Air Attenuation Research Articles

The role of early postoperative CT scan following surgery for herniated lumbar disc.

40 patients underwent lumbosacral microdiscectomy in a 12 months period covering the years 1988-1989 in our center. For the purpose of the present study, we considered the 30 cases who underwent CT control of the operated interspaces and of the adjacent vertebral endplates. This was routinely carried out on the 3rd post-operative day. The present study failed to show correlations between early post-operative CT data and subsequent clinical results in this unselected series of patients. In fact an image suggesting persistent disc herniation was as a rule observed in spite of satisfactory clinical results, and other CT findings that have been considered of clinical significance, such as intraspinal air and low attenuation of disc space, were commonly observed in patients with an uncomplicated post-operative course. The present study suggests that early post-operative CT appears to be of no value in the management of potential early complications of low-back surgery. The introduction of MRI appears to offer other possibilities in the evaluation and management of failed back surgery syndrome (FBSS).

Auditorium acoustics simulation for the Macintosh.

Geometrical ray-tracing techniques have been used as educational and analysis tools in a Macintosh program written in ThinkTM C. This program takes as an input file a standard DXF-format file created from any of several popular CAD programs. From this data, a wire-frame representation of the auditorium is displayed and the user is able to specify the acoustic reflection characteristics of each surface. The program then allows the user to specify the position, frequency characteristics, and directionality of sound sources in the hall, and to run a ray-tracing simulation represented in 3-D color graphics of the behavior of sound rays emanating for the sources. Diffraction effects and air attenuation are accounted for in the propagation algorithm. Sound level contours through arbitrary planes are available after the simulation terminates, as are a variety of acoustic descriptors for user-specified listener locations. Hopefully this normal-mode analysis for steady excitations can be incorporated into newer versions of the program.

Effects of melt-blowing process conditions on morphological and mechanical properties of polypropylene webs

Isotactic polypropylene (iPP) melt-blown webs have been studied in terms of the relationships among the processing conditions, structure and mechanical properties. The melt-blowing process conditions investigated in this study were die and air temperatures, die-to-collector distance ( DCD) and attenuation air flow rate at the die. The macroscopic web structure was characterized in terms of interfibrous bonding and fibre diameter using scanning electron microscopy. The degree of fibre entanglements was found to increase with decreasing DCD, with increasing die temperature, or with increasing air flow rate. Using wide-angle X-ray diffraction and differential scanning calorimetry (d.s.c.), the microscopic molecular structure of iPP has been investigated. The fibres in a web consist of the α-form crystal, smectic phase and amorphous regions. The crystallinity of the iPP webs obtained from d.s.c. was found to be similar (39–44%) since the smectic phase transforms to crystals on heating in a d.s.c. A wide range of tensile properties and flexural rigidity values in the machine direction and cross direction were obtained at various processing conditions. The results have been analysed in terms of the degree of fibre entanglements, fibre orientation in the machine direction and the fraction of smectic phase.

Long-distance sound propagation over an impedance discontinuity

A calculation method is presented for sound propagation over an impedance discontinuity which is an improvement over previous methods, being particularly efficient for long distances. A homogeneous, still atmosphere and flat ground are assumed. The starting point is to make an approximation in a two-dimensional boundary integral equation formulation of the problem. This leads to an expression of the solution as the solution for homogeneous ground plus an integral over part of the boundary. This integral is recognized as a generalized Fourier integral, to which standard techniques of asymptotic analysis can be applied. An asymptotic approximation is found which can replace the part of the integral which is most expensive to evaluate by numerical quadrature. The remaining integral is evaluated by numerical quadrature. Monofrequency excess attenuation results for propagation from a point source above rigid ground to a receiver above absorbing ground are discussed. When the source and receiver are low enough, very orderly trends are observed. The results are applied, with air attenuation and A-weighting, to a notional jet engine noise source; simple trends are noted.

Examination of surface exposure ages of Antarctic moraines using in situ produced 10Be and 26Al

Concentrations of 10Be (t 1 2 = 1.5 × 10 6 y ) and 26 A1 (t 1 2 = 0.72 × 10 6 y ) have been determined by accelerator mass spectrometry (AMS) in a suite of quartz samples taken from sandstone boulders in several moraines in Arena Valley, a dry valley adjacent to the Taylor Glacier in the Quatermain Mountains, Southern Victoria Land, East Antarctica. These isotopes are produced in surficial quartz by cosmic ray spallation of O and Si. The concentrations in these samples ranged from 6.1 × 10 5 to 3.0 × 10 7 at g −1 for 10Be and from 9.4 × 10 6 to 1.2 × 10 8 at g −1 for 26A1, depending upon the extent of exposure at the surface. Analyses of 10Be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons; the value of 142 +96 −41 g cm −2 is consistent with other published values. Production rates of 17 +6 −4 at g −1 y −1 for 10Be and 113 +54 −16 at g −1 y −1 for 26A1 at 1300 m and 87°S and a 26Al: 10Be production ratio of 6.5 +1.3 −1.3 were calculated from the data. These values correspond to sea-level production rates at high geomagnetic latitude of 6.4 at g −1 y −1 and 41.7 at g −1 y −1 for 10Be and 26A1, respectively, consistent with determinations based on ~ 11 Ky glacially polished surfaces in the Sierra Nevada in California. These production rates imply exposure ages for the various moraines ranging from 50 Ky to 2.5 My, in accordance with other geological evidence. The 10Be and 26Al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3He. Examination of the 3He concentrations in conjunction with the present data set yields an estimate of 3He production of 230 +85 40 at g −1 y −1, corresponding to 85 +31 −15 at g −1 y −1 at sea level. This is lower than previously reported values of 240 ± 60 at g −1 y −1 and ~190 at g −1 y −1 (scaled to sea-level and high geomagnetic latitude) based, respectively, on volcanic flows less than 2000 years old at low latitude, and samples collected from 14,400-year-old volcanic flows. This discrepancy is likely due to diffusive losses which may become significant (up to ~50%) for surface exposure ages on the order of 2.0 My. Nevertheless, the general correlations among the three isotopes suggest that 3He does not suffer gross losses through diffusion.

Structure and filtration properties of melt blown polypropylene webs

AbstractPolypropylene melt blown webs have been studied in terms of the relationships among the processing conditions, structure, and filtration efficiency. The effects of the processing conditions on filtration efficiency to aerosolized latex particles, pore size, fiber diameter, and air permeability have been investigated. The melt blowing process conditions investigated in this study were die and air temperatures, die‐to‐collector distance, and attenuation air flow rate at the die. The filtration efficiency has been found to increase linearly as mean pore size decreased. The degree of fiber‐entanglement increased, therefore, the pore size and air permeability decreased with increasing processing temperature, increasing air flow rate at the die, or decreasing die‐to‐collector distance. Average fiber diameter appeared to change little with die‐to‐collector distance, but decreased with increasing die temperature or with increasing air flow rate.

An air attenuation correction factor for use with long counters

A correction based on a well-collimated beam geometry is usually applied to allow for the effect of air attenuation between a neutron source and a long counter when making fluence measurements; such a correction represents an upper limit to the actual attenuation. In this paper a simple formula representing a realistic lower limit estimate is derived. Both estimators are then applied to the correction of fission spectrum and 14 MeV fluence determinations. The discrepancy between the two methods is used to provide an indication of the systematic modelling error associated with these corrections. The results indicate that systematic effects of this type are negligible in comparison with other sources of uncertainty in most practical situations.

Digital evaluation of the acoustics of small models: The midas package

Compared with more fashionable computer modeling, acoustical modeling is far from being superseded. In fact, there is often no alternative to acoustical modeling when designing an auditorium. Careful analysis of the laws of similarity shows that there remains some possibility of improving the acoustical testing of small models. This improvement does not solely concern transducers but the processing of impulse responses as well. Digital processing provides both accuracy of computation and compensation for air attenuation. Based on these principles, a computer-aided testing plant was designed and a computer package was written to drive it. A modular structure was selected as giving the maximum flexibility. Thus a digital evaluation of objective room-acoustical parameters can be achieved within a few minutes. So far, computer-aided acoustical modeling has proved well adapted for research and designing purposes.

Fast Neutron Streaming through Duct in Concrete Shields

Measurements on the neutron and γ-ray attenuation in multi-legged air filled ducts have been carried out using a Cockroft-Walton type neutron generator. The measured spectra were obtained with an NE-213 liquid scintillator using pulse shape discrimination to resolve neutron and γ-ray pulse height data and using a spectral unfolding code to convert these data to energy spectra. Results are given for two rectangular duct geometries. A series of integral measurements have also been carried out with a long counter for fast neutrons and a radiation survey meter to measure the γ-ray dose. The biological dose arising due to the streaming of neutrons was measured with a rem counter. The results show the relative decrement of the integral counts along the axis of the duct. The spectral measurements reveal the presence of resonant windows of oxygen present in the concrete medium. Also the 14 MeV component is found to be dominant in the first leg of the duct whereas degraded neutrons dominate in the second leg. The ...

Sound attenuation in air over water on Puget Sound during temperature inversion and noninversion conditions

Excess attenuation of sound over a 7500-ft distance was measured on 6 days to determine effects of temperature inversions and wind direction. The tests were conducted using one-third octaves of pink noise at center frequencies of 250, 500, and 1000 Hz. During temperature inversions, excess attenuation downwind was found to be significantly reduced and negative. Results of the tests were used to predict sound levels that would be received at island residences and a bird sanctuary under a proposed industrial use of the mainland shoreline.

Depth profiling of defects in epilayer semiconductor materials by using synchrotron x-radiation topography

Depth profiling of defect structures can be accomplished by taking advantage of the increased penetration of x rays with decreasing wavelength. Topographs can then be taken at various integrated depths giving a nondestructive profile of the microstructure. This is usually accomplished via a monochromator, which is a slow and expensive procedure, even at a synchrotron source. Appropriate filters can be used in white beam synchrotron topography which reduce fluorescence while at the same time giving a limited, though not monochromatic, bandpass. This allows observations of the microstructure over a narrow energy range, diminishing the complications arising from harmonics in interpreting white beam images. In studies of GaAs crystals it was found possible to use GaAs or Cu prefilters in conjunction with deliberate air attenuation of the long wavelength components of the synchrotron spectrum. Studies were made on GaAs crystals that had been processed in a variety of ways. Examples will be shown for GaAs substrates with low grown-in dislocation density and with Zn-doped GaAs epilayers. This selective energy filtering provides a simple, rapid method of recording reflections varying from approximately 0.2 to 1.6 Å. The combination of this technique with a precision goniometer tilt stage permitted observations of the specimen defect structure from a few microns below the surface to 0.3 mm depth, thus presenting a visual profile of the effect of processing on the defect structure of the sample.

An updated model for millimeter wave propagation in moist air

A practical atmospheric Millimeter‐Wave Propagation Model (MPM) is formulated that predicts attenuation, delay, and noise properties of moist air for frequencies up to 1000 GHz. Input variables are height distributions (0–30 km) of pressure, temperature, humidity, and suspended droplet concentration along an anticipated radio path. Spectroscopic data consist of more than 450 parameters describing local O2 and H2O absorption lines complemented by continuum spectra for dry air, water vapor, and hydrosols. For a model (MPM*) limited to frequencies below 300 GHz, the number of spectroscopic parameters can be reduced to less than 200. Recent laboratory measurements by us at 138 GHz of absolute attenuation rates for simulated air with water vapor pressures up to saturation allow the formulation of an improved, though empirical water vapor continuum. Model predictions are compared with selected (2.5–430 GHz) data from both laboratory and field experiments. In general, good agreement is obtained.

The prediction of sound fields inside non-diffuse spaces: Transmission loss considerations

The prediction of sound levels inside internally complex enclosures is extended to include transmission through walls. The sound field is estimated from the spatial variation of phonon density which is calculated by using a random walk technique. This statistical geometrical method also includes consideration of air attenuation and specular reflection of sound from walls whose individual random incidence absorption coefficients are individually known. Solutions agree with experimental results obtained in models, to within, on average, 4 dB.

Attenuation of shock parameters in air and water

This paper describes the results of shock pressure measurements in the range of 1–25 MPa in water and in the range of 60–500 kPa in air. Pressure pulses were generated by exploding wire technique and measured with a quartz piezo-electric transducer. The attenuation with distance of shock overpressure, impulse and energy in shock front has been studied. Experimental data on shock attenuation in air is scarce and the results presented here confirm the attenuation behaviour derived from theoretical considerations.

Methods Of Sensitometry For Low Energy Radiography

Inverse-square sensitometry for low kVp techniques is limited by air attenuation and beam hardening. It is thus difficult to obtain accurate H and D curves for mammographic screen-film combinations at technique factors used clinically. Methods are described for the determination of characteristic curves at low x-ray energies which provide composite intensity-scale curves using small changes in source-to-receptor distance and concomitant changes in either exposure time, beam attenuation, or kVp. H and D curves obtained at low kVp's using the various sensitometric approaches are compared with those obtained using inverse-square sensitometry; the relative merits of the different methods are discussed.

Shock attenuation in air during the explostion of a plane high-explosive charge in a shock tube

Shock attenuation in air during the explostion of a plane high-explosive charge in a shock tube

The detection of ultra-cold neutrons by the activation of manganese

The activation of manganese foils has been used to detect ultra-cold neutrons and to measure their attenuation in helium and air.

A new technique for reactor siting dose calculations.

The COMRADEX code was developed to calculate radiation doses from accidents postulated for advanced reactors with sophisticated engineered safeguards. The use of a direct numerical technique, designed especially for quasilinear differential equations, to solve the leakage and decay chains, permits the inclusion of several special features. These include the capability for considering up to four levels of containment, each with an independent time-varying leak rate. Provision is also included for filtration, particle coagulation and fallout, and special cleanup systems. Shielding may be introduced between the various containment shells. A full fission product inventory (plus special isotopes) is used with input release fractions. The meteorology model is that of Pasquill and Gifford. Internal doses are determined with the ICRP model. The cloud gamma dose is obtained by integrating over the anisotropic Gaussian cloud; this calculation is simplified by assuming a single air attenuation kernel for all gammas.

Comparison of Exposure Standards in the 10–50 kV X-Ray Region

Direct comparisons of the low-energy X-ray exposure standards of the Bureau International des Poids et Mesures (BIPM) and the National Research Council of Canada (NRC), and also of the BIPM and the National Bureau of Standards of the USA (NBS), were made, using X rays generated at 10, 30 and 50 kV. The half-value layers for these qualities of radiation were 0.036, 0.176, and 2.257 mm of aluminum respectively. The results indicate that measurements with two free-air chambers can be made in the low-energy X-ray region with a discrepancy of less than 0.3%. The uncertainties and errors in the geometric and correction factors used for the comparison measurements, as well as other systematic and statistical errors, were examined in detail and are reported. The results of preliminary work on air attenuation corrections, diaphragm comparisons and saturation corrections are included.

Calculated Calibrations for Ion Chambers Fabricated from Plastics Simulating Air and Muscle; Determination of W and Γ Ra

An experiment is described utilizing two 16-liter ionization chambers, fabricated from electrically conducting plastics which closely simulate air and human muscle, designed to minimize most of the errors inherent in the use of cavity chambers. A careful calibration was done, using a 226Ra source in 0.5 mm platinum, in an almost scatter-free environment which permitted the derivation of accurate corrections for scatter and air attenuation. Calibrations calculated from the physical measurements of the ion chambers are compared with the experimental calibrations. Values of Wbeta for air, and muscle gas and of tauRa are derived.