Synthetic Impulse Research Articles

Gaussian beam summation in shallow waveguides

The complex beam parameter ϵ controls the beamwidth and the curvature of the wavefront of a Gaussian beam. Here some strategies for choosing ϵ are analysed for a shallow cylindrically symmetric waveguide and examplified by three choices of ϵ used in the calculation of synthetic impulse responses by Gaussian beam summation. The geometry of the shallow waveguide puts a restriction on the feasible width of the beams, since beams that are too wide cause problems with end effects in the summation. Best results in the numerical examples are observed when choosing ϵ to give plane wavefronts and narrow beams at the points contributing to the field, so that the end effects are negligible. The expansion of a point source into beams for launch-angle-dependent ϵ is shown to agree with the commonly used expansion, corresponding to launch-angle-independent ϵ.

Three-dimensional interactions of a free jet with a perpendicular synthetic jet

Three-dimensional interaction of an axisymmetric free jet with a single synthetic jet that is oriented perpendicular to the free jet was investigated experimentally using PIV. The main objective was to investigate the effect of the upstream location of the synthetic jet, inside the nozzle of the main jet ( Re U e = 6600). The interaction location of the synthetic jet (for a range of momentum coefficients) was varied from 0.4 to 1.4 orifice diameters upstream of the main jet exit plane. The synthetic jet was driven at a frequency of 1000 Hz, which corresponds to a Strouhal number of 0.16 (within the range of unstable modes of the free jet). To better explore the complex flow field resulting from the interaction, a reduction technique was used where 3D flow renderings were calculated from multiple 2D measurement planes. The interaction is affected by two mechanisms: (1) direct impact of the synthetic jet into the main jet, and (2) amplification of unstable modes of the main jet. When the momentum coefficient is increased, the synthetic jet penetrates deeper into the main jet flow creating larger streamwise vortical structures and increasing the main jet width. Increasing the distance between the synthetic jet exit and the main jet nozzle exit the effectiveness of the synthetic jet's impulse decreases, resulting in smaller jet vectoring, weaker streamwise vortical structures, and decreased turbulence levels. When compared to a continuous control jet of the same momentum coefficient and upstream location, the effects are significantly different.

HFSWR based on synthetic impulse and aperture processing

A novel high-frequency surface wave radar (HFSWR) has been proposed, which is based on the principles of synthetic impulse and aperture processing. It uses multiple omnidirectional transmit antennae to simultaneously transmit a set of orthogonal waveforms, and the echoed signals are received and processed by one or multiple reception arrays. Although the transmit beam pattern is omnidirectional, by proper processing, the received signal multiple equivalent directional transmit beam patterns can be simultaneously formed. The signal processing scheme to obtain equivalent directional transmit beam pattern is investigated in detail. Considerations for system parameter selection to achieve an overall best system performance are proposed and discussed. In particular, for the novel HFSWR, target range and angle estimates are coupled together due to the orthogonality of the transmitted waveforms. A necessary and sufficient condition on the transmit antenna array geometry and transmit frequencies, which ensures that target range and angle estimates are uncoupled, is presented.

Measurement and subjective evaluation of the spatial modulation of late reverberation

The acoustical characterization of a large rectangular space with uniformly distributed absorption is facilitated by a predictably linear reverberant decay of decibel sound pressure level. By contrast, large spaces with irregular shapes yield correspondingly irregular reverberant decays. A contributing factor is coupled space effects where the several reverberant decays occur simultaneously. When the mean free path for reflections is relatively large, as in a cathedral, temporal thresholds can be exceeded to the point where it is possible to hear spatially separated decays. The phenomenon of a slow amplitude modulation between separate locations can be described as moving late reverberation, which is audible, e.g., in a cathedral when an organ note is stopped. Measurements of this phenomenon were made in San Franciscos Grace Cathedral, a French Gothic-style Episcopal church, using multiple calibrated microphone impulse response recordings. Data on the temporal-spatial diffusion of reverberation from various spatial perspectives are presented, along with sound examples. Preliminary listening test data involving subjective evaluation of late reverberation movement is presented, where the temporal and spatial components of a synthetic impulse response is manipulated in a manner characteristic of similar large spaces.

Application of the mode isolation algorithm for experimental modal analysis to a system with close natural frequencies

Experimental modal analysis techniques in current use have difficulty with modes whose natural frequencies are very close or whose drive point mobility is very low. How well these issues are addressed by the Mode Isolation Algorithm (MIA), see M. V. Drexel and J. H. Ginsberg, ‘‘Modal parameter isolation using state space mode isolation,’’ in Proceedings of the 19th International Modal Analysis Conference, Orlando, Florida, February 5–8, 2001, is the topic of this presentation. MIA is an iterative procedure that employs frequency domain response data. The application of MIA to a system consisting of a beam with three suspended spring-mass-dashpot subsystems is described. Judicious choice of the system parameters leads to two modes having close natural frequencies, and two other modes in which impulse excitation at the free end of the beam results essentially in vibration of the subsystems away from the end. Synthetic impulse response data for the analysis is contaminating with white noise. It is shown that MIA accurately identifies all natural frequencies and modal damping ratios if the frequency resolution of the response data is sufficiently fine. Furthermore, the two normal mode shapes associated with the proximate natural frequencies are correctly identified, but the two with low drive point mobility are not. [Work supported by the National Science Foundation, Grant No. CMS-0098539.]

Performance analysis of pulse compression using phase-coded signals for sparse-array synthetic impulse and aperture radar

Sparse-array Synthetic Impulse and Aperture Radar (SIAR) can isotropically radiate by employing multiple frequencies (synthetic pulse) and multiple antennas (synthetic antenna). According to Ambiguity Function(AF), its range resolution depends only on bandwidth of transmitted signals, however, the distance grating lobes emerge when increasing the time-bandwidth product of transmitted signals. The performance of pulse compression is analyzed with the transmitted signals modulated by phase-coded sequences. It is seen that greater ratio of pulse compression and suppression of range sidelobe in SIAR can be obtained, and its effective range and range resolution is increased as well.

Time-space 3D matched filtering for performance analysis of synthetic impulse and antenna radar

The principle and performance of Synthetic Impulse and Antenna Radar(SIAR) are analyzed with the concept of 3D matched filtering. The discussion here is concentrated on the characteristics of SIAR in the case of three dimensions. The results obtained are helpful for designing this new style radar.

FILM SYNTHETIQUE AVEC REFLEXIONS MULTIPLES THEORIE ET CALCUL PRATIQUE*

AbstractOur purpose is to give a short summary of the theory of synthetic seismograms, including all multiple reflections and to show the method of their construction with the use of an electronic computer.The waves to be considered in reflection seismic being approximately plane and horizontal it is generally admitted that in most cases the propagation phenomena can be described with the equation image (u, displacement; V(z), velocity; p, density). Moreover, for all practical purposes, the velocity V (z) is not a continuous function of the depth z. In fact, the earth can be divided up into more or less thin layers, with constant velocity inside each layer and sudden variations at each interface. It is therefore reasonable to substitute to the single equation (1) a series of simple propagation equations with constant coefficients image provided a set of boundary conditions is adjoined to them in order to ensure the continuity of displacement and tension.As with all seismic problems, this is essentially a transient system and a very convenient method to resolve equation (2) is to resort to the Laplace transformation, by writing image Then the general integral of (2) is: image A and B being two constants. This expression is valid inside of a layer, including the two faces. If the propagation velocity in an adjoining layer is V we get an equation similar to (4), say (4′), but with different constants C and D. At a point on the interface, both expressions are valid. Consequently, if we take three points M, P and N into consideration, respectively at the depths z– V T, z and z + V T, we can write four expressions (4) and(4′). The continuity of the tension gives a fifth expression. The constants A, B, C and D can be eliminated from these equations. The result of the elimination is image is the reflection coefficient. Going back to the original functions we find a recurrence expression with four terms image In order to make use of this expression, we set out from curve image which divides the plane Ozt into two domains:1) a domain contiguous to the axis Oz where u is identical to zero;2) the remainder of the plane where for z = o, u(t, o) =s(t)–“the signal”–is given in a narrow interval in the proximity of the origin.The numerical calculation is carried out at the intersection points of two sets of straight lines:1) equidistant parallels to Oz (with spacing x) and2) parallels to Ot through the intersection of the curve T with the straights of the first family.Computations having been carried out for all points (z, t) of the second domain, they lead finally to the values of the function u at the surface, u(t, o), outside the interval where the signal is given. This function u(t, o) is the requested synthetic seismogram.The shape of the signal enters into the calculations. As a matter of fact it is always necessary to try several signals, hence to construct several synthetic seismograms. However, the operation consisting in the modification of the response is simpler than the calculation of the initial film. This leads to the notion of the synthetic impulse seismogram, which is constructed by assuming that the signal is a pure impulse. This impulse seismogram being calculated, it is easy to construct as many synthetic records as there are signals to be taken into account.