CA1252652A - Stringed musical instrument - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A stringed musical instrument has a plurality of strings stretched between two fixed points and over a bridge. The strings are of substantially circular cross section and made from an elastomeric material such as silicone rubber;all strings are monolithic.
The bridge incorporates one or more piezo-electric or other transducer mounted as a bimorphic element in a simple beam mounting between fixed supports on the bridge and in acoustic contact with the body of the instrument through resilient baffle pads in contact with the face of the piezo-electric element remote from the supports. By using elastomeric strings the length of the fingerboard of the instrument can be signifi-cantly reduced whilst nevertheless providing pitch values within a conventional musical range and a new depth of tone without requiring a large resonance chamber or long strings.

Description

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.

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 l~ ;t~

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.

1 a OBJECTS OF THE INVENTION

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.

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 ~ ;r~

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 t;S'~

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.

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.

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.

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.

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.

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 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.

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.

_ g _ 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) 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

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.

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.

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.

33 The thic1~ness of the acoustic transrnission members - 12 ~

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.

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 ~'3~

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 Xf~i5,~, 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.

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Claims (10)

WHAT IS CLAIMED IS:

1. 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 resiliently 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 said bridge body member and said bridge saddle member and said first and second transducer mounts being in connection with the other of said bridge body member and said bridge saddle member, and wherein, said at least one string being stretched over said bridge saddle member whereby to apply vibrations of said one string to said saddle member and thence to said at least one transducer element via said resilient transducer mounting means.

2. The stringed musical instrument of Claim 1, wherein said at least one string is composed of a homogeneous monolithic elastomeric element.

3. The stringed musical instrument of Claim 1, wherein the diameter of said at least one string lies in the range 1.5 to 6 mm when unstressed.

4. The stringed musical instrument of Claim 1, wherein the length of said at least one string is in the region of 450 mm.

5. The stringed musical instrument of Claim 1, wherein the Shore hardness of said at least one string lies in the range 30 to 100.

6. The stringed musical instrument of Claim 1, wherein said string is of substantially circular cross-section throughout its length.

7. The stringed musical instrument of Claim 1 wherein there are a plurality of said strings each composed of silicone rubber and stretched over a fingerboard along a neck of said instrument.

8. The stringed musical instrument of Claim 1, including an oscillator circuit, and wherein said at least one elastomeric string is impregnated with electrically conductive particles and said at least one transducer element is part of an oscillator circuit generating a carrier frequency signal modulated by the vibrations of said at least one string.

9. A stringed musical instrument comprising an instrument body, a neck projecting from said body, a bridge assembly carried by said instrument body, at least one elastomeric string stretched along said neck and over and in vibration transmitting contact with said bridge assembly, resilient mounting means within and in vibration receiving contact with said bridge, at least one flexible transducer element, said one flexible transducer element being mounted for flexing movement by said resilient mounting means under influence of vibrations transmitted from said string to said resilient mounting means through said bridge whereby, said one flexible transducer element will generate electrical signals under the influence of vibration of said at least one elastomeric string.

10. The stringed musical instrument of Claim 9 wherein said resilient mounting means comprises first means bearing against one face of said transducer element inwardly of said transducer element's two ends with respect to said one face, and second mounting means in contact with an opposite face of said transducer element with respect to the first mentioned face and bearing against said transmission element adjacent the ends of said opposite face in opposition to said first mounting means.