Abstract: A broadband electroacoustic transducer is disclosed which provides an improved response over a relatively broad range of frequencies. The transducer includes a plurality of tines extending between the outer circumferential surfaces of a pair of piezoelectric cylinders. The cylinders are disposed about a common axis and are separated by a gap. The tines extend longitudinally across the gap from one cylinder to the other. Each tine is separated from the adjacent tines by a space which is sized to prevent contact between the tines when the cylinders radially oscillate. A pre-stress wrap is disposed over the tines on both sides of the gap to provide a static, inwardly radial force on the cylinders.

Abstract: The provided invention is a novel electrostrictive driving device which comprises a sonic wave projector element having alternating electrodes and polymer material film layers. The device provides when subjected to a high bias voltage and a superimposed A.C. voltage, a high Angstroms/Volt response. Also, provided is a process for projecting sonic waves using the electrostrictive driving device of this invention.

Abstract: A composite split cylinder transducer comprises an electromechanical driver and a cylindrical shell having a longitudinal gap. The shell further has a portion, disposed opposite the gap, comprised of a high strength material having increased stiffness. Transducers of this configuration are capable of being employed at greater ocean depths where high hydrostatic pressure conditions exist with little effect on acoustic performance.

Abstract: A bonded insulator flexural transducer for use in sonar applications includes a hollow metal tube having an inner surface, a segmented ceramic electromechanical driver disposed along an inner surface of the tube and a bonded insulator disposed between the segmented driver and the metal tube. The bonded insulator includes a high dielectric strength film, a layer of a binding fiber material, and a layer of epoxy. The dielectric film provides the electrical isolation between the driver and the shell and the binding fiber material provides a reliable bond by effectively trapping the epoxy between the driver and shell.

Abstract: An ultrasonic probe includes a piezoelectric material layer having a pair of electrodes provided on both main surfaces thereof for applying voltage thereto, and a backing material provided on one electrode. The backing material has an acoustic impedance lower than that of the piezoelectric material layer. Interposed between the backing material and one electrode is an acoustic reflecting material layer which has a thick first portion and a thin second portion. The second portion may have a substantially zero thickness to allow the backing material to be in partial contact with one electrode. Thereby, the ultrasonic probe can transmit and receive ultrasonic waves at its resonance frequencies. Also provided is an ultrasonic diagnostic apparatus which displays an image resultant from combining images having the frequencies obtained by driving the ultrasonic probe.

Abstract: A radially-polarized piezoelectric cylindrical transducer is provided. The ransducer has an electrically conductive end cap affixed with a conductive epoxy to one end of a piezoelectric body and a second cap affixed to the other end. The piezoelectric body comprises a layer of piezoelectric material located between an inner and an outer electrode. The outer electrode has an electrode gap which electrically insulates the end cap from the outer electrode. The unit is electrically conductive and permits the attachment of two electrical leads to its exterior, one to the first end cap and the other to the outer electrode of the piezoelectric body. In an alternative embodiment, two conductive end caps are affixed with conductive epoxy to opposite ends of the piezoelectric body. In this embodiment, the inner electrode has an electrode gap which electrically insulates the second end cap from the inner electrode. One electrical lead attaches to each end cap in this alternative embodiment.

Abstract: An apparatus for detecting a person in a body of water includes a transmitter for generating an electrical swept frequency signal having a frequency that continuously varies between upper and lower limits, at least one pair of transducers disposed in a body of water including a transmitting transducer connected to receive the swept frequency signal for launching a beam of acoustical waves in response to the swept frequency signal in the body of water and a receiving transducer disposed opposite the transmitting transducer for receiving the acoustical waves and for converting received acoustical waves into an electrical received signal, and a receiver connected to the receiving transducer for producing a detected signal in response to the received signal, for producing a threshold signal in response to the detected signal, and for initiating an alarm when the detected signal falls below the threshold for at least a predetermined length of time.

Abstract: There is disclosed an ultrasonic shockwave transducer for use in lithotripsy, hypothermia and like treatments for generating ultrasonic shock waves and transmitting them to a concretion or tissue to be destroyed. The transducer is arranged to focus the energy of the ultrasonic shock waves proportionally onto at least two points disposed on a line disposed about the main axis of, and being spaced from the radiation surface of, the transducer, the line being arbitrarily curved in three dimensions.

Abstract: A piezoelectric ceramic hydrostatic sound sensor or transducer having high ensitivity to hydrostatic pressure is made by placing a flat plastic disc between two flat layers of green ceramic material, compressing and fusing the layers, heating to a first temperature at which the plastic decomposes, leaving a flat void in the ceramic, and heating to a second temperature at which the ceramic sinters. The transducer is provided with electrodes on its top and bottom surfaces. In a further improvement, ceramic particles are provided which are entrapped in the void; they render the sound sensor sensitive to inertial forces. In yet another improvement, the inside walls of the void are coated with a conductive noble metal connected to a terminal wire, whereby an additional electrode is provided for sensing the electromechanical response of the transducer.

Abstract: A transducer assembly providing a conversion between the electrical and sonic energy, the transducer assembly being adapted for operating submerged at great depths in ocean waters. The transducer assembly is formed by a front and back plate which are affixed in one embodiment to the ends of a first mechanically distributed impedance member, a second mechanically distributed impedance member located between the front and the back plate and a transducer of electroacoustical material mechanically coupling the first and second members for imparting sonic vibrations to these members. In a preferred embodiment, the transducer is bonded at one end thereof to the back plate and at the other end thereof to the second member. External hydrostatic pressure present at the front and back plates does not compress the electroacoustical material.

Abstract: A solid-state tunable transducer has been developed by incorporating an elastically nonlinear material, silicone rubber, into an electroacoustic transducer made from piezoelectric ceramics. The resonant frequency and mechanical Q of the transducer are tuned mechanically by applying a uniaxial compressive stress to the composite. The resonant frequency is tuned electrically by placing a piezoelectric actuator into the composite and varying the magnitude of the d.c. bias.

Abstract: A composite ultrasound transducer (array) contains piezoelectric ceramic transducer elements which radiate substantially in the longitudinal direction, and are embedded in a polymer matrix. The transducer elements have such a geometrical structure, and are arranged relative to each other, so that the development of oscillation modes perpendicular to the longitudinal direction of the transducer elements is suppressed. A method for manufacturing the composite ultrasound transducer includes the steps of filling a plastic form, having a "negative" of the desired shape of the transducer elements, with ceramic slurry, and pyrolyzing the plastic form after the slurry dries therein. The resulting voids between the hardened ceramic elements are filled with polymer to create the overall composite transducer.

Abstract: A hydrophone for calibrating lithotripters includes a spot poled disc of piezoelectric material which is mounted on a brass cylinder backing. A coaxial cable is electrically connected to one surface of the disc through the brass backing and the opposing surface of the disc. A mixture of polymeric material and microballoons is provided to protect the interconnection of the center conductor and the opposing surface of the disc and prevent damage due to the incident ultrasonic shock waves. The brass backing is placed in a suitable housing of phenolic material. A process in fabricating the piezoelectric disc and assembling the hydrophone is disclosed.

Abstract: A method of forming a laminated piezoelectric structure is disclosed wherein a plurality of polarized piezoelectric films of vinylidene fluoride and trifluoroethylene are solvent welded together. The solvent welding solution comprises a copolymer of vinylidene fluoride and trifluoroethylene and a suitable solvent. The transducers produced from such a laminated structure are also disclosed.

Abstract: An acoustical lens for focusing ultrasonic energy having a transducer, a curved focussing lens and a voltage generator coupled with the transducer. The transducer is comprised of an array of piezoelectric crystal elements. The focussing lens has a concave upper surface and a planar bottom surface allowing the array of piezoelectric elements to be coupled to the bottom surface of the array. The piezoelectric elements receive voltage from the voltage generator and transmit an energy ray through the focussing lens producing a focussed energy beam.

Abstract: An acoustic field transducer (10) comprises an elastic membrane (15A) made of piezo-electric material and peripherally held in stretched condition with static strain. A major surface area of the membrane (15A) is free to move in response to acoustic field variations which are coupled to the membrane (15A). Electrical conductors (17) are connected to the membrane (15A) for collecting signals piezo-electrically generated by the membrane due to strain variations and which are a measure of the acoustic field variations. The acoustic field variations are coupled to the membrane (15A) in a variety of different arrangements.

Abstract: An improved electromechanical transducer is provided for use in an acoustic telemetry system. The transducer of this invention comprises a stack of ferroelectric ceramic disks interleaved with a plurality of spaced electrodes which are used to electrically pole the ceramic disks. The ceramic stack is housed in a metal tubular drill collar segment. Thick metal spacer plate are selectively sandwiched between electrodes in order to promote thermal cooling of the ceramic stack. In accordance with an important feature of this invention, the thick metal spacer plates are comprised of a material (such as copper alloys, aluminum alloys or the like) which is softer than the relatively hard, brittle ceramic disks thus reducing the stresses upon the disks when the assembly is subjected to bending, torsion and the like, and thereby minimizing the risk of structural failure of the disks when in operation within a downhole acoustic signal generator.

Abstract: There is disclosed, a piezoelectric transducer for generating focussed ultrasonic shock waves for use in lithotripsy. The ultrasonic shock waves are emitted in pulsed form and can be transmitted by way of a coupling medium to the body of a patient to be treated. The transducer comprises a substantial member of individual piezoelectric transducer elements of ceramic or like material which are connected to the poles of a pulse generator and are fixed to a support in mosaic form and with their sides electrically insulated from one another. The acoustic termination of the transducer elements is essentially free from reflection. An intermediate medium of at least one layer, the acoustic impedance of which lies between that of the ceramic of the transducer element and that of the coupling medium, is provided between the transducer elements and the coupling medium. The thickness d of the layer is chosen in accordance with the relationship d>.tau..sub.k .multidot.c.sub.LA, where .tau..sub.

Abstract: The present invention comprises an audio alarm for providing a back-up acoustic output in response to a sensed parameter including a first audio alarm circuit having a first transducer for providing a first acoustic output in response to the sensed parameter, a second audio alarm circuit having a second transducer which provides the back-up acoustic output in response to a predetermined fault condition, and which is acoustically coupled to the first transducer. The second audio alarm circuit provides an electrical signal by converting the first acoustic output of the first transducer into an electrical signal. A detector circuit is connected to the first and second audio alarm circuits and responds to the sensed parameter and to the electrical signal from the second audio alarm circuit to provide the predetermined fault condition by interpreting the electrical signal from the second audio alarm circuit.

Abstract: The FLEXBAR is a sonar transducer element that is basically a piezoceramic "free-free" flexure bar, but modified so as to radiate as a "monopole" rather than as a "dipole", and retaining the unique properties of being nodally mounted and dynamically balanced. One of the important consequences of this simple modification is the fact that the reaction forces on the FLEXBAR mountings and, the concomitant structure-borne vibrations, are virtually eliminated. This property, and related properties, result in unexpected but significant improvement in the performance of low-frequency, high-power, board-band sonar transducer arrays.

Abstract: The sonar transducer includes a metal core about which is situated a cylindrical piezoelectric ceramic ring element with paper interposed therebetween. In order to improve resistance to fracture resulting from mechanical shock, in one embodiment, the core is reduced in diameter, such that a potting compound can be situated between the paper and the ring. In a second embodiment, the ring element is divided into two individual, axially aligned cylindrical ring subelements with a gasket interposed therebetween.

Abstract: An electroacoustic transducer according to the invention comprises a stacking of annular plates (13) made of piezoelectric ceramics alternating with annular electrodes (14) and pressed by an axial prestressing stem (15). The contact faces between each plate and the adjacent ones are made of two flat parts (16a, 16b) whose surface is smaller than the maximum cross section, due to the chamfers (19a, 19b, 20a, 20b) which truncate the edges of each plate. One application is the construction of low frequency sonar transmitters. Referenced elements refer to FIG. 3.

Abstract: This invention describes an acoustic transducer assembly wherein an extremely high figure of merit (d.sub.h g.sub.h) is obtained as a result of converting incoming acoustic axial stress into radial extensional stress thereby multiplying its effect. The piezoelectric active element is encased in a metal sandwich enclosing two semilunar air spaces which allow the device to withstand extremely high hydrostatic pressure yet still respond to low level sound waves when acting as a hydrophone. The mechanical prestress induced by the differential coefficients of expansion between the metal case and the piezoelectric ceramic element also serves to prevent depolarization aging.

Abstract: A piezoelectric or ferroelectric stress sensing material in intimate electrical communication with an electroluminescent material produces light at an amplitude dependent on the stress applied to the stress sensing material. The light signal is transmitted from the electroluminescent material by fiber optic cable to an optical signal detector. In the preferred embodiments the electroluminescent material comprises a light emitting diode as a small electrical short circuit load across two otherwise insulated faces of a piezoelectric or ferroelectric element. The embodiments include a composite stress sensing and electroluminescent material, separate stress sensing and electroluminescent materials joined by conductive film, foil or wire and additional means to bias, amplify and control the optical signal produced by the electroluminescent material and transmitted by the fiber optic cable.

Abstract: A deep submergence, acoustically stable directional transducer uses a wavide and an encapsulant to channel sound. The waveguide forms an air space with a housing so that radiated sound is totally reflected at the air space interface. The encapsulant is matched to the acoustic impedance of the sea water at the intended operating depth to minimize reflection at the encapsulant-to-sea water interface.

Abstract: A hydrophone device is provided with a piezoelectric ceramic-polymer composite of 0-3 connectivity in which the piezoelectric filler comprises a solid solution of PBTiO.sub.3 -BiFeO.sub.3 wherein the solid solution contains 50-80 wt. % of the BiFeO.sub.3. The piezoelectric ceramic-polymer composite has large hydrostatic piezoelectric charge and voltage coefficients which are desirable in hydrophone devices.

Abstract: An electroacoustic transducer having a truncated hemispheric propagation or esponse pattern is disclosed. The transducer comprises an assembly of fish-bone shaped piezoelectric elements, each characterized by a plurality of rod portions extending from a spine portion. The assembly is pivotally mounted within a liquid filled acoustic window means.

Abstract: Electrical signal-receiving transducers for logging tools include a silicon plate having piezoresistors formed thereon and response to deflection caused by pressure waves acting on said plate to generate an electrical signal through a bridge circuit including balancing resistors. Another embodiment of the transducer comprises a curved plate which is supported in a tool cavity for response to a pressure wave to deflect and exert forces on a piezoelectric element to generate a signal proportional to the deflection and a pressure wave causing such deflection. Monopole and quadrupole arrangements of the curved plate may be used for sensing or generating symmetric and asymmetric wave forms.

Abstract: A piezoelectric signaling device has an oscillator circuit which includes a transistor amplifier and a piezoelectric transducer. There is a Zener diode in the transistor circuit. The Zener voltage of the diode determines the start-up voltage of the device. By choosing diodes with different Zener voltages, the start-up voltage may be preselected. In other embodiments, the diode may be connected in the base circuit of the transistor or in other portions of the oscillator circuit.

Abstract: A hydrophone mounted on the hull of a ship comprises an electromechanical ansducer made of void-containing ceramic material having a high piezoelectric sensitivity to water-borne acoustic sound signals, a transducer made of solid ceramic material having a low piezoelectric sensitivity to hydrostatic acoustic signals, both transducers having similar piezoelectric sensitivity to shipboard noise transmitted via the ship's hull and transducer mounting; and means for sensing and manipulating voltage signals generated in response to water-borne acoustic signals and mount-transmitted shipboard noise such that the mount transmitted noise signals are largely cancelled out and a relatively noise-free signal representing hydrostatically transmitted sound is obtained.

Abstract: An accelerometer-based direction finding and ranging transducer such as a hydrophone uses piezoelectric ceramic as the active material. Orthogonally arranged accelerometers share an inertial mass and resonance control system and the accelerometers together with a pressure hydrophone and electronics are all housed within one pressure vessel.

Abstract: A hydrophone having a self-contained means for warning an operator that the hydrophone has reached or exceeded a safe design depth limit. The active elements of the hydrophone are bender-type piezoelectric wafers. An internal stop is provided such that in the presence of a hydrostatic pressure that exceeds a safe design limit, the wafers bottom out against the stop. The stop short-circuits the electrical output signals of the wafers, warning the operator that the hydrophone is in danger of destruction.

Abstract: An acoustic element assembly used in underwater receivers and projectors comprises two piezoelectric elements bonded to opposite sides of a hybrid circuit substrate (16). The hybrid circuit substrate (16) includes radial slots (32, 34, and 36) which have been cut therethrough. The radial slots (32, 34, and 36) permit a center section (31) to move bidirectionally. Integral micro-electronic circuitry which makes up the receiver or projector is fabricated in the center section (31) on both sides of an alumina ceramic disk (30) using thick and thin film technologies. Piezoelectric cylinders (12 and 14) or piezoelectric hemispheres (22 and 24) are bonded to opposite sides of disk (30) with both conductive and non-conductive bonding materials. The microelectronic circuitry is electrically connected to said cylinders or hemispheres (12, 14, 22, and 24) by conductive lines (44). The acoustic element assembly may be formed by automated techniques and does not require any internal wiring.

Abstract: A piezoelectric hydrophone is designed with overload protectors (1a, 11a, 11b) which protects the hydrophone against internal and external overpressures. The hydrophone assembly proper and a first overload protector (1a) are made in one piece, while other overload protectors (11a, 11b) are mounted outside the hydrophone assembly (1) above the pressure sensitive diaphragms (4a, 4b).The separate metallic parts of the hydrophone are joined by means of laser welding which may be performed in a pressure chamber. Likewise the final sealing of the hydrophone assembly may take place in a pressure chamber under a pressure corresponding to the water depth at which the hydrophone is operated.

Abstract: Underwater piezoelectric arrangement in which it comprises two piezoelectric elements made of piezoelectric ceramic-polymer composites and juxtaposed to form a single layer structure, one of the element(s) having larger hydrophone constant than that of the other element(s), and by noise cancelling of the piezoelectric assembly there can be selectively detected only charges or voltages generated by acoustic waves which acts on each element.

Abstract: A method of testing piezo-electric transducers includes locating one end (11) of a rod (10) of solid material over the active head (20) of the transducer, the rod (10) simulating the normal acoustic impedance presented to the transducer by the normal medium, in which the transducer is intended to operate, energizing the transducer and detecting the acoustic wave propagated through the rod at the other end of the rod. A loading rod for use in testing a piezo-electric transducer is an elongated solid body so dimensioned as to simulate the normal operating acoustic impedance presented to a piezo-electric transducer to be tested.

Abstract: In a detection device of the presence of frost and/or measuring the thickness of the frost by ultrasound, a probe with a piezoelectric ultrasonic transducer generates an ultra-sonic acoustic emission wave and detects ultrasonic acoustic echo waves reflected by the frost. An actuating device for the probe is arranged to provide to the transducer electrical energizing signals. The probe comprises: a frost detector placed in front of the transducer and constituted of a material having an acoustic impedance between 30.times.10.sup.5 and 44.times.10.sup.5 kg m.sup.-2 s.sup.-1. This detector has a surface designed to be exposed to the surrounding atmosphere to be covered with frost. An acoustic delay device is adapted to ensure acoustic coupling and adaptation between the transducer and the detector and to separate in time the ultrasonic emission add reflected waves. A body of heat insulating material covers the assembly of the detector, the transducer and the acoustic delay device.

Abstract: An apparatus and method for the measuring the transverse dimensions of a hole such as in an oil well including a sonde on which an electro-acoustic transducer is mounted in an off centered position in relation to the axis of the sonde. This transducer has two opposite active faces so as to simultaneously transmit acoustic pulses in diametrically opposite directions from the sonde. When the sonde is centered in the hole, the transducer picks up the echoes retransmitted by the wall of the hole in the two directions, at different times. A recording is made of these times and preferably also of an indication of the amplitude of each of these echoes to obtain both a diameter measurement and an indication of the condition of the hole surface. Several transducers are provided, superposed and oriented along various diameters distributed around the axis of the sonde.

Abstract: For increasing the bandwidth of an electroacoustic omnidirectional transducer a piezoceramic tube is provided at its outer circumference with segment-shaped additional mass members having a larger outer surface area than the piezoceramic tube.

Abstract: A piezoelectric polymer (PVDF) hydrophone array consists of multiple layers disposed symmetrically about a stiffener layer. The stiffener layer prevents flexural modes in the operating frequency band and provides a mounting structure for acceleration noise cancellation. The piezoelectric polymer layers are attached to the stiffener layer either directly or through intervening layers which provide mechanical vibration isolation of the polymer and stiffener layers.

Abstract: An underwater acoustical transducer includes an annular, spring steel band supporting a diaphragm of a woven fabric held taut by the band. A flexible piezoelectric transducing element is attached with respect to the band, typically in contiguous relation to a portion of the fabric overlying the radially outward surface of the band. The transducing element can include an electrically conductive core surrounded by flexible piezoelectric material, or alternatively, a substantially flat strip of flexible piezoelectric material.

Abstract: An acoustically transparent voided, polyvinylidene fluoride (PVDF) hydrope made of material whose impedance matches the characteristic acoustic impedance (.rho.c) of sea water, having drastically reduced diffraction and resonance effects. The frequency response is thus flat at frequencies less than one-half elastic wavelength in the PVDF material. An array of such hydrophones in front of a projector saves space without affecting projector performance.

Abstract: Cubelets are made from a piezoelectrically active ceramic material having holes therein which may be cross-cored. The holes are selectively filled with a liquid curable resin, such as an epoxy. The cubelets may also be formed from bonded or sintered "half-shapes" containing half cores, wherein a face bisects one hole or an array of holes or cross-cored holes to form one or an array of semicircular grooves or crossed semicircular grooves. The cubelets, whether filled with resin or having unfilled internal volume, are arranged in a precision array within the snug-fitting cavities of a lattice, whereby the holes are sealed off, to form a cubelet/lattice assembly which is then electroded and poled in an electric field and encapsulated in polymer to form a part of a hydrophone assembly, for example.

Abstract: The ultrasound sensor (2) of the invention comprises a polymer foil (4) which is supported in its outer region and is piezoelectrically activated at least in one portion (42). The portion (42) is coupled electrically to a first electrode (200) in the form of an adjacent, i.e., touching pin. A second electrode (8), in the form of a grid (214) connected to ground and/or a ring (216) connected to ground, is physically separated from the activated portion (42). The pin (200) is connected to the first input of an amplifier (210). The second input thereof is connected to ground. The metallic take-off at the activated zone (42) results in high sensitivity of the ultra-sound sensor (2) which is provided particularly for the measurement of shock waves with a high pressure amplitude and which finds application in lithotripsy.

Abstract: Permeation of sea water through the protective rubber housing to the conductive portions of the exterior surface of the ceramic or other pressure-sensitive element results in the development of a low insulation resistance which leads to failure of the unit. The operational life of the unit is extended through the application of an acoustically transparent water impervious layer, interposed and bonded between the exterior surface of the ceramic or other pressure-sensitive element and the interior surface of the rubber housing.

Abstract: An electro-acoustic transducer has a transducer plate having a piezo-electric layer attached thereto. The electro-acoustic transucer is clamped between bearing members at an annular edge region which is free of the piezo-electric layer. The piezo-electric layer is provided with electrodes which extend into the annular free region of the transducer plate. At least one of the bearing members has electrically conductive zones which press directly or indirectly against the electrodes and which press against electrical plug elements on a housing for the transducer.

Abstract: The insulation resistance between the electroded surfaces of an underwater transducer and the water within which the transducer is immersed is greatly improved by coating the electroded surface of the transducer with a layer of air-free, high-resistance waterproof material before bonding an elastomer covering over the assembly to serve as the outer housing.

Abstract: A miniature hydrophone is disclosed which contains a piezoelectric foil as a base element (2) with a linear arrangement of multiplicity of detector elements (10). In accordance with the invention, the electrodes (5) are arranged on a top surface (15) of one of the flat sides of a support (16) and are capacitively coupled by a bonding layer (6) to the base element (2). The detector elements (10) are associated with an amplifier (32) which, with equal length connecting leads (11, 26 and/or 12, 27), is fastened to a side face of the support (16). The electrical interconnection between the detector element (10) and the associated amplifier (32) employs a groove (24) through the corners of support 16. Additional such modules can form an assembly unit in a row arrangement of a size equal to that of the detector elements (10).

Abstract: A directional flextensional transducer including a transducer shell capable of operation in odd and even drive modes and a transduction drive bar or the like. The transduction drive bar is excited in an even mode to impart extensional motion thereto and is simultaneously excited in an odd mode to impart inextensional motion thereto. The combined excitation causes the flextensional transducer shell to move unidirectionally.

Abstract: A sensor and method for performing immunoassays using surface acoustic waves on a piezoelectric crystal is provided. A specific antigen or antibody is bonded to the surface of the piezoelectric crystal and its resonant frequency in a blank solution is determined. The sensor is then exposed to a test solution containing an antigen or antibody corresponding to the substance bonded to the surface of the crystal. As the antigen and antibody react, the resonant frequency of the crystal is altered. This change can be correlated to determine the amount of antigen or antibody in the test solution. A nonreactive reference sensor can also be used to compensate for any nonselective adsorption.