Note: Descriptions are shown in the official language in which they were submitted.
1 BACKGROUND OF THE INVENTION
3 It is known for stringed electrical musical instruments
4 to employ at leas~ one transducer or pick-up sensitive
to the vibrations of strings stretched over a bridge to
6 generate electrical signals which can be amplified, and
7 in some cases further modified, for reproduction. The
8 majority of such pick-ups utilise electrsmagnetic
9 transducers which necessitate that the strings be
ferromagnetic, although piezo-electric transducers
11 capable of producing electrical signals resulting
12 solely from mechanical vibrations have been known.
13 Typical examples of known such piezo-electric
14 transducers for stringed musical instruments are
discussed in British Patent No. 1 524 833 and British
16 Patent application No. 2 070 313. Various problems
17 have been associated with known piezo-electric
18 transducers; one such problem has been that of
19 faithfully reproducing in the electrical signals the
tonal qualities of the vibrating string of the musical
21 instrument. United States Patent 4 030 396 discloses
22 one prior art attempt to so construct a pick-up for a
23 stringed musical instrument incorporating a piezo-
24 electric transducer that the electrical signal will
reproduce faithfully, or more nearly faithfully, the
26 tonal qualities of the original vibrating string.
27
28 In prior art stringed musical instruments the major
29 factor influencing the necessary length of the
vibrating strings i5 the modulus of elasticity of the
31 material of which the string is made. This also
32 influences the tension under which the string must be
33 placed in order to vibrate at a given frequency in
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1 order to be able to obtain a desired range of pitch.
2 In known musical instruments this tension, particularly
3 when applied to four or 5ix strings, has been
4 sufficiently high as to make it necessary, at least in
S the case oE instruments such as guitars, for the
6 instrument neck, carrying the finger board or
7 fretboard, to be reinforced with a metal rod to prevent
8 bowing~
Although, by using rather thick and stiff strings, it
11 has in the past been possible to generate notes in the
12 lower register with instruments such as, for example,
13 an electric bass guitar without a resonance chamber,
14 instruments such as the double bass have still required
a resonance chamber and rather long strings in order to
16 produce the low notes required.
17
1 a OBJECTS OF THE INVENTION
1 9
A primary object of the present invention is to provide
21 an electrical musical instrument in which bass notes
22 can be produced without a resonance chamber and using
23 shorter strings than has hithertofore been the case.
24 Another object of the invention is to provide a new
instrument with monolithic strings and novel tonal and
26 playing qualities.
27
28 A further object of the present invention is to provide
29 a stringed musical. instrument in which it is possible
to generate such low notes as were previously only
31 produced from a large instrument such as the double
32 bass, using an instrument of small dimensions, the
33 vibrations of the strings of which are detected by
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electro-acoustic transducer~ and sui~ably amplified.
Still another object of the present invention is 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
wei~ht as well as small in scale.
SUMMARY OF THE INVENTION
According to one as?ect of the present invention,
there is provided a stringed musical instrument
having at least one vibratible
string stretched between a first point at one end of the string and
a second point at the other end thereof,
a bridge over which said at least one string passes, said
bridge including:
- a bridge body member,
- a bridge saddle member in direct contact with said string,
and
'- means re~iliently interconnecting said bridge saddle member
and said bridge body member,
at least one elongate flexible transducer element having a
first end and a second end,
resilient transducer mounting means mounting said at least
one transducer element on said bridge, said resilient transducer
mounting means comprising:
- first and~second transducer mounts in contact with one face
of said at least one transducer element at said first and second
ends thereof respectively, and
- a resilient third transducer mount in contact with the face
of said at least one transducer element opposite said one face
thereof and intermediate said first and second ends thereof, said
resilient third transducer mount being in contact with one of szid
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bridge b~dy member and ~aid bridge ~addle mer~er and said fir~t ~nd
~econd transducer mounts being in connection with the other o~ said
bridge body member and said bridge saddle member, and wherein,
said at lea~t one string being stretched over said bridge
saddle me~ber whereby to apply vibrations of qaid one ~tring to
~aid saddle member and thence to said at least one transducer
element via 6aid resilient ~ransducer mounting means.
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, banjo, dulcimer and even the piano, harpsichord
or like such encased stringed instruments.
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1 Preferably the string or strings have a substan~ially
2 circular cross-section. This latter term includes
3 octagonal, hexagonal and elliptical sections but
4 excludes flat band or strip like strings as used on
children's instruments in place of proper strings.
7 By using an elastomeric material of the above-defined
8 cross-sectional shape in place of the conventional
9 steel or nylon materials used for musical instrument
strings (which are only of circular section), it is
11 possible to yenerate notes of the required register
12 without placing the strings under the very high tension
13 which has previously been unavoidable. The use of such
14 low tension is also made possible by employing a highly
sensitive piezo-electric transducer fitted into the
16 bridge to detect the vibrations of such strings.
17 Conversely, such a delicate and sensitive transducer
18 can only be used in an arrangement where it is not
19 subjected to the high forces exerted due to the high
tension of a conventional stringed instrument and thus
21 ~he present invention favours this also. It is to be
22 noted that typically the modulus of elasticity of steel
23 is in the region of 20 x 1011 dynes /cm2 whilst the
24 modulus of elasticity of elastomeric materials may be a
factor of 109 less than this and tests conducted on a
26 practical instrument constructed in accordance with the
27 present invention have demonstrated that strings having
28 a modulus of elasticity in the region of between 5 and
29 15 x 102 dynes /cm2 can produce very acceptable results
comparable to a bass guitar or even to a double bass.
31
32 In a preferred embodiment of the invention the length,
33 diameter and material of the strings are chosen such
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1 that the tension in the or each string does not exceed
2 a few kg when the string is tuned to the required pitch
3 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.
7 In prior art ins~ruments the strings intended for
8 producing the lower notes are of composite con-
9 struction, incorporating a central elongate core around
which is wound a winding of steel or other metal the
11 properties of which are such as to modify the vibrating
12 characteristics of the string~ in particular to damp
13 out the higher harmonics and to stiffen the string such
14 that it vibrates with a low fundarnental and the lower
harmonics only. In the stringed instrument of the
16 present invention, however, such composite string
17 structures are unnecessary and, even for the lowermost
18 notes, it is sufficient to use strings which are each
19 composed of an homogeneous monolithic element. This
not only simplifies the string manufacturing techniques
21 but also economises on the cost of replacement strings
22 should these become necessary after an extended period
23 of use.
24
Typically, the diameter of strings for the stringed
26 instrument of the present invention may lie in the
27 range of 1.5 - Ç mm when unstressed. Because
28 elastomeric material having a very low Young 15 modulus
29 is used the diameter of the string will, of course, be
substantially modified when placing it under tension,
31 and a diameter reduction in excess of 50 per cent has
32 in some cases been observed using the elastomeric
33 materials described hereinbelow.
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1 Another advantage accruing from the use of elastomeric
2 strings lies in the fact ~hat a range of low notes can
3 be produced using strings very much shorter than has
4 hithertofore ~een found necessary. Indeed, an
electrical musical instrument capable of producing
6 notes having a pitch corresponding to that of a bass
7 guitar or a double bass can be produced using strings
8 having a length, between the bridge and the fixed stop
9 of the nut no greater than 0.45 m.
1 0
11 It is preferred that the pick-up includes one or more
12 piezo-electric transducers of a bimorphic type housed
13 in a casing which is capable of accommodating the
14 stresses of string tension whilst nevertheless
acoustically coupling the transducer to the strings via
16 appropriately formed acoustic transmission elements.
17 Such an arrangement makes the use of highly sensitive,
18 delicate transducers possible without the risk of
19 damage. Previous piezo-electric transducers have
required relatively high tension in the strings and
21 have been specifically adapted to o~erate at such high
22 tension. By using a bimorph structure it has been made
23 possible to detect vibrations of relatively low energy
24 due to the greater sensitivity of such a structure.
26 Other features and advantages of the invention will
27 become apparent,from a study of the following
28 description, in which reference is made to the
~9 accompanying drawings.
31
32
33
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2 BRIEF DESCRIPTION OF THE DRAWINGS
4 Figure 1 is a perspect.ive view of an instrument
formed as an embodiment of the invention;
6 Figure 2 is a plan view of a component of the
7 bridge illustrated in Figure 1;
8 Figure 3 is a sectional view taken on the line
9 III-III of Figure 1 illustrating the construction of
the combined bridge and pick-up.
11 Figure 4 is a block schematic diagram of the
12 electrical circùit of such an instrument; and
13 Figure 5 is a circuit diagram illustrating another
14 embodiment of the invention.
16
17 DESCRIPTIGN OF THE PREFERRED EMBODIMENTS
18
19 Referring first to Figure 1 there is shown a stringed
musical instrument ~tructured as a bass guitar
21 generally indicated 11 and comprising a neck 12
22 extending from a rudimentary body 13 to a headstock 14.
23 As discussed above, the invention is equally applicable
24 to other types of musical instrument.
26 In the illustrated instrument four strings 15, 16, 17
27 and 18 are stre,tched along the neck 12 between a bridge
28 19 and a fixed neck stop or nut 20. The strings 15-18
29 are anchored to the body 13 by passing through
~0 appropriate apertures 21, 22, 23, 24 and are secured at
31 the headstock 14 on capstan-type tension adjusting
32 members 25, 26, 27, 28 which because of the relatively
33 low tension and elasticity in the strings, do not
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1 require the complex worm screw and worm wheel
2 arrangement conventionally used on stringed musical
3 instruments such as the guitar, but may be no more than
4 friction pegs.
6 The neck 12, between the bridge 19 and the nut 20, has
7 a length of about 0.45 m and, because of the low
8 tension applied by the strings, can be made from a
9 monolithic light~eight element such as wood or plastics
without requiring any reinforcement such as the steel
11 rods which have been used in prior art instruments.
12 Moreover, the neck of the instrument does not require
13 fret bars as have hithertofore been required for
14 guitars in order to define the two end points of the
vibrating string since the pressure of a player's
16 finger on the string, pressing it to the finger board,
17 is sufficient to "stop" the string at the appropriate
18 point. Although frets as such are not required it may
19 be found convenient, as illustrated in Fi~ure 1, to
provide the finger board with a plurality of markings
21 indicating the position of the fingers when stopping
22 the appropriate notes.
23
2~ The strings 15 - 18 of the instrument illustrated in
Figure 1 are made from a silicone rubber elastomer and
26 the unstressed diameters of the strings 15,16,17 and 18
27 are 3 mm, 2.64 mm, 2.4 mm and 1.78 mm respectively.
28 The strings can be ~uned to the notes E A D and G by
29 applying a tension in the region of 45 gms, 55 gms, 60
gms and 70 gms respectively with a 0.38 m scale length
31 (that is the length from the nut stop 20 to the highest
32 fret position indicated 29 in Figure 1.
33
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1 On tests conducted on the strings described above the
2 relation between the e~tension and the applied load is
3 given by the following table:
String Diameter (mm) 3 2.64 2.4 1.78
6 Natural length (cm) 25 25 25 25
7 Extended length 1 (cm) ~0 44 66 55
8 (Load 50 gms)
9 Extended length 2 (cm) 56 56 94 70
(Load 100 gms)
1 1
12 The Shore hardness of the silicone ruober material used
13 ~or the strings under test lay between 30 and 100
14 and calculations based on these figures resulted in a
Young's modulus varying between 7 x 102 dynes /cm2 and
16 12.99 x 102 dynes /cm2. As compared, for example, with
17 steel strings having a Young's modulus of 20 x 1011
18 dynes /cm2 it will be appreciated that the modulus of
19 elasticity of the strings employed in the instrument of
the present invention is many orders of magnitude lower
21 and it is the use of such strings which makes it
22 possible ~o produce a lightweight, unreinforced,
23 instrument having a short scale length. In an
24 alternative embodiment the string diameters are 6 mm,
5.5 mm, 5.00 mm and 4.5 mm respectively, and the joint
26 tension applied by all four strings is just under l0
27 kg. as describe~O
28
29 Also visible in Figure 1 are three control knobs
30,31,32 for volume and tone control and a switch 33
31 which selectively connects or disconnects a passivetone
32 control circuit interposed between the piezo-electric
33 transducer and an amplifier; with the passive tone
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1 control circuit switched out oE circuit by the switch
2 33 the transducer is connec-ted directly to an
3 amplifier. As is usual, connection of the instrument
4 to an amplifier can be made via a jack plug socket 34
which can just be seen in Figure 1.
7 Conversion of the mechanical vibrations of the strings
8 15 - 18 into electrical signals is effected utilising a
9 piezo-electric transducer incorporated in the bridge
19. The structure of the bridge 19 is illustrated in
11 cross-section in Figure 3 where it can ~e seen that the
12 bridge 19, mounted on the instrument body 13 comprises
13 an outer casing 35 of channel section closed at the top
14 by a cover plate 36 which is secured by adhesive to the
channel section base with the interposition of a
16 resilient gasket 37 the shape of which is illustrated
17 in Figure 2.
18
19 The gasket 37 is a flat strip of elastomeric material
having a plurality of recesses or notches 38 along each
21 edge such that when applied over the channel section
22 member 35 the recesses 38 give the assembly a high
23 compliance. Over the cover 36 is fitted the bridge
24 member 39 which is contacted by the strings 15 - 18,
illustrated as being housed in appropriately sized
26 recesses although, in practice, the strings may rest
27 directly on a s~raight upper edge of the bridge 39.
28 Secured to the underface of the gasket 37 are four
29 mounting pads 40l pairs of which are spanned by
respective piezo-electric transducers 41, 42. These
31 transducers are of a twin plate bimorph type and a
32 suitable transducer is that sold by Vernitron Limited
33 and identified by type PZT5B. lrhis comprises a
1 modiEied lead zirconate titanate ceramic having a high
2 electromechanical couplincJ coefficient and a high
3 charge sensitivity.
The bimorph structure i5 one involving a flexing type
6 piezo-electric element consisting of two transverse
7 expander plates secured together face to face and
8 provided with electrodes on the outwardly facing
9 surfaces. Mechanical bending of the element causes a
corresponding voltage to be developed between the
11 electrodes. This bimorph construction provides a high
12 overall compliance and capacity, thereby making the
18 transducer extremely sensitive. Such a structure is,
14 however, too delicate to bear directly the forces
exerted by a string under tenslon.
16
17 Acoustic couplin~ of the transducers 41,42 to the
18 channel base 35 is effected by means of a malLeable
19 bead 43,44. In the specific embodiment a synthetic
plastic material is used since this simplifies
21 construction, the beads 43,44 being made slightly
22 oversize and then being plastically deformed as the
23 piezo-electric bimorph transducers 41,42 are pressed
24 down onto them, to be sealed in position by attachment
of the cover plate 36. This construction ensures that
26 the vibrations of the strings are detected by the
27 transducers and converted into varying voltages ~hich
28 are transmitte~ vla a cable 45 to the passive tone
29 control circuits controlled by the knobs 30-32 and
leading to the jack plug socket 34 for connection to an
31 amplifier.
32
33 The thic1~ness of the acoustic transrnission members
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1 43,44 has been shown slightly greater than that of the
2 support pads 40. In practice, these may be about the
3 same thickness although results may be improved by
4 utilising thicker support pads 40 and thinner acoustic
transmission elements 43~44O In any event the
6 thickness of both the support pads 40 and the acoustic
7 transmission members 43,44 should be at least 1 mm to
8 allow sufficient space for flexure of the pie~o-
9 electric transducer comprising the transducer elements
41,42.
1 1
12 The electronic circuits on the intrument itself are
13 shown in Figure 4. These comprise a high input
14 impedance passive tone control circuit 46 which passes
the electrical signal from the transducer 41 to a
16 volume control unit 47 (controlled by the knob 30) from
17 where it is passd to a low gain pre-amp 48 before being
18 applied to the jack plug 34. The pre-amp 48 may have
19 an optional fixed bass boost. In an alternative
embodiment (not shown) the pre~amp g8 is dispensed
21 with. The passive to~e control circuit 46 mounted on
22 the instrument body 13 may be of any known type,
23 particularly incorporating high pass and low pass
24 filters as passive R~ circuits for modifying the signal
characteristics prior to amplification. If the passive
26 control circuit is not required this can be switched
27 out of circuit py use of the switch 33 (which is not
28 shown in Figure 4). In a practical embodiment
29 constructed to test ~he invention it was found that
suitable adjustment of the passive control circuit
31 enabled the instrument to produce signals resembling
32 those of a bass guitar or a double bass. Because the
33 tension in the strings is low the pressure required on
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1 the finger board by the player's fingers is also
2 correspondingly low and this ma~es it possibl~ for the
3 instrument to be played very rapidly, unlike prior art
4 low note instruments in which relatively high finger
pressure and large movements between notes were
6 required, because of the length of the strings, such
7 large movements between notes made playing the
8 instrument at high speed rather difficult and this is
9 overcome in the instrument of the present invention.
11 Other structures than those specifically illustrated
12 may be employed. For example, the elastomer of the
13 strings may be impregnated with ferrous material to
14 enable a magnetic transducer to be used. Again,
however, the active element of the transducer would be
16 maintained out of contact with the strings. Likewise,
17 with such strings, a capacitative transducer or
18 transducers may be employed, and these may be
19 positioned to detect orthogonal vibrations of the
strings. Other types of transducer, including
21 electrostatic types, laser types, and others involving
22 no direct contact with the string may be used. Figure
23 5 illustrates an advantageous circuit configuration in
24 which the vibrations of a metal impregnated elastomer
string 15 is detected by two orthogonally positioned
26 capacitor plates 49,80, one of which is grounded by
27 line 51 and the, other of which is connected to a
28 tapping of an inductance 52 o a resonant oscillator
29 circuit including a variable trimmer capacitor 53
shunted across the inductance 53 and connected, with
31 the inductance 53 to the collector of an NPN transistor
32 54 the emitter of which is connected to ground through
33 an inductance 55 and parallel connected resistor 56 and
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l capacitor 57. A trimmer capacitor 58 is connected
2 across the collec~or-emitter junction of the transistor
3 54, and its base is biased by a voltage divider
4 comprising two series connected resistors 59,60
connected between a positive supply line and the ground
6 line. The resistor 60 is shunted to ground by a
7 capacitor 61. This oscillator circuit generates a high
8 frequency "carrier" signal which can be modulated at
9 audio frequency by the capacitative variations of the
plates 49,80 at the frequency of vibration of the
11 string 15. This audio frequency modulated carrier may
12 be transmitted via an antenna schematically indicated
13 50. The broadcast signal is then received by an
14 appropriately tuned receiver and amplified, modified,
etc., in the usual way. Such a system has the
16 considerable advantage of avoiding the necessity for a
17 large number of wires on stage at a performance thereby
18 simplifying the P.A. system very considerably.
19
21
22
23
24
26
27
28
29
31
32
33
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