Chapter 23 Experimental Mapping of the Acoustic Field Generated by Ultrasonic Transducers Songmao Chen, Christopher Niezrecki, and Peter Avitabile Abstract In recent years, a laboratory noncontact excitation method based on the focused ultrasound radiation force, generated by an ultrasonic transducer has been exploited to excite vibrations within structures with size ranging from the micro to macro scale. The excitation frequency has a range from a few kHz to 1 MHz and can potentially be used for modal testing. However, the inability to monitor the real time acoustic radiation force prevents this approach from being used as a practical technique for measuring the frequency response functions in modal testing. This work deals with understanding the acoustic field generated from ultrasonic transducers in order to monitor and control the acoustic radiation pressure and force imparted to a structure. In this paper, the acoustic field generated by circular ultrasonic transducers was calculated based on the Rayleigh Integral and a boundary element method. The vibration velocity distribution of the vibration surface of an ultrasonic transducer at certain frequencies was measured, linearly interpolated, and mapped to the radially discretized elements of transducer surface, which was then used to map the acoustic pressure generated. A microphone array was built to measure the frequency response functions (FRFs) of acoustic pressure with respect to the drive voltage at several spatial locations within three vertical and one horizontal planes in front of the transducer. The comparison between the simulation results and experimental results is presented and shows good agreement. Keywords Ultrasound radiation force • Modal testing • Mapping • Boundary element method • Rayleigh integral 23.1 Introduction In modern modal testing practices, excitations imparted using an impact hammer or shakers [1] are commonly used. These conventional excitation techniques require that the excitation sources be physically in contact with the structures to be tested, resulting in mass loading, stiffness loading effects [2] and other physical issues that may distort the structure’s true dynamic characteristics. In some cases, this type of contact excitation may even be physically impossible due to space or bandwidth excitation limitations, like excitation of arteries [3] and micro structures [4]. Providing physical excitation of a structure in the ultrasonic range (>20 kHz) can be a challenge. In recent years, a laboratory noncontact excitation method based on the focused ultrasound radiation force has been exploited to excite vibrations within structures with size ranging from the micro to macro scale. The ultrasound radiation force is generated by the superposition of two modulated ultrasound signals applied to an ultrasound transducer that can be used to drive structures at the difference frequency [5]. The excitation frequency has a range from a few kHz to 1 MHz and can potentially be used for modal testing. Huber et al., used this ultrasound radiation force for non-contact modal excitation in air and measured the frequencies and corresponding operating deflection shapes [6] for a brass reed used in air [7]. The technique was also used for objects such as microcantilevers with resonance frequencies over 1 MHz [4], hard drive suspensions [8], and a classical guitar with resonance frequencies below 100 Hz [9]. Roozen et al., illustrated the application of acoustic radiation force to excite a sphere on rod and tried to quantify the force by using an inverse method from the induced structural deformation [10]. Some theoretical works have been published to simulate the dynamic acoustic radiation force acting on spheres [11] and cylindrical shells [12]. However, only a limited amount of work has been done to fully understand the acoustic radiation of an operating ultrasound transducer in air. The inability to monitor the real time acoustic radiation force prevents this approach from being used as a practical technique for measuring the frequency response functions in modal testing. S. Chen ( ) • C. Niezrecki • P. Avitabile Structural Dynamics and Acoustic Systems Laboratory, University of Massachusetts Lowell, One University Avenue, Lowell, MA 01854, USA e-mail: Songmao_Chen@student.uml.edu © The Society for Experimental Mechanics, Inc. 2016 J. De Clerck, D.S. Epp (eds.), Rotating Machinery, Hybrid Test Methods, Vibro-Acoustics & Laser Vibrometry, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-30084-9_23 243
RkJQdWJsaXNoZXIy MTMzNzEzMQ==