Empirical Study of the Effect of Diffraction on Velocity of Propagation of High‐Frequency Ultrasonic Waves

1960 ◽  
Vol 32 (11) ◽  
pp. 1401-1404 ◽  
Author(s):  
H. J. McSkimin
Keyword(s):  
Empirical Study ◽  
High Frequency ◽  
Ultrasonic Waves ◽  
Velocity Of Propagation

scholarly journals High Frequency Ultrasonic Wave Propagation in  Anisotropic Materials

2021 ◽  
Author(s):  
◽  
Andrew Paul Dawson
Keyword(s):  
Wave Propagation ◽  
Ultrasonic Wave ◽  
High Frequency ◽  
Guided Wave ◽  
Ultrasonic Waves ◽  
Tissue Characterisation ◽  
Ultrasonic Wave Propagation ◽  
Matching Layer ◽  
Fibrous Tissues ◽  
Wave Modes

<p>The influence of highly regular, anisotropic, microstructured materials on high frequency ultrasonic wave propagation was investigated in this work. Microstructure, often only treated as a source of scattering, significantly influences high frequency ultrasonic waves, resulting in unexpected guided wave modes. Tissues, such as skin or muscle, are treated as homogeneous by current medical ultrasound systems, but actually consist of highly anisotropic micron-sized fibres. As these systems increase towards 100 MHz, these fibres will significantly influence propagating waves leading to guided wave modes. The effect of these modes on image quality must be considered. However, before studies can be undertaken on fibrous tissues, wave propagation in more ideal structures must be first understood. After the construction of a suitable high frequency ultrasound experimental system, finite element modelling and experimental characterisation of high frequency (20-200 MHz) ultrasonic waves in ideal, collinear, nanostructured alumina was carried out. These results revealed interesting waveguiding phenomena, and also identified the potential and significant advantages of using a microstructured material as an alternative acoustic matching layer in ultrasonic transducer design. Tailorable acoustic impedances were achieved from 4-17 MRayl, covering the impedance range of 7-12 MRayl most commonly required by transducer matching layers. Attenuation coefficients as low as 3.5 dBmm-1 were measured at 100 MHz, which is excellent when compared with 500 dBmm-1 that was measured for a state of the art loaded epoxy matching layer at the same frequency. Reception of ultrasound without the restriction of critical angles was also achieved, and no dispersion was observed in these structures (unlike current matching layers) until at least 200 MHz. In addition, to make a significant step forward towards high frequency tissue characterisation, novel microstructured poly(vinyl alcohol) tissue-mimicking phantoms were also developed. These phantoms possessed acoustic and microstructural properties representative of fibrous tissues, much more realistic than currently used homogeneous phantoms. The attenuation coefficient measured along the direction of PVA alignment in an example phantom was 8 dBmm-1 at 30 MHz, in excellent agreement with healthy human myocardium. This method will allow the fabrication of more realistic and repeatable phantoms for future high frequency tissue characterisation studies.</p>

The Velocity of Propagation of Ultrasonic Waves and the Form of the Molecules of High Polymers

1950 ◽  
Vol 23 (1) ◽  
pp. 151-162
Author(s):  
Giulio Natta ◽  
Mario Baccaredda
Keyword(s):  
Molecular Weight ◽  
Form Factor ◽  
Experimental Error ◽  
Ultrasonic Waves ◽  
Low Molecular Weight ◽  
Side Chains ◽  
Polyethylene Oxides ◽  
Velocity Of Propagation ◽  
Secondary Side ◽  
Linear Macromolecules

Abstract The velocity of propagation of ultrasonic waves in numerous substances of high molecular weight was determined. For substances not fusible at temperatures below 100° C, this velocity was determined by extrapolation from solutions considered ideal. For linear macromolecules without side chains, the ultrasonic velocity appears to be practically equal, within the limits of experimental error, to that calculated by the formula of Rao and on a basis of the additive values of the bond velocity of Lagemann and Corry. For molecules which have many side chains, the velocity is lower than the calculated value, whereas for compounds of low molecular weight this deviation is relatively small, viz., less than 10 per cent; it becomes much higher, viz., almost up to 40 per cent, for macromolecules. The form factor is defined as the ratio of the velocity determined experimentally to the velocity calculated by the formula of Rao. This form factor is equal to 1 for polymers without side chains or with very few side chains, such as paraffins, polyethylenes, Nylon, polyethylene oxides, and polyoxymethylenes; is only 0.89–0.90 for natural rubber; only 0.82–0.84 for Buna and for hydrogenated Buna, poly-α-butylenes, and polystyrenes; only 0.79–0.80 for polyisobutylenes; only 0.89 for polymethacrylates; only 0.78 for polyvinylisobutyl ethers; only 0.65 for Butyl rubber; and only 0.63 for polymethyl methacrylates. The form factor is thus affected by the frequency and length of the side chains, and by any secondary side chains which may be present.

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