Free-field Radiation Research Articles

Active control of structural volume velocity using shaped PVDF sensors

In low frequency, the sound power radiated from planar structures is simply related to the net structural volume velocity. A cost function based on the volume velocity in active control of structural radiation has the advantage of keeping the control simple (one error sensor). The implementation of volume velocity error sensors in feedforward control of flexural beams and plates using shaped PVDF films is presented. For a beam, a single extended strip of prescribed shape is needed, while in the case of a panel, a number of shaped strips related to the number of flexural modes contributing to the volume velocity is required. The sensor obtained is independent of the type and frequency of excitation. A procedure for deriving the appropriate sensor shape, based on analytical or experimental modes, is discussed. The experimental implementation of volume velocity sensors is addressed and results of active control using piezoceramic (PZT) actuators are presented in the case of a simply supported beam, and simply supported or clamped panels. The strategy of minimizing the volume velocity is shown to provide significant acoustic attenuation for structural free-field radiation or transmission problems.

Theoretical Background of the Dosimetry System 86 (DS86)

DS86 is a calculation code system for the estimation of the radiation dose of atomic bomb survivors at Hiroshima and Nagasaki. The DS86 calculation process is as follows: 1. Compute free-field radiation environment: radiation transport calculation can be done for the free air field at Hiroshima and Nagasaki. 2. Compute shielded radiation environment: depending on each shielding situation, the shielding can be calculated by using either the Nine-Shielding Parameter or the Globe Shielding Data. 3. Compute radiation environment at the organ site: organ kerma and fluence spectra can be calculated using a transport calculation code depending on each survivor sex, age, orientation and posture.

Free-field correction factor for spherical acoustic waves impinging on cylinders

The scattering and diffraction of spherical waves by an infinite rigid cylinder is analytically studied for monopole and dipole acoustic sources. A correction factor at the cylinder surface for the change in the free-field radiation levels due to the presence of the cylinder is calculated. The results indicate that the correction factor is sensitive to source frequency and location. Regions of negative correction factor, due to shadowing and creeping wave behavior are identified.

Generalized method of predicting optimal performance of active noisecontrollers

In this investigation, the formulation for studying the performance of an optimal active noise controller for harmonic, enclosed sound fields is derived using an indirect boundary-element method. The optimal active noise controller is defined as the volume velocity secondary source strengths that minimize the sound pressure level at a number of discrete interior locations. In addition, the formulation for studying the performance of an optimal active noise controller that minimizes the free-field power radiated from a generalized, distributed noise source is presented. Representative results are provided for each formulation. For the enclosure problem, results are given for global control and local control schemes using a rectangular enclosure. For the free-field radiation problem, the effects of secondary source location and quantity on active noise controller performance are investigated. The relative merits of each formulation are discussed.

Measurement of Shielding Effectiveness of Microwave-Protective Suits

With more restrictive exposure standards and greater public concern about safety from nonionizing radiation, protective clothing to shield workers from dangerous levels of electromagnetic radiation has become imminently important. Tests were conducted of the shielding effectiveness of a number of microwave suits. Fabric attenuations at 10 selected locations on the suit were measured by the waveguide-transmission-loss method to evaluate the material, and relative field strengths at the surface of a full-sized phantom man model exposed to 2450-MHz free-field radiation fields with and without the suit were measured to evaluate the entire suit. The ratio of the incident power required to produce the same output from a diode E-field sensor placed against the synthetic tissue of the model exposed with and without the suit was used as a measure of attenuation. The waveguide measurements are important for a comparative evaluation of the different fabrics, and the free-field measurements are important for evaluating the fabric as well as the suit design and configuration in shielding effectiveness. The Navy suit provided the best performance in terms of shielding effectiveness, with attenuation varying from 34.5 to 48.7 dB, but its flammability constitutes a safety hazard. The Wave Guarde suit was second best, with attenuation varying from 13.2 to 35 dB. The Milliken (20.3-33.7 dB attenuation) and Invascreen (18.7-24.5 dB attenuation) suits provide acceptable protection for E-field polarization parallel to the long axis of the body, but poor protection (2.0- 19.2 dB for the former and 0.4-21.1 dB for the latter) for E-field polarization perpendicular to the body. Localized high-power-density exposures of up to 1000 mW/cm/sup 2/ for 1 h from a waveguide aperture did not cause material damage to any of the suit fabrics. The Milliken was the most fire retardant, but its shielding performance was the poorest, owing to the large number of openings at the pockets, cuffs, sleeves, and collar. The Invascreen suit is well designed to eliminate the problem, but the shieldlng of the fabric is less effective.

Control of Colonial Hydroid Macrofouling by Free-Field Ultrasonic Radiation

Free-field ultrasonic radiation inhibited the feeding of the macrofouling hydroid Garveia franciscana by causing tentacle contraction at the sonic degasification threshold. Within the frequency range of 250 to 2000 kilohertz, the threshold sound intensity (sonic degasification threshold) that caused tentacle contraction was directly proportional to frequency, with the minimum observed being 0.6 watt per square centimeter at 250 kilohertz. A pulse length of 0.2 second and interpulse period of 102 seconds gave the lowest average power required to produce tentacle contraction at a peak pulse sound intensity of 6.2 watts per square centimeter with a frequency of 250 kilohertz. Twenty-four hour exposures to the sound regime caused destruction of the hydranths and regression of tissue in the stolons.

Operant behavior and colonic temperature of Macaca mulatta exposed to radio frequency fields at and above resonant frequencies.

Five food-deprived rhesus monkeys were exposed to 225-MHz continuous-wave, and 1.3-GHz, and 5.8-GHz pulsed radiation to determine the minimal power densities affecting performance. The monkeys were trained to press a lever (observing-response) thereby producing signals that indicated availability of food. In the presence of the aperiodically appearing food signals, a detection response on a different lever was reinforced by a food pellet. Continuous, stable responding during 60-min sessions developed and was followed by repeated exposures to radiofrequency radiation. The subjects, restrained in a Styrofoam chair, were exposed to free-field radiation while performing the task. Colonic temperature was simultaneously obtained. Observing-response performance was impaired at increasingly higher power densities as frequency increased from the near-resonance 225 MHz to the above-resonance 5.8 GHz. The threshold power density of disrupted response rate at 225 MHz was 8.1 mW/cm2; at 1.3 GHz it was 57 mW/cm2, and at 5.8 GHz it was 140 mW/cm2. These power densities were associated with reliable increases in colonic temperatures above sham-exposure levels. The mean increase was typically in the range of 1 degree C, and response-rate changes were not observed in the absence of concomitant temperature increases. In these experiments increase of colonic temperature was a much better predictor of behavioral disruption than was either the power density of the incident field or estimates of whole-body-averaged rates of energy absorption.

A Model for Photodetection of Single-mode Cavity Radiation

A model for photodetection is presented which permits evolution of the detected field during the detection process. The integrated univariate and bivariate statistics of the photocount are obtained exactly for detection of single-mode thermal, coherent and free-field cavity radiation. The scheme encompasses the results of three hitherto disparate photocounting models, and demonstrates explicitly the relation between the univariate photon counting statistics and the photon statistics of the field. First-order photocount correlation functions are derived and compared with the corresponding cavity photon population correlations. Photocount antibunching is examined and found to be possible in this model.

Propagation of plane waves through two parallel dielectric sheets.

A study is presented of the standing waves created in the region between two parallel dielectric sheets when a plane electromagnetic wave propagates through the system in a direction normal to the dielectric boundaries. This model closely resembles the exposure of experimental animals, housed within restrainers of dielectric materials, to free-field microwave radiation. Data are presented giving the ratio of the standing waves set up for various sheet thicknesses and different dielectric materials, and it is demonstrated that significant changes in estimated power density levels are created by the restrainers.

Sound-Absorbing Pad for Minimizing the Acoustical Effect of the Ground on Noise Test Rigs

The inaccuracies that occur if acoustical measurements to determine the free-field radiation characteristics of a source are made in the presence of the ground or other partially reflecting surface are first discussed. On test rigs where measurements are made close to the source, interference between direct waves and waves reflected from the surface is the main source of inaccuracy. A correction for this effect cannot easily be calculated for most practical sources. A solution to this problem is to place an acoustically nonreflecting pad on the surface between the source and the microphone. Such a pad has been designed from model tests and constructed for use with an experimental fan used for noise research. Full-scale tests show that the ground interference effect is greatly reduced, giving a maximum error of ±2 dB at 20 ft from a pure-tone source at frequencies between 100 Hz and 20 kHz.