Abstract: The present invention provides a low frequency acoustic transducer for non-destructive testing of a test structure. The transducer is arranged for conversion between electrical energy and acoustic energy associated with an acoustic wave propagating through a portion of the test structure. The transducer comprises a bending actuator for generating the acoustic wave by generating a vibration from an electrical signal or for generating an electrical signal from a vibration generated by the received acoustic wave. The bending actuator has a vibration surface and a contact area surrounded by a portion of the vibration surface. The transducer further comprises a mode setting member that has a rigid portion which is in direct mechanical contact with the bending actuator at the contact area such that, within the contact area, an amplitude of the vibration is substantially suppressed. The location and shape of the contact area determine a bending mode associated with a resonance frequency of the bending actuator.

Abstract: The present invention provides a low frequency acoustic transducer for non-destructive testing of a test structure. The transducer is arranged for conversion between electrical energy and acoustic energy associated with an acoustic wave propagating through a portion of the test structure. The transducer comprises a bending actuator for generating the acoustic wave by generating a vibration from an electrical signal or for generating an electrical signal from a vibration generated by the received acoustic wave. The bending actuator has a vibration surface and a contact area surrounded by a portion of the vibration surface. The transducer further comprises a mode setting member that has a rigid portion which is in direct mechanical contact with the bending actuator at the contact area such that, within the contact area, an amplitude of the vibration is substantially suppressed. The location and shape of the contact area determine a bending mode associated with a resonance frequency of the bending actuator.

Abstract: A liquid flow meter for directly measuring the velocity of a liquid is disclosed. The liquid flow meter includes a pair transducers arranged facing each other in a conduit through which the liquid flows. The liquid flow meter also includes a transmitter means for causing the transducers to simultaneously transmit an acoustic wave packet directed for reception at the other transducer. A differential receiver means is also included whereby the differential receiver means has inputs each coupled to a corresponding one of the transducers for detecting an acoustic signal received thereby and determining a difference between the two received signals. The difference being related to the velocity of the liquid within the conduit. The transmitter means and the differential receiver means are each matched to the transducers to ensure substantial reciprocity to thereby substantially avoid phase and/or amplitude variations in the received signal. A method for measuring the velocity of a liquid is also disclosed.