Abstract: The apparatus consists of a circular, flexible disc held at a constant distance from a curved absorbing surface by pins or flexible ribs. Sound from a loudspeaker is fed to the center of the disc and allowed to propagate radially in the space between disc and absorber. Radial arrays of microphones on the disc surface sense sound pressure amplitude and phase, from which impedance is calculated.

Abstract: A non-destructive evaluation technique particularly suited for in situ testing utilizes significant changes in internal friction damping (IFD) as an indicating factor. A baseline for the specific damping capacity of an object is determined and a tolerance range is established from the baseline. Measurements of the specific damping capacity are periodically made and checked to see if they are within the tolerance range. Unexpected changes in specific damping capacity indicate the presence of an incipient flaw in the object. The evaluation technique can also be used to locate flaws, monitor crack growth and predict useful life. Apparatus for measuring specific damping capacity is also disclosed.

Abstract: An apparatus for measuring mechanical characteristics of a material capable of transmitting vibration, particularly skin of a living subject. An impulse from a generator such as a small hammer causes a disturbance to propagate a wave along the material. A receiver includes a source of radiation and two photocells spaced apart and isolated from each other. The photocells receive the radiation after reflection from spaced locations on the surface of the material. The travel time of the propagated wave between the two reflection points is measured to determine the speed of wave propagation in the material.

Abstract: An apparatus for non-destructively testing concrete comprises a tri-wheel platform having a transducer mounted adjacent each wheel and mechanically coupled to the concrete to be responsive to vibration. An aperture is formed in the center of the platform equispaced from the three transducers. Vibrations are imparted to the concrete that emanate outwardly toward the transducers by manually striking the concrete through the center aperture with a rubber tipped impact rod. The transducers generate electrical signals responsive to the amplitudes of detected vibrations which are a function of the structural continuity of the concrete. Electronic circuitry monitors the outputs of the transducers and makes comparisons of the signals to determine whether a flaw exists in the concrete within the region bounded by the three transducers.

Abstract: In an ultrasonic wave generator or receiver using an ultrasonic wave transducer, a negative immittance converter is inserted in a circuit of the generator or receiver in order to cancel components which impede the damping characteristics of the ultrasonic transducer.