CONTACT MICROPHONES FOR GUITARS AND LIKE INSTRUMENTS

Abstract: The microphone comprises a sheet (10) of polyvinylidene fluoride or other polymeric piezoelectric material bearing an electrode pattern (12). The material is uniaxial with its axis of piezoelectric material aligned with the strings of the instrument such that the microphone is sensitive to vibration produced by the strings when played but insensitive to other vibration such as that produced by the player handling the instrument. The microphone is wrapped around the sides and base of the bridge (14) of the instrument before the latter is mounted in the saddle plate. In an alternative embodiment (Fig.4), the microphone is positioned between the body and the saddle plate of the instrument.

Description: Contact Microphones for guitars and like instruments This invention relates to contact microphones for guitars and like instruments, that is instruments having strings stretched over a bridge, and principally those where the strings are plucked, for example guitars, banjos and mandolins. The use of contact microphones on instruments has been known for some time, but hitherto they have been subject to a number of disadvantages with respect to sound quality when compared with good quality conventional microphones. Also, the noises of the player handling the instrument and moving his fingers on the strings would often seriously interfere with the sound desired; this makes the instrument very hard to play. Nevertheless, contact microphones are commonly used because of their convenience when compared with separate microphones with supporting stands etc. The present invention deals with a contact microphone system which addresses . he problems outlined above with a view to producing a contact microphone which is actually superior to conventional (non-contact) microphones. The invention provides a contact microphone for a guitar or like instrument, comprising a sheet of polymeric piezoelectric material adapted to be interposed between the bridge and the body of the instrument and having electrode areas arranged to sense voltages induced in ≤aid sheet by deformation thereof caused by predetermined modes of vibration while being insensitive to other modes of vibration. The required sensitivity may be achieved by shaping of the electrode areas, by the use of a piezoelectric material having a predominantly uniaxial response aligned with the strings of the instrument, or both of these. The invention provides two types of embodiment which can be used separately or together depending on the fidelity required. The first is applied to the bridge of . the instrument in such a manner as to produce an output corresponding to vibration perpendicular to the strings but no output for vibration axially of the strings. The second is applied to the saddle plate of the instrument to give an output reproducing low frequencies associated with body resonance. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 shows a microphone of the first type of embodiment; Fig. 2 is a side view of the microphone of Fig. 1 applied to the bridge of a guitar; Fig. 3 is a perspective view showing the assembly of Fig. 2 in position on the saddle plate of the guitar; and Fig. is a plan view illustrating a microphone of the second type applied to the saddle plate of a guitar. Referring to Fig. 1, the microphone comprises a shaped sheet 10 of a polymeric piezoelectric material, preferably/ polyviny/lidene fluoride (PVDF). As discussed in more detail below, the sheet 10 is most suitably of uniaxial PVDF having its axis of piezoelectric activity .in the direction A-A whereby in use said axis is aligned with the strings of the guitar. An electrode pattern 12 is printed on the sheet 10, suitably with silver ink, the pattern 12 consisting of a central bar 12a and oppositely-extending truncated triangles 12b at each string position. The reverse of the sheet 10 is covered with a continuous metallised conductive ground plane and conductors (not shown) are connected to the pattern 12 and to the ground plane to provide the output signal. The assembly of Fig. 1 is wrapped around the bridge 1 as seen in Fig. 2, and this is then inserted in the customary slot in the saddle plate 16 of the guitar (Fig. 3). As alternatives, the sahped conductor 12 may be positioned next the bridge 14 with the ground plane to the outside, or the continuous ground plane may be replaced by another conductor of the same shape and positioning as the conductor 12. A specific example of this embodiment will now be described with more particularity. The base film is prepared from vinylidene fluoride homo olymer, film grade resin ( YNAR (trade mark) 9816-30 from Pennwalt Corporation) by melt extrusion. The base film is uniaxially stretch oriented at a stretch ratio ranging between about 4 and 5 to 1 to produce a film having a thickness of about 28 microns. This thickness is not critical but is preferred for the present use. The film is then electrically polarized (poled) in known manner by subjecting the film to a pressure of about 300 psi, a temperature of about 65 C, and voltage of 15-l8kV. The temperature is held at 65°C for 10 minutes and the film is permitted to cool at a rate of about 2°C/mi over a period of 18-20 minutes. At the end of this cooling period, the voltage is decreased to zero and the film is removed and stabilized under a pressure of about 350-400 psi and temperature of 4O-6O C. The poled film is then electroded, with silver ink, by silk screen painting multiple patterns of the type shown in Fig. 1 on one side of the film and then repeating this painting process on the other side of the film to provide a corresponding and aligned pattern of silver ink on the opposite side of the film. Leads are attached to the electrode pattern on each side of the film and two-side-coated adhesive plastic film is applied to the electrically positive side of the film. The laminate is die cut over the electrode patterns to provide a transducer which is about as long as the bridge and about as wide as the total width of the two sides and the base of the bridge to which it is to be attached. After fastening the film transducer to the bridge piece by contact with the exposed adhesive surface and fastening the bridge to the slot in the saddle plate, the pickup is connected through the leads to a power source and amplifier, as is conventionally arranged. It is preferred that the bridge 14 be at least thicker than one-third the thickness of the thickest string, and preferably thicker than the thickest string, passing over it. This is desirable to cause one of the design's primary advantages, since direct flexing of the saddle plate forward or back in line with the string will produce little or no output signal, tension on one side of the bridge being cancelled by compression on the other. Similarly, much of the handling noise of ■ the instrument results in flexing of the bridge and this can be seen to cancel. Only that handling noise which produces a piston movement (movement transverse to the strings) of the top plate will produce an output, representing only a small percentage of the noise picked up by known contact microphones or ceramic pickups. Since the pickup of the type shown in Figs. 1-3 is between the strings and the body, and since its movement is defined by the natural bridge material, it reads the output of the difference between string and body movement. This results in the output being defined by the structure of the body, and thus ' faithfully reproducing tonal qualities resulting from the body structure. There is however some infidelity caused by the fact that the bridge is at times located at a nodal point of body resonance. This means that while the microphone so far described already produces very high fidelity and transparency, there is an error in the output at low frequencies. While for most musicians this would not be significant, for those who desire even higher fidelity a second pickup is used. This is illustrated in Fig. 4. The saddle plate of the guitar (such as the saddle plate 16 of Fig. 3) has secured between it and the guitar body a relatively large sheet of uniaxial PVDF, suitably the same material as that described in detail above, with the axis of sensitivity aligned with the strings. Holes are provided at A or B for passage of the strings; in the example shown both are provided to enable the pickup to fit various guitars. A conductive pattern 22 is printed on the top surface of the sheet 20 to act as a sensing electrode, and the rear surface again is provided over its whole surface with a conductive ground plane. Holes 24 through the sheet are plated through to provide electrical interconnection at these points between the pattern 22 and the ground plane. This has the effect that the areas of the conductive pattern act as individual pickups connected in series. As seen in Fig. 4, a suitable pattern 22 consists of four strips transverse to the strings, together with printed connectors. Suitably, these strips are each 8.5 n_ wide and are spaced 3.0 mm apart. It would be possible for both sides of the sheet 20 to be provided with a continuous metallised conductive layer. However, such a transducer would have a very large capacit- ance, and the pattern described is preferred to reduce the capacitance and produce an impedance similar to that of the bridge pickup of Figs. 1-3. The sheet 20 produces signals resulting from compression under the bridge and stretching in the string direction, and its sensitivity is approximately equal between the change in length caused by the swing of a node and the compression signal from the bridge. Once again the signal is largely defined by the load rather than actual movement of the guitar body, and thus the output from handling is still relatively low as the bridge acoustic impedance is higher than any other similar area of the body. The pickup of Fig. 4 ma be used on its own, but the damping of transients by the significant masses in the way makes it less suitable than the pickup of Figs. 1-3. When used together, however, the bridge pickup provides the transient and major harmonic information and the second pickup provides the low frequency information which gives the instrument its solidity. Thus as the bridge pickup alone is lacking only the absolute solidity of these low frequencies, the second pickup canbe dispensed with for all but the most critical applications such as the amplification and recording of classical guitar or for solo work on record. It is probably not necessary for most group work on record or stage work. The combination of the two pickups can produce a sound which by all practical means of observation is identical to the unamplified guitar but louder (assuming, of course, that the amplification and speaker systems are of top quality). Although described particularly with reference to a guitar, the invention can be used with other stringed instruments having a bridge resting in a slot.

Claims: CLAIMS 1. A contact microphone for a guitar or like instrument, comprising a sheet of polymeric piezoelectric material adapted to be interposed between the bridge and the body of the instrument, said sheet having disposed thereon electrode areas arranged to sense electrical energy induced in the sheet by deformation thereof caused by predetermined modes of vibration while being insensitive to other modes of vibration. 2. The microphone of claim 1, in which said sheet is of a uniaxially oriented material such that, in use, its axis of orientation is aligned with the strings of the instrument. 3. The microphone of claim 2, in which said sheet is of polyvinylidene fluoride prepared from a vinylidene resin which requires orientation to improve its piezoelectric activity. 4. The microphone of claim 3, in which said sheet is prepared from a homopolymer of vinylidene fluoride by melt extrusion followed by unidirectional stretching and electrical polarization. 5. The microphone of claim 1, in which said sheet is adapted to be secured to the base and opposed sides of an instrument bridge, and in which the electrode areas on at least one face of the sheet comprise a central conductive portion for positioning between said base and the saddle plate of the instrument and a plurality of portions extending from and to either side of said central portion so as to be aligned with each string. 6. The microphone of claim 5, in which said sideways extending portions each have the form of a truncated triangle. 7. The microphone of claim 1, in which said sheet is shaped and dimensioned to be positioned between the saddle plate and the body of an instrument. 8. The microphone of claim 7, in which said electrodes comprise a continuous ground plane on one face of the sheet and, on the opposite face, a plurality of strips transverse to the string connection and connected in series. 9. The microphone of claim 8, in which the connections between the strips are also electrically connected to said ground plane.

Inventors: STRACHAN JOHN SCOTT, STRACHAN, JOHN, SCOTT
Applicants: SYRINX INNOVATIONS, SYRINX INNOVATIONS LIMITED
Application year: 1986
Main classification: G10H3
IPC codes: G10H3