George W. Swenson

A brief tutorial on the fast field program (FFP) as applied to sound propagation in the air

Abstract The Fast Field Program (FFP), a computational technique originally developed for predicting acoustic wave propagation in the sea, has proved useful for calculating sound propagation in the air above the ground. The procedure involves the Hankel transformation of the Helmholtz equation in circular cylindrical co-ordinates, and the integration of the resulting ordinary differential equation by analogy with electrical transmission lines. Variation of the sound speed in the vertical co-ordinate is represented by horizontal stratification of the air into discrete layers, each with a different sound speed homogeneous and isotropic within the layer. The heights of source and detection points can be arbitrarily assigned, as can the ground impedance. It is assumed that there is no azimuthal variation of sound pressure and that there is no range dependence of ground impedance or atmospheric parameters. The computation yields the sound pressure, at a given detector height, for a continuous range of source to detector radii. This tutorial paper gives a detailed account of the theory of the FFP, describes the current implementation in terms of discrete variables, and references the original sources of the technique.

Radio-astronomy precedent for optical interferometer imaging

Radio-astronomical correlator arrays have demonstrated the ability of producing images of cosmic radio sources with extremely high resolution, dynamic range, and precision. A large body of experience and theory has been developed in connection with this technology that may form useful precedents for the development of similar technology in the optical and infrared regions of the electromagnetic spectrum. The production of images of sources from measurements of their mutual coherence functions is demonstrated, and the capabilities and limitations of the technique are discussed. A prototype optical instrument is proposed, and its advantages and liabilities are identified.

Determination of the wingsnap sonation mechanism of the golden-collared manakin (Manacus vitellinus)

ABSTRACT Male golden-collared manakins (Manacus vitellinus), small suboscine passeriform birds of Panamanian forests, communicate acoustically using a variety of non-vocal sonations. The most prominent sonations are single or multiple intense ‘wingsnaps’ with a dominant acoustic frequency around 5 kHz. Several hypotheses have been proposed addressing the source of the sound, ranging from purely aerodynamic origins (due to a rapid jet of air formed by the wings or by a ‘whiplike’ motion) to purely structural origins (such as physical contact of the wings), but without definitive assessment. Using anatomical analysis as well as high-speed video and synchronized audio recordings, we show that compared with related species, M. vitellinus radii are morphologically unique and confirm that they collide over the back of the bird at the moment (±1 ms) the wingsnap is produced. Using aeroacoustic theory, we quantitatively estimate the acoustic signatures from several sonation mechanisms. We conclude that only the physical contact hypothesis, wherein the wing collisions create the sound, is consistent with the measured sonation. Summary: The intense Manacus ‘wingsnap’ involves modified wing bones that, upon collision, are themselves the source of this unusual acoustic signal.

The Paraboloidal Reflector Antenna in Radio Astronomy and Communication: Theory and Practice

and historical development.- Geometry of reflector antennas.- Electromagnetic theory of the reflector antenna.- Antenna characteristics in practical applications.- Measurement of antenna parameters.- Miscellaneous subjects.- Design features of some radio telescopes.

A standing-wave facility for low-frequency impedance/absorption measurement

Abstract A facility for normal-incidence acoustic impedance and absorption co-efficient measurement in the frequency range 20–85 Hz has been developed at the US Army Construction Engineering Research Laboratories. The system consists of an enclosed waveguide operating in the fundamental mode, with an array of microphones and data processing electronics for determining the complex reflection coefficient of a surface two meters square.

Antennas and Arrays

This chapter opens with a brief review of some basic considerations of antennas. The main part of the chapter is concerned with the configurations of antennas in interferometers and synthesis arrays. It is convenient to classify array designs as follows: 1. Arrays with nontracking antennas 2. Interferometers and arrays with antennas that track the sidereal motion of a source: Linear arrays Arrays with open-ended arms (crosses, T-shaped arrays, and Y-shaped arrays) Arrays with closed configurations (circles, ellipses, and Reuleaux triangles) VLBI arrays Planar arrays.

Very-Long-Baseline Interferometry

The technique of very long baseline interferometry (VLBI) has undergone two decades of steady growth and refinement since its inception in 1967. In the beginning, only crude measurements of visibility on single baselines were possible. Now 18-station arrays have been used to produce images with dynamic ranges exceeding 2000:1; relative motions of cosmic masers have been tracked at the microarcsecond level of accuracy; and angular size measurements have been made with baseline lengths up to 2 two earth diameters with an orbiting satellite as a receiving element.

Optical Synthesis Telescopes

A new type of optical/infrared telescope is suggested, based on a combination of the principles of radio astronomical interferometric image synthesis and computational phase retrieval. Physical configurations, design principles, necessary technical developments, possible modes of operation and important possible uses of the telescope are discussed. The design philosophy is that it should not be necessary to maintain tighter mechanical tolerances than those required conventionally for radio telescopes. Because it is intended to realize milli-arc-second resolution, implying a telescope aperture of the order of 100 m, it is recognized that it is impractical to achieve optical tolerances, so that the adopted design principles should not rely on them. The design goals can be attained by relying on heterodyne detection followed by purely digital processing, thereby permitting useful signal-to-noise ratios to be conveniently obtained by multiplexing parallel channels with the aid of large-scale-integration techniques. Besides offering highly resolved images of many of the more important astrophysical objects, the telescope could be used for imaging certain types of space vehicles. The kind of telescope proposed herein could be usefully constructed either on the surface of the earth or outside the Earths atmosphere.

Propagation Effects: Ionized Media

Three distinct ionized media, or plasmas, affect the propagation of radio signals passing through them: the Earth’s ionosphere; the interplanetary medium, also known as the solar wind; and the interstellar medium of our Galaxy. The effects of scattering in other galaxies or in the media between galaxies are not usually important. There are several essential differences between neutral and ionized media with regard to propagation. For neutral media, the index of refraction is greater than unity and is unaffected by magnetic fields. In ionized media, the index of refraction is less than unity and is strongly affected by magnetic fields. Most plasma phenomena scale as ν−2, and their effects can be avoided or mitigated, if desired, by observations at high frequency. Absorption plays an important role in neutral media but very little in ionized media since most radio astronomical observations occur at frequencies far above the plasma frequency. Descriptions of scattering phenomena in both types of media are based on Kolmogorov theory. However, the situation in the neutral troposphere is greatly simplified because the turbulent layer lies close to the observer, and only phase fluctuations develop. The ionized media lie far from the observer, and both phase and amplitude fluctuations are often present in the wavefront when it reaches the observer.

Calibration and Imaging

This chapter is concerned with the calibration and Fourier transformation of visibility data, mainly as applied to Earth-rotation synthesis. Methods for the evaluation of the visibility measurements on a rectangular grid of points, necessary for the use of the discrete Fourier transform as implemented with the fast Fourier transform (FFT) algorithm, are discussed. Phase and amplitude closure conditions, which are valuable calibration tools, are also described. Analysis of the causes of certain types of image defects is given. Special consideration is given for certain observing modes, such as spectral line, and conversion of frequency to velocity is described. In addition, methods of extracting astronomical information directly from visibility data by model fitting are described. These techniques are important even with arrays having excellent (u, v) coverage. Some methods of calculating Fourier transforms before the advent of the FFT are discussed in Appendix 10.3.