We claim: 1. An acoustic resonator for measuring force comprising: a cylindrical body having: a central section with a first radius (a) comprising a material having a plane-wave shear velocity .nu..sub.a ; and two distal sections with a second radius (b) comprising a material having a plane-wave shear velocity .nu..sub.b wherein the ratio a/.nu..sub.a differs from the ratio b/.nu..sub.b by an amount selected to trap some acoustic resonant modes in the central section. 2. The acoustic resonator of claim 1 wherein .nu..sub.a <.nu..sub.b and a=b. 3. The acoustic resonator of claim 1 wherein a resonant mode exists in the central section such that displacement amplitude of the resonant mode decays exponentially in the distal sections. 4. The acoustic resonator of claim 1 wherein the cylindrical body comprises a hollow metal tube. 5. The acoustic resonator of claim 1 wherein the central section comprises a material adapted to receive electromagnetic energy and convert the received electromagnetic energy into mechanical energy. 6. An acoustic resonator for measuring force comprising: a cylindrical body having: a central section with a first radius (a) comprising a material having a plane-wave shear velocity .nu..sub.a ; and two distal sections with a second radius (b) comprising a material having a plane-wave shear velocity .nu..sub.b wherein the ratio a/.nu..sub.a differs from the ratio b/.nu..sub.b, .nu..sub.a =.nu..sub.b and a>b. 7. An acoustic resonator for measuring force comprising: a cylindrical body having: a central section with a first radius (a) comprising a material having a plane-wave shear velocity .nu..sub.a ; two distal sections with a second radius (b) comprising a material having a plane-wave shear velocity .nu..sub.b wherein the ratio a/.nu..sub.a differs from the ratio b/.nu..sub.b ; and means for applying force to the cylindrical body through at least one of the distal sections. 8. An acoustic resonator for measuring force comprising: a cylindrical body having: a central section with a first radius (a) comprising a material having a plane-wave shear velocity .nu..sub.a ; two distal sections with a second radius (b) comprising a material having a plane-wave shear velocity .nu..sub.b wherein the ratio a/.nu..sub.a differs from the ratio b/.nu..sub.b ; and transition regions between the central section and each of the two distal sections wherein the diameter of the transition regions is graduated from the first radius (a) to the second radius (b). 9. An acoustic resonator for measuring force comprising: a cylindrical body, wherein the cylindrical body comprises a solid metal cylinder having: a central section with a first radius (a) comprising a material having a plane-wave shear velocity .nu..sub.a ; and two distal sections with a second radius (b) comprising a material having a plane-wave shear velocity .nu..sub.b. 10. An acoustic resonator for measuring force comprising: a cylindrical body having: a central section with a first radius (a) comprising a material having a plane-wave shear velocity .nu..sub.a ; two distal sections with a second radius (b) comprising a material having a plane-wave shear velocity .nu..sub.b wherein the ratio a/.nu..sub.a differs from the ratio b/.nu..sub.b ; a plurality of permanent magnets surrounding the central section so that alternating North-South poles of the permanent magnets face a surface of the central section; and an electromechanical acoustic transducer (EMAT) sensor comprising both a first coil for torsional modes and a second coil for flexural modes, the EMAT sensor being positioned around the central section between the surface of the central section and the plurality of permanent magnets. 11. A force sensor comprising: a cylindrical body having a central section and two distal sections wherein a diameter of the central section is larger than a diameter of the distal sections; two permanent magnets positioned about the circumference of the central section such that alternating north-south poles of the permanent magnets face the surface of the central section; a coil positioned around the central section between the surface of the central section and the two permanent magnets, the coil having inputs for receiving a first excitation signal. 12. The force sensor of claim 11 wherein the coil is a spiral coil having sections that are substantially oriented in an axial direction with respect to the cylindrical body and aligned to the pole end of the magnets. 13. The force sensor of claim 11 wherein the coil is a solenoid coil. 14. The force sensor of claim 11 wherein the coil is a spiral coil having sections that are substantially oriented in an axial direction with respect to the cylindrical body and aligned equidistant between the pole ends of the magnets. 15. The force sensor of claim 12 further comprising: a solenoid coil, positioned around the central section between the surface of the central section and the plurality of permanent magnets, the solenoid coil having inputs for receiving a second excitation signal. 16. The force sensor of claim 11 wherein the cylindrical body comprises metal. 17. An acoustic resonator comprising: a cylindrical body having a central section and two distal sections; and means for substantially trapping selected acoustic resonant modes in the central section. 18. The acoustic resonator of claim 17 wherein the means for substantially trapping comprises a gradual change in diameter of the cylindrical body at an interface between the central section and each of the two distal sections. 19. The acoustic resonator of claim 17 wherein the means for substantially trapping comprises an elastic constant change at an interface between the central section and each of the two distal sections. 20. An acoustic resonator comprising: a cylindrical body having a central section and two distal sections; means for substantially trapping selected acoustic resonant modes in the central section, wherein the means for substantially trapping comprises an abrupt change in diameter of the cylindrical body at an interface between the central section and each of the two distal sections. 21. An acoustic resonator comprising: a cylindrical body having a central section and two distal sections; means for substantially trapping selected acoustic resonant modes in the central section, wherein the means for substantially trapping comprises a thin film formed in intimate contact with an exterior surface of the central section. 22. An acoustic resonator comprising: a cylindrical body having a central section and two distal sections; means for substantially trapping selected acoustic resonant modes in the central section; and mounting means for coupling a load to each of the distal portions. 23. A force sensor comprising: a cylindrical body having a central section and two distal sections wherein selected resonant modes are substantially trapped in the central section and exponentially decay in the distal sections; means for exciting the selected resonant modes in the central section; means for detecting a resonant frequency of the selected resonant modes in the central section. 24. The force sensor of claim 23 wherein the central section of the cylindrical body is hollow and encloses a pressurized fluid and the measured resonant frequency changes with pressure of the enclosed fluid. 25. The force sensor of claim 23 further comprising: means on the distal ends for applying torque to the cylindrical body whereby the measured resonant frequency changes with applied torque. 26. The force sensor of claim 23 further comprising means on the distal ends for applying axial force to the cylindrical body whereby the measured resonant frequency changes with applied axial force. 27. The force sensor of claim 23 wherein the non-contact means for exciting excites both torsional modes and flexural modes and the non-contact means for measuring measures resonant frequency of the torsional modes and flexural modes, and the force sensor further comprises: means for mathematically combining the measured resonant frequencies of the torsional and flexural modes to provide temperature compensation. 28. The force sensor of claim 23 wherein the means for measuring comprises a direct digital measurement of resonant frequency.