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PubMed2001

Second-harmonic generation in sound beams reflected from, and transmitted through, immersed elastic solids.

Second-harmonic generation in sound beams reflected from and transmitted through thick isotropic elastic solids is investigated experimentally. Measurements of diffraction patterns are compared with a theoretical model based on integral solutions for harmonic generation in sound beams. The solutions are connected by classical linear theory for reflection and transmission at fluid-solid interfaces. Nonspecular phenomena associated with rapid phase variations near critical angles are accurately described. The principal restriction is that the solid is sufficiently thick that internal reflections may be ignored.

Journal Article↗

PubMed2001

Wave reflections from duct terminations.

The reflection coefficients and inertial end corrections of several duct terminations, including finite length duct extensions perpendicular to an infinite wall, as well as at a number of angles, curved interface surfaces, and annular cavities, are determined and analyzed in the absence of flow by employing the boundary element method. Predictions for the classical unflanged and flanged circular ducts show good agreement with analytical and computational results available in the literature. The predictions for curved interface surfaces (bellmouth or horn) are also consistent with the available experimental data. In view of its high reflection coefficient, the duct termination with an annular cavity may be suggested for the suppression of noise radiation in a specific frequency band or for an effective wave reflection from the termination.

Journal Article↗

PubMed2002

Low-frequency reflection characteristics of the s0 lamb wave from a rectangular notch in a plate.

An analysis of the reflection of the fundamental Lamb mode s0 from surface-breaking rectangular notches in isotropic plates is presented. The results are obtained from finite element time domain simulations together with experimental measurements. Very good agreement is found between the simulations and the measurements. Results are shown for a range of notch widths and depths, including the special case of a crack, defined as a zero-width notch. An interference phenomenon is identified which explains the periodic nature of the reflection coefficient when plotted as a function of notch width. Finally, an analysis using the S-parameter approach and both low and high frequency asymptotic analyses yields physical explanations of the nature of the reflection behavior from the cracks. It is found that the low frequency (quasistatic) approximation may be used accurately for cracks up to about a quarter of the plate thickness, provided that the quasistatic crack-opening function is chosen such that bending of the plate is omitted. At higher frequencies and depths the functions tend towards the high frequency (ray theory) predictions but these are never accurate models within the nondispersive frequency range of the s0 mode.

Humans↗

PubMed2002

A functional equation for the specular reflection of rays.

This paper aims to generalize the "radiosity method" when applied to specular reflection. Within the field of thermics, the radiosity method is also called the "standard procedure." The integral equation for incident energy, which is usually derived for diffuse reflection, is replaced by a more appropriate functional equation. The latter is used to solve some specific problems and it is shown that all the classical features of specular reflection, for example, the existence of image sources, are embodied within this equation. This equation can be solved with the ray-tracing technique, despite the implemented mathematics being quite different. Several interesting features of the energy field are presented.

Journal Article↗

PubMed2003

Complex reflection phase gradient as an inversion parameter for the prediction of shallow water propagation and the characterization of sea-bottoms.

In this paper a quantity is proposed, referred to as the complex reflection phase gradient, whose use in a matched field inversion procedure allows for the rapid extraction of first order geo-acoustic information about the sea-bottom. It is based on the observation that at low grazing angles the reflection phase and bottom loss for a wide range of sea-bottom types commonly exhibits an approximate linear relationship to the vertical component of the acoustic wave number at the seabed. The real part of this quantity specifies the rate at which the reflection phase varies with vertical acoustic wave number while the imaginary part quantifies the rate of change of bottom loss. Despite being defined with just two real parameters it is shown that it provides an accurate prediction of the sound field for a wide range of bottom types. In addition, its measurement permits an estimate to be made for the input impedance to the seabed in the zero grazing angle limit and, in the case of a homogeneous elastic half-space of known density, the compressional and shear wave speed. The main advantage of the two-parameter seabottom representation is that each parameter is readily inverted from comparatively few acoustic pressure measurements. The usefulness of the technique is illustrated by the results from computer simulated acoustic pressure measurements made at just eleven sensors in a simple shallow water channel, and results from a 10 cm deep laboratory channel at frequencies between 10 kHz and 75 kHz.

Journal Article↗

PubMed2003

On the significance of reflection coefficients produced by active surfaces bounding one-dimensional sound fields.

Active boundary surfaces intended to control reverberation or other characteristics of enclosed sound fields have often been investigated using plane wave tubes. This paper presents an analysis of actively terminated semi-infinite and finite-length plane wave tubes to provide needed clarification of the effects of these surfaces. By considering relationships between complex pressure-amplitude reflection coefficients and acoustic energy quantities, the investigation reveals that increases in reflection coefficient moduli at terminations do not always produce corresponding increases in total energy or energy flux in adjacent fields. These relationships are shown to depend upon physical properties of the acoustic spaces, sources, and source positions. The investigation also demonstrates how the impact of reflection coefficients with moduli exceeding unity may be easily misinterpreted.

Journal Article↗

PubMed2003

Seabed reflection measurement uncertainty.

The seabed reflection coefficient is a fundamental property of the ocean waveguide. Measurements of the frequency and angular dependence of the reflection coefficient can provide information about the geoacoustic properties of the seabed or can be used as an input to propagation models. The uncertainty of the measurements must be known in order to determine prediction uncertainties for the acoustic field and/or the geoacoustic properties. Analysis indicates that the reflection measurements have a standard deviation from +/- 0.5-1 dB at full angular resolution depending on frequency and experiment geometry. The dominant contribution to the error is source amplitude variability, and a new processing approach was developed that reduces the error for frequencies above a few hundred Hz. Further reduction in the uncertainty can be obtained by averaging in angle, for example, a +/- 1 degrees angle averaging leads to a standard deviation of less than +/- 0.5 dB. Errors in the angle estimate are a few tenths of a degree from 0-34 degrees grazing angle: the crucial angular range for predicting long-range propagation or for geoacoustic property inversion.

Journal Article↗

PubMed2004

Study of acoustic nonlinearity parameter imaging methods in reflection mode for biological tissues.

Three novel methods for acoustic nonlinearity parameter B/A imaging in reflection mode are developed in this paper. They are: (1) the acoustic nonlinearity parameter B/A tomography by detecting reflective second harmonic wave, (2) the B/A tomography in reflection mode via the measurement of the difference frequency wave generated by a parametric array, and (3) the C-scan imaging of B/A via the measurement of the echo second-harmonic signal. A theoretical analysis and the experimental imaging of normal and pathological biological tissues by using these methods are also present and discussed. Results show that using the acoustic nonlinearity parameter imaging we can more easily distinguish the diseased tissue from the normal one than using the linear acoustic parameters.

Adipose Tissue↗

PubMed2005

Application of acoustic reflection tomography to sonar imaging.

Computer-aided tomography is a technique for providing a two-dimensional cross-sectional view of a three-dimensional object through the digital processing of many one-dimensional views (or projections) taken at different look directions. In acoustic reflection tomography, insonifying the object and then recording the backscattered signal provides the projection information for a given look direction (or aspect angle). Processing the projection information for all possible aspect angles enables an image to be reconstructed that represents the two-dimensional spatial distribution of the object's acoustic reflectivity function when projected on the imaging plane. The shape of an idealized object, which is an elliptical cylinder, is reconstructed by applying standard backprojection, Radon transform inversion (using both convolution and filtered backprojections), and direct Fourier inversion to simulated projection data. The relative merits of the various reconstruction algorithms are assessed and the resulting shape estimates compared. For bandpass sonar data, however, the wave number components of the acoustic reflectivity function that are outside the passband are absent. This leads to the consideration of image reconstruction for bandpass data. Tomographic image reconstruction is applied to real data collected with an ultra-wideband sonar transducer to form high-resolution acoustic images of various underwater objects when the sonar and object are widely separated.

Acoustics↗

PubMed1985

Localization of sound in rooms, II: The effects of a single reflecting surface.

Auditory localization was studied in a room bounded by a single acoustically reflective surface. The position of that surface was varied so as to stimulate a floor, a ceiling, and left and right side walls. The surface was eliminated in one condition so that we could examine localization in free field for purposes of comparison. Using a source identification method we assessed the influences of these various room configurations on the localization of both slow-onset and impulsive sine tones of low frequency (500 Hz). We also measured the steady-state interaural-time-difference (ITD) and interaural-intensity-difference (IID) cues available to subjects in the different room configurations and compared these data with the perceptual judgments. Our results indicate the following: (1) A sound must include transients if the precedence effect is to operate as an aid to its localization in rooms. (2) Even if transients are present the precedence effect does not eliminate all influences of room reflections. (3) Due to the interference of reflections large interaural intensity differences may occur in a room and these have a considerable influence on localization; this is true even at low frequencies for which IID cues do not exist in a free field. (4) Listeners appear to have certain expectations about the reliability and plausibility of various directional cues and perceptually weight the cues accordingly; we suggest that this may explain, in part, the large variation in time-intensity trading ratios reported in the literature and also the differing reports regarding the importance of onsets for localization. (5) In this study we find that onset cues are of some importance to localization even in free field.

Acoustic Stimulation↗

PubMed1986

A computer model for simulating reflected ultrasound signals.

This letter describes a model for simulating an ultrasound signal reflected from a medium composed of randomly distributed scatterers, as typified by soft biological tissue, such as liver. The model is specified in terms of the effective transducer beam radius B and the mean scatterer spacing S. The novel feature of the model is that the transducer field is partitioned by packing cylinders, called microbeams, into concentric annular regions that lie parallel to the transducer axis. The radii of the microbeams and the annuli are related to S. An independent reflector sequence is generated for each microbeam, the microbeam sequences in each annulus are summed and convolved with the impulse response of a point reflector. The reflected waveform sequence is then generated by summing the annular contributions and convolving with a band-limited pulse waveform. Simulated signals were generated for different values of B and S and compared with actual signals reflected from two in vivo livers and a tissue-equivalent phantom. Estimates of the kurtosis for the simulated signals indicate the range of signals that can be generated by varying the values for S and B.

Computer Simulation↗

PubMed1990

Revision of estimates of acoustic energy reflectance at the human eardrum.

An improved analysis procedure has been applied to standing wave patterns measured previously [B. W. Lawton and M. R. Stinson, J. Acoust. Soc. Am. 79, 1003-1009 (1986)] in human ear canals. Revised acoustic energy reflection coefficients, at the eardrum, are obtained for 20 ears for frequencies between 3 and 13 kHz. The new analysis addresses anomalous features of the standing wave patterns, apparent at frequencies above 8 kHz, due primarily to the curvature of the ear canal. Much better agreement is now found, at these higher frequencies, between the theoretical form assumed for the standing wave patterns and the experimental data. The revised values of eardrum reflectance are somewhat smaller, especially for frequencies above 11 kHz. The reflectance rises from about 0.25 at 4 kHz up to 0.7 at 8 kHz, falls to a minimum of 0.5 at 11 kHz, then rises to 0.6 at 13 kHz. Considerable intersubject variability in the results is noted.

Acoustic Impedance Tests↗

PubMed1993

Ear-canal impedance and reflection coefficient in human infants and adults.

The ear-canal impedance and reflection coefficient were measured in an adult group and in groups of infants of age 1, 3, 6, 12, and 24 months over frequency range 125-10,700 Hz. The development of the external ear canal and middle ear strongly affect input impedance and reflection coefficient responses, and this development is not yet complete at age 24 months. Contributing factors include growth of the area and length of the ear canal, a resonance in the ear-canal walls of younger infants, and a probable influence of growth of the middle-ear cavities. The middle-ear compliance is lower in infants than adults, and the middle-ear resistance is higher. The power transfer into the middle ear of the infant is much less than into that of the adult. Such differences in power transfer directly influence both behavioral and physiological measurements of hearing. The difficulties of interpretation of neonatal tympanograms are shown to be a consequence of ear-canal wall vibration. Impedance and reflectance measurements in the 2-4-kHz range are recommended as a potentially useful clinical tool for circumventing these difficulties.

Acoustic Impedance Tests↗

PubMed2000

A boundary that sustains a negligible specular reflection coefficient over a wide frequency band

In a previous paper the authors analyzed and discussed the specular reflection coefficient of a plane boundary comprised of a plate, a compliant layer, and a fluid. The analysis showed that a negligible specular reflection coefficient may be derived provided specific resonance conditions are met. The resonance of concern is that between the surface mass of the plate and the surface stiffness of the compliant layer. The conditions of resonance included the value that must be assigned to the loss factor in the compliant layer. In the present paper, an attempt is made to determine the conditions that must be placed on the surface stiffness of the compliant layer in order to increase the frequency range over which a negligible specular reflection coefficient may be maintained. The tolerances in these conditions are also estimated.

Journal Article↗

PubMed2004

Changes in Earth's reflectance over the past two decades.

We correlate an overlapping period of earthshine measurements of Earth's reflectance (from 1999 through mid-2001) with satellite observations of global cloud properties to construct from the latter a proxy measure of Earth's global shortwave reflectance. This proxy shows a steady decrease in Earth's reflectance from 1984 to 2000, with a strong climatologically significant drop after 1995. From 2001 to 2003, only earthshine data are available, and they indicate a complete reversal of the decline. Understanding how the causes of these decadal changes are apportioned between natural variability, direct forcing, and feedbacks is fundamental to confidently assessing and predicting climate change.

Journal Article↗

PubMed1995

Comparison of two reflectance photometers in the assessment of neonatal hypoglycaemia.

To assess the accuracy and reliability of reflectance photometers in estimating blood glucose concentrations, two were assessed: the Ames Glucometer 3 (Bayer Diagnostics) with Glucofilm Test strips; and the Reflolux S (Boehringer Mannheim) with BM Test Glycemia 20-800 strips. These were compared with laboratory blood glucose estimations in 100 assays (50 comparisons for each machine, measuring the difference (d) between the glucose reading and the mean of the reflectance photometer and the laboratory value). The Ames Glucometer 3 (mean d = + 0.7 mmol/l, (SD 1.1) mmol/l) was less accurate than the Boehringer Reflolux S (mean d = 0.2 mmol/l, (SD (0.7) mmol/l). The range of error of both machines is wide (Ames 2 SD range + 2.9 mmol/l to -1.5 mmol/l true readings; Boehringer + 1.8 mmol/l to -1.2 mmol/l of true readings). Because of this, any reflectance photometer readings that are even slightly low should be checked with laboratory estimations. The clinical value of such machines is limited in infants with low blood glucose concentrations.

Blood Glucose↗

PubMed2006

Abnormal oral mucosal light reflectance: a new clinical marker of high risk for colorectal cancer.

BACKGROUND: A familial predisposition to colorectal cancer (CRC) has been clearly established, consisting of familial clustering in 15-20% and clear hereditary aetiology in 5-10% of overall CRC cases. Early identification of families and individuals at high risk is essential as intensive surveillance has been demonstrated to reduce cancer incidence and overall mortality. In the present study, the value of oral mucosal light reflectance in identifying hereditary non-polyposis colorectal cancer (HNPCC) carriers was investigated. METHODS: Twenty members of six different genetically unrelated HNPCC kindred and 30 genetically unrelated age and sex matched healthy controls were examined. Lower gingival and vestibular oral mucosal reflectance was measured using an imaging spectrophotometer. RESULTS: HNPCC carriers showed significantly lower values in the 590-700 nm wavelength range (p<or=0.0004). A reflectance cut off value <or=47.9% at the 700 nm wavelength discriminated between HNPCC carriers and controls, with 100% sensitivity and 100% specificity. CONCLUSIONS: These findings may provide an additional phenotypic sign in HNPCC carriers, which could be used in first level CRC population screening programmes.

Adaptor Proteins, Signal Transducing↗

PubMed2006

Prolonged mechanical systole and increased arterial wave reflections in diastolic dysfunction.

OBJECTIVE: To evaluate whether left ventricular ejection time indexed for heart rate (left ventricular ejection time index (LVETI)) and arterial wave reflections (augmented pressure (AP)) are increased in patients with diastolic dysfunction (DD). DESIGN: Prospective observational study. SETTING: University teaching hospital providing primary and tertiary care. SUBJECTS: 235 consecutive patients undergoing left heart catheterisation were categorised as having definite DD, possible DD or no DD (controls) on the basis of their left ventricular end diastolic pressures and N-terminal brain natriuretic peptide concentrations. MAIN OUTCOME MEASURES: LVETI and AP were prospectively assessed non-invasively by radial applanation tonometry. In addition, all patients underwent comprehensive echocardiography, including tissue Doppler imaging of mitral annulus velocity in early diastole (E'). RESULTS: LVETI was longer in patients with definite DD than in patients with possible DD and in controls (433.6 (SD 17.2), 425.9 (17.9) and 414.3 (13.6) ms, respectively, p < 0.000001). Arterial wave reflections were higher in definite DD than in possible DD and control groups (AP was 19.4 (SD 8.9), 15.2 (8.0) and 10.7 (6.8) mm Hg, respectively, p < 0.000001). In receiver operating characteristic curve analysis, LVETI detected DD as well as echocardiography (E:E'). Area under the curve for LVETI to differentiate patients with definite DD from normal controls was 0.81 (95% CI 0.72 to 0.89, p < 0.0001). In multivariable logistic regression analysis, LVETI added significant independent power to clinical and echocardiographic variables for prediction of DD. CONCLUSIONS: Mechanical systole is prolonged and arterial wave reflections are increased in most patients with DD. Rapid non-invasive assessment of these parameters may aid in confirming or excluding DD.

Aged↗