Determination of object resonances by vibro-acoustography and their associated modes

• Published in: Ultrasonics Vol. 42; no. 1; pp. 537 - 543
• Main Authors: Mitri, F.G, Fellah, Z.E.A, Closset, E, Trompette, P, Chapelon, J.Y
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 01.04.2004; Elsevier Science; Elsevier
• Subjects: Acoustics; Engineering Sciences; Exact sciences and technology; Finite-element simulations; Fundamental areas of phenomenology (including applications); Physics; Radiation force; Resonance frequency; Ultrasonics, quantum acoustics, and physical effects of sound; Vibro-acoustography; Vibro-acoustography; Radiation force; Resonance frequency; Finite-element simulations; Water; Acoustic emission testing; Acoustic measurement; Frequency sweeping; Numerical method; Experimental study; Low frequency; Finite element method; Sphere; Cylinder; Difference frequency; Medical imagery; Vibrational mode; Non destructive test; Elastography; Annular antenna; Transducer network; Storage tank; Hydrophone; Non contact measurement
• ISSN: 0041-624X; 1874-9968
• DOI: 10.1016/j.ultras.2004.01.050

Vibro-acoustography technique known by its noncontact excitation was used to detect resonance frequencies of objects in water. Two intersecting ultrasound beams generated by a 40 mm-diameter annular array transducer, focused at 35 mm and driven at f 1=2.2 MHz and f 2=2.22 MHz respectively, were targeted inside the object under test to produce a radiation force beating at the difference frequency f 2− f 1. This low frequency radiation force was used to excite the resonance vibration modes of the object by sweeping the frequency f 2 between 2.22 and 2.275 MHz. The amplitude of the acoustic emission produced by the vibrations of the object was detected by a low frequency hydrophone (BW=60 kHz). By this approach, it was possible to detect resonance frequencies through amplitude variations of the measured acoustic emission. Experiments were conducted in a water tank for objects of different shapes and sizes. With a chalk sphere (15 mm-diameter) two resonance frequencies were detected at 45.75 and 68.75 kHz, and with a cylinder (10.38 mm-diameter and 32.20 mm-length) four principal resonance frequencies were identified in the 60 kHz-bandwidth of the hydrophone. It was shown with finite element calculations performed with Ansys, in which both solid and fluid parts were modelled, that the measured resonance frequencies corresponded to compressional or dilatation vibration modes of the object. It was verified that shear waves generated by torsional vibration modes were not propagated in water, as it is well known. The use of this technique to characterize heterogeneities in different media seems to be relatively more advantageous to other ultrasonic methods.