A Multi-Element Low-Frequency Ultrasonic Transducer as a Source of High-Intensity Focused Ultrasound in Air

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Abstract

The acoustic and electrical properties of a 128-element ultrasonic transducer designed to generate high-intensity focused ultrasound in air in the low-frequency ultrasonic range are investigated. To reduce parasitic grating maxima of the acoustic field, a spiral arrangement of piezoelectric elements on a spherical base was used. The operating frequency of the transducer was 35.5 kHz, and the diameter of the source and focal length were approximately 50 cm, significantly exceeding the wavelength (approximately 1 cm). This selection of parameters allowed for effective focusing, with localization of wave energy in a small focal region, thereby achieving extremely high levels of ultrasonic intensity. The parameters of the ultrasonic field were studied using a combined approach that included microphone recording of the acoustic pressure and measuring the acoustic radiation force acting on a conical reflector. Acoustic source parameters were determined from the two-dimensional spatial distribution of the acoustic pressure waveform, which was measured by scanning the microphone in a transverse plane in front of the source. Numerical modeling of nonlinear wave propagation was also used based on the Westervelt equation to simulate the behavior of intense waves. The acoustic pressure level reached 173 dB, with a focal spot size comparable to the wavelength.

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About the authors

S. A. Asfandiyarov

Lomonosov Moscow State University

Author for correspondence.
Email: asfandiiarov.sa14@physics.msu.ru
Russian Federation, Moscow

S. A. Tsysar

Lomonosov Moscow State University

Email: asfandiiarov.sa14@physics.msu.ru
Russian Federation, Moscow

O. A. Sapozhnikov

Lomonosov Moscow State University

Email: asfandiiarov.sa14@physics.msu.ru
Russian Federation, Moscow

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Supplementary files

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2. Fig. 1. Photograph of the fabricated 128-element focusing grating and associated equipment. 1 - grating, 2 - household power amplifier Atoll AM 200, 3 - signal generator Agilent 33120A

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3. Fig. 2. (a) - frequency dependences of the real (solid line) and imaginary (dashed line) parts of the impedance Z1 of one grid element, (b) - impedance ZΣ of 128 grid elements connected in parallel, (c) - impedance Z of the grid with the connected matching device; the inset shows the scheme of the device matching the voltage source U with the electric load ZΣ

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4. Fig. 3. (a) - scheme of measurements with a microphone moving along the hologram plane in front of the radiating antenna array at a distance of 240 mm from its centre. (b) - typical time profile of the electrical signal on the microphone at one of the points of the hologram at pulse excitation of the antenna array. (c) - distribution of the acoustic pressure amplitude along the hologram surface measured at the grating operating frequency of 35.5 kHz

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5. Fig. 4. Photograph of the experimental setup for measuring the electroacoustic efficiency of the ultrasonic grating based on the acoustic radiation force measurement. 1 - ultrasonic grating, 2 - conical reflector, 3 - precision scales, 4 - wattmeter

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6. Fig. 5. Distributions of the amplitude Aν and phase φν of the normal component of the vibrational velocity on the grating surface at the operating frequency of 35.5 kHz and at 36 kHz

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7. Fig. 6. Distribution of the intensity Iac of acoustic radiation, normalised to the total acoustic power Wac of the radiation, on the x-axis in the focal plane at frequencies 35.5 kHz (solid line) and 36 kHz (dashed line)

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8. Fig. 7. Frequency dependences of the acoustic power at a voltage of 1 V on the elements of the grating: solid line - calculation from the full hologram, dots - the result of measurement by the acoustic radiometer method, dashed line - calculation from a section of the hologram in the form of a circle with a diameter of 180 mm

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9. Fig. 8. Frequency dependence of the electroacoustic efficiency of the grating

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10. Fig. 9. Acoustic pressure profiles in the focus measured at different levels of excitation of the elements. Dashed line - profile in the linear regime at the amplitude of electric voltage 0.5 V on the elements (pressure scale - left). Solid lines - profiles in the nonlinear regime; thick line - at a voltage amplitude of 5.8 V, thin line at 7.7 V (pressure scale - right)

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11. Fig. 10. Acoustic wave profiles at the focus obtained by numerical modelling using the ‘HIFU beam’ complex at the voltage amplitude at the transmitter of 7 V (dashed line) and 10 V (solid line)

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