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Nonlinearly coded signals for harmonic imaging

100%

Nowicki A. , Wójcik J. , Kujawska T.

tom Vol. 34, No. 1

63-74

In this paper a new method utilizing nonlinear properties of tissues to improve contrast-to-noise ratio is presented. In our novel method the focused circular transducer is excited with two-tone bursts (including the 2.2MHz fundamental and 4.4MHz second harmonic frequencies) with specially coded polarization of each tone. This new approach was named Multitone Nonlinear Coding (MNC) because the choice of both tones polarization and amplitude law, allowing optimization of the probe receiving properties, depends on nonlinear properties of tissue. The numerical simulations of nonlinear fields in water and in tissue-like medium with absorption coefficient of 7Np/(m MHz) are performed. The comparison between the proposed method and the Pulse Inverse (PI) method is presented. The concept of the virtual fields was introduced to explain properties of both the Pulse Inversion and MNC methods and to compare their abilities. It was shown that for the same on-source pressure an application of the MNC method allows to decrease the mechanical index about 40%, to improve lateral resolution from 10 to 30% and to gain the signal-to-noise ratio up to 8 times with respect to the PI method.

Trombone Transfer Functions: Comparison Between Frequency-Swept Sine Wave and Human Performer Input

88%

Beauchamp J. W.

Archives of Acoustics

2012

tom Vol. 37, No. 4

447-454

Source/filter models have frequently been used to model sound production of the vocal apparatus and musical instruments. Beginning in 1968, in an effort to measure the transfer function (i.e., transmission response or filter characteristic) of a trombone while being played by expert musicians, sound pressure signals from the mouthpiece and the trombone bell output were recorded in an anechoic room and then subjected to harmonic spectrum analysis. Output/input ratios of the signals’ harmonic amplitudes plotted vs. harmonic frequency then became points on the trombone’s transfer function. The first such recordings were made on analog 1/4 inch stereo magnetic tape. In 2000 digital recordings of trombone mouthpiece and anechoic output signals were made that provide a more accurate measurement of the trombone filter characteristic. Results show that the filter is a high-pass type with a cutoff frequency around 1000 Hz. Whereas the characteristic below cutoff is quite stable, above cutoff it is extremely variable, depending on level. In addition, measurements made using a swept-sine-wave system in 1972 verified the high-pass behavior, but they also showed a series of resonances whose minima correspond to the harmonic frequencies which occur under performance conditions. For frequencies below cutoff the two types of measurements corresponded well, but above cutoff there was a considerable difference. The general effect is that output harmonics above cutoff are greater than would be expected from linear filter theory, and this effect becomes stronger as input pressure increases. In the 1990s and early 2000s this nonlinear effect was verified by theory and measurements which showed that nonlinear propagation takes place in the trombone, causing a wave steepening effect at high amplitudes, thus increasing the relative strengths of the upper harmonics.

Symulation numerique de la propagation non -lineaire d’une onde longue dans un canal de profondeur constante. Comparaison de quelques methodes aux differences finies

63%

Le Provost C. , Saenz M.

tom 09