of producing
notes having a pitch corresponding to that of a bass guitar or a double bass can be
produced using strings having a length, between the bridge and the fixed stop of the
nut no greater than 0.45 m.
[0015] It is preferred that the pick-up includes one or more piezo-electric transducers
of a bimorphic type housed in a casing which is capable of accommodating the stresses
of string tension whilst nevertheless acoustically coupling the transducer to the
strings via appropriately formed acoustic transmission elements. Such an arrangement
makes the use of highly sensitive, delicate transducers possible without the risk
of damage. Previous piezo-electric transducers have required relatively high tension
in the strings and have been specifically adapted to operate at such high tension.
By using a bimorph structure it has been made possible to detect vibrations of relatively
low energy due to the greater sensitivity of such a structure.
[0016] Embodiments of the present invention will now be more particularly described by way
of example, with reference to the accompanying drawings, in which:
Figure 1 is a perspective view of an instrument formed as an embodiment of the invention
Figure 2 is a plan view of a component of the bridge illustrated in Figure 1;
Figure 3 is a sectional view taken on the line III-III- of Figure 1 illustrating the
construction of the combined bridge and pick-up.
Figure 4 is a block schematic diagram of the electrical circuit of such an instrument;
and
Figure 5 is a circuit diagram illustrating another embodiment of the invention.
[0017] Referring first to Figure 1 there is shown a stringed musical instrument structured
as a bass guitar generally indicated 11 and comprising a neck 12 extending from a
rudimentary body 13 to a headstock 14. As discussed above, the invention is equally
applicable to other types of musical instrument.
[0018] In the illustrated instrument four strings 15, 16, 17 and 18 are stretched along
the neck 12 between a bridge 19 and a fixed neck stop or nut 20. The strings 15-18
are anchored to the body 13 by passing through appropriate apertures 21, 22, 23, 24
and are secured at the headstock 14 on capstan-type tension adjusting members 25,
26, 27, 28 which because of the relatively low tension and elasticity in the strings,
do not require the complex worm screw and worm wheel arrangement conventionally used
on stringed musical instruments such as the guitar, but may be no more than friction
pegs.
[0019] The neck 12, between the bridge 19 and the nut 20, has a length of about 0.45 m and,
because of the low tension applied by the strings, can be made from a monolithic lightweight
element such as wood or plastics without requiring any reinforcement such as the steel
rods which have been used in prior art instruments. Moreover, the neck of the instrument
does not require fret bars as have hithertofore been required for guitars in order
to define the two end points of the vibrating string since the pressure of a player's
finger on the string, pressing it to the finger board, is sufficient to "stop" the
string at the appropriate point. Although frets as such are not required it may be
found convenient, as illustrated in Figure 1, to provide the finger board with a plurality
of markings indicating the position of the fingers when stopping the appropriate notes.
[0020] The strings 15 - 18 of the instrument illustrated in Figure 1 are made from a silicone
rubber elastomer and the unstressed diameters of the strings 15,16,17 and 18 are 3
mm, 2.64 mm, 2.4 mm and 1.78 mm respectively. The strings can be tuned to the notes
E A D and G by applying a tension in the region of 45 gms, 55 gms, 60 gms and 70 gms
respectively with a 0.38 m scale length (that is the length from the nut stop 20 to
the highest fret position indicated 29 in Figure 1.
[0021] On tests conducted on the strings described above the relation between the extension
and the applied load is given by the following table:
[0022] The Shore hardness of the silicone rubber material used for the strings under test
lay between 30 and 100 and calculations based on these figures resulted in a Young's
modulus varying between 7 x 10
2 dynes /cm
2 and 12.99 x 10 dynes /cm
2. As compared, for example, with steel strings having a
Young's modulus of 20
x 1011 dynes /cm
2 it will be appreciated that the modulus of elasticity of the strings employed in
the instrument of the present invention is many orders of magnitude lower and it is
the use of such strings which makes it possible to produce a lightweight, unreinforced,
instrument having a short scale length. In an alternative embodiment the string diameters
are 6 mm, 5.5 mm, 5.00 mm and 4.5 mm respectively tuned to produce notes E, A, D and
G of a bass guitar, and the joint tension applied by all four strings is just under
10 kg. as described.
[0023] Also visible in Figure 1 are three control knobs 30,31,32 for volume and tone control
and a switch 33 which selectively connects or disconnects a passive tone control circuit
interposed between the piezo-electric transducer and an amplifier; with the passive
tone control circuit switched out of circuit by the switch 33 the transducer is connected
directly to an amplifier. As is usual, connection of the instrument to an amplifier
can be made via a jack plug socket 34 which can just be seen in Figure 1.
[0024] Conversion of the mechanical vibrations of the strings 15 - 18 into electrical signals
is effected utilising a piezo-electric transducer incorporated in the bridge 19. The
structure of the bridge 19 is illustrated in cross-section in Figure 3 where it can
be seen that the bridge 19, mounted on the instrument body 13 comprises an outer casing
35 of channel section closed at the top by a cover plate 36 which is secured by adhesive
to the channel section base with the interposition of a resilient gasket 37 the shape
of which is illustrated in Figure 2.
[0025] The gasket 37 is a flat strip of elastomeric material having a plurality of recesses
or notches 38 along each edge such that when applied over the channel section member
35 the recesses 38 give the assembly a high compliance. Over the cover 36 is fitted
the bridge member 39 which is contacted by the strings 15 - 18, illustrated as being
housed in appropriately sized recesses although, in practice, the strings may rest
directly on a straight upper edge of the bridge 39. Secured to the underface of the
gasket 37 are four mounting pads 40, pairs of which are spanned by respective piezo-electric
transducers 41, 42. These transducers are of a twin plate bimorph type and a suitable
transducer is that sold by Vernitron Limited and identified by type PZT5B. This comprises
a modified lead zirconate titanate ceramic having a high electromechanical coupling
coefficient and a high charge sensitivity.
[0026] The bimorph structure is one involving a flexing type piezo-electric element consisting
of two transverse expander plates secured together face to face and provided with
electrodes on the outwardly facing surfaces. Mechanical bending of the element causes
a corresponding voltage to be developed between the electrodes. This bimorph construction
provides a high overall compliance and capacity, thereby making the transducer extremely
sensitive. Such a structure is, however, too delicate to bear directly the forces
exerted by a string under tension.
[0027] Acoustic coupling of the transducers 41,42 to the channel base 35 is effected by
means of a malleable bead 43,44. In the specific embodiment a synthetic plasticsmaterial
is used since this simplifies construction, the beads 43,44 being made slightly oversize
and then being plastically deformed as the piezo-electric bimorph transducers 41,42
are pressed down onto them, to be sealed in position by attachment of the cover plate
36. This construction ensures that the vibrations of the strings are detected by the
transducers and converted into varying voltages which are transmitted via a cable
45 to the passive tone control circuits controlled by the knobs 30-32 and leading
to the jack plug socket 34 for connection to an amplifier.
[0028] The thickness of the acoustic transmission members 43,44 has been shown slightly
greater than that of the support pads 40. In practice, these may be about the same
thickness although results may be improved by utilising thicker support pads 40 and
thinner acoustic transmission elements 43,44. In any event the thickness of both the
support pads 40 and the acoustic transmission members 43,44 should be at least 1 mm
to allow sufficient space for flexure of the piezo-electric transducer comprising
the transducer elements 41,42.
[0029] The electronic circuits on the intrument itself are shown in Figure 4. These comprise
a high input impedance passive tone control circuit 46 which passes the electrical
signal from the transducer 41 to a volume control unit 47 (controlled by the knob
30) from where it is passd to a low gain pre-amp 48 before being applied to the jack
plug 34. The pre-amp 48 may have an optional fixed bass boost. In an alternative embodiment
(not shown) the pre-amp 48 is dispensed with. The passive tone control circuit 46
mounted on the instrument body 13 may be of any known type, particularly incorporating
high pass and low pass filters as passive RC circuits for modifying the signal characteristics
prior to amplification. If the passive control circuit is not required this can be
switched out of circuit by use of the switch 33 (which is not shown in Figure 4).
In a practical embodiment constructed to test the invention it was found that suitable
adjustment of the passive control circuit enabled the instrument to produce signals
resembling those of a bass guitar or a double bass. Because the tension in the strings
is low the pressure required on the finger board by the player's fingers is also correspondingly
low and this makes it possible for the instrument to be played very rapidly, unlike
prior art low note instruments in which relatively high finger pressure and large
movements between notes were required, because of the length of the strings, such
large movements between notes, made playing the instrument at high speed rather difficult
and this is overcome in the instrument of the present invention.
[0030] Other structures than those specifically illustrated may be employed. For example,
the elastomer of the strings may be impregnated with ferrous material to enable a
magnetic transducer to be used. Again, however, the active element of the transducer
would be maintained out of contact with the strings. Likewise, with such strings,
a capacitative transducer or transducers may be employed, and these may be positioned
to detect orthogonal vibrations of the strings. Other types of transducer, including
electrostatic types, laser types, and others involving no direct contact with the
string may be used. Figure 5 illustrates an advantageous circuit configuration in
which the vibrations of a metal impregnated elastomer string 15 is detected by two
orthogonally positioned capacitor plates 49,80, one of which is grounded by line 51
and the other of which is connected to a tapping of an inductance 52 of a resonant
oscillator circuit including a variable trimmer capacitor 53 shunted across the inductance
53 and connected, with the inductance 53 to the collector of an NPN transistor 54
the emitter of which is connected to ground through an inductance 55 and parallel
connected resistor 56 and capacitor 57. A trimmer capacitor 58 is connected across
the collector-emitter junction of the transistor 54, and its base is biased by a voltage
divider comprising two series connected resistors 59,60 connected between a positive
supply line and the ground line. The resistor 60 is shunted to ground by a capacitor
61. This oscillator circuit generates a high frequency "carrier" signal which can
be modulated at audio frequency by the capacitative variations of the plates 49,80
at the frequency of vibration of the string 15. This audio frequency modulated carrier
may be transmitted via an antenna schematically indicated 50. The broadcast signal
is then received by an appropriately tuned receiver and amplified, modified, etc.,
in the usual way. Such a system has the considerable advantage of avoiding the necessity
for a large number of wires on stage at a performance thereby simplifying the P.A.
system very considerably.
1. A stringed musical instrument having one or a plurality of strings (15,16,17,18)
stretched between a first point (21,22,23,24) at one end of the string to a second
point (25,26,27,28) at the other end (20) thereof and passing over a bridge (19) incorporating
or having associated therewith one or more transducer elements (41,42) for converting
mechanical vibrations of the strings (15,16,17,18) into electrical signals, characterised
in that the string or strings (15,16,-17,18) is or are composed of an elastomeric
material and the transducer elements (41,42) are maintained out of direct mechanical
connection therewith.
2. A stringed musical instrument according to Claim 1 characterised in that the strings
(15,16,17,18) are each composed of an homogeneous monolithic elastomeric element.
3. A stringed musical instrument according to Claim 1 or Claim 2 characterised in
that the diameter of the or each string (15,16,17,18) lies in the range 1.5 to 6 mm
when unstressed.
4. A stringed musical instrument according to any preceding Claim, characterised in
that the length of the or each string (15,16,17,18) is in the region of 450 mm.
5. A stringed musical instrument according to any preceding Claim, characterised in
that the Shore hardness of the or each string lies in the range 30 to 100.
6. A stringed musical instrument according to any preceding Claim, characterised in
that the transducer (19) includes at least one piezo-electric crystal element (41,42)
mounted as a simple beam between supports (40) mechanically linked to the or a string
(15,16,17,18) whereby to transmit vibration from the or a string (15,16,17,18) to
the said transducer element (41,42).
7. A stringed musical instrument according to Claim 6, characterised in that the or
each said piezo-electric crystal element (41,42) is in contact with a resilient buffer
element (43,44) on the side thereof remote from the said transducer supports (40).
8. A stringed musical instrument according to any preceding Claim, characterised in
that the or each said string (15,16,17,18) is of substantially circular cross-section
throughout its length.
9. A stringed musical instrument according to any preceding Claim in the form of a
guitar, characterised in that the strings (15,16,17,18) are composed of silicone rubber
and are stretched over a fingerboard (12) along a neck (11) of the instrument, which
fingerboard has no frets upstanding therefrom.
10. A stringed musical instrument as claimed in any preceding Claim, characterised
in that the string or strings (15,16,17,18) is or are impregnated with electrically
conductive particles and the transducer element (49,80) is part of a tuned high frequency
oscillator circuit operable to generate a carrier signal modulated by the audio frequency
vibrations of the string or strings.