Rapid Prototyping of Pyramidal Structured Absorbers for Ultrasound

Abstract

Acoustic measurements or ultrasonic testing can be strongly affected by reflections or echoes from test tank walls. In order to create a non-reflecting environment equivalent to infinite medium, a pyramidal structured absorber (PSA) can be used to coat the walls of an ultrasonic tank. In this work, we model an array of tetragonal pyramid ultrasonic wave absorbers. This model is based on two coupled first-order equations describing the stress and particle velocity within an isotropic medium. For absorbing media, the Kelvin-Voigt model of viscoelasticity is used. The equations are discretized in 2D using an efficient time-stepping pseudo-spectral scheme that takes in consideration both, the acoustic properties and attenuation characteristics of the composite materials. We then built a 3D printed PSA using a Stratasys Objet500 Connex 3D printer, which allows to combine photopolymers in specific concentrations and microstructures. We designed PSA covering the frequency ranges from 0.5 MHz to 5 MHz and from 1 MHz to 10 MHz, with double homogeneous layer: a core of rubber material with a skin of a variety of elastomers by combining rigid and flexible materials. Each single pyramid contains two major parts: the ground of the pyramid (9.4 mm base × 4.7 mm height, for 0.5 MHz and 4.7 mm base × 2.35 mm height, for 1 MHz) and the body of the pyramid (23.5 mm height, for 0.5 MHZ and 11.75 mm height, for 1 MHz). The measured echo-reduction was greater than 35 dB at the covering frequency range and the transmission loss was estimated by 20 dB. Echoes increase rapidly for frequencies below the minimum frequency of the covering range. The modeling and 3D printing of PSA with different sizes, in a wide range of frequencies, is a cost-effective custom solution for a wide range of applications including for example, radiation force balances, hydrophone mounts and medical ultrasound equipment.

A polydimethylsiloxane-based axicon lens for focused ultrasonic brain stimulation techniques

“…The objective was to validate our model using experimental measurements of the US field produced by the tFUS-axicon transducer. The experiments were performed in a 230 Â 230 Â 216 mm test tank, with anechoic coating performed using 3D-printed pyramidal structured absorbers (PSA) [20]. A needle hydrophone (Force Technology MH28) was used to measure the beam pattern of the axicon lens.…”

Measurement of the ultrasound attenuation and dispersion in 3D-printed photopolymer materials from 1 to 3.5 MHz

“…This allows both for their behaviour to be accurately predicted and for their efficiency to be maximised for a particular application. 10 However, while a number of works have reported these parameters, 7,[11][12][13][14][15][16][17][18][19] these measurements have typically been made at a single frequency, reported only sound speed, and or been carried out on a single or limited range of materials. A more recent work carried out detailed measurements on the properties of 3D-printed thermoplastics.…”

“…Beyond this specific application to wavefront shaping, the photopolymers characterised in this work have also been used for a range of other biomedical applications, including the generation of complex 3-D bone mimicking imaging phantoms, 21,23 teaching phantoms, 24 and acoustic absorbers. 19…”

Demonstrating acoustic camouflage with ultrasonic sensors in the laboratory

“…At the same time, we were surprised that the accordion-folded paper, which most closely resembles the wedge shapes common in anechoic chambers, performed so poorly in terms of camouflage. Anechoic wedge depths are often quite deep compared to the wavelength being absorbed [8], so it is possible that a deeper V-shape or a different geometry would be more successful. We might characterize the furry cloth as unsuccessful in the sense that, if someone were monitoring the sensor and knew that the backdrop was 151 cm away, a reading of >151 cm would indicate a large object was present and blocking the sensor.…”