Complex Echo Research Articles

The preception of complex echoes by an echolocating dolphin

Dolphins echolocate with short broadband acoustic signals which have good time resolution properties. Received echoes are often complex, with many resolvable highlights or components caused by reflection of the incident signal from external and internal boundaries of a target and from different propagational modes within a target. A series of experiments was performed to investigate how dolphins perceive complex echoes. Echoes were produced by a microprocessor‐controlled electronic target simulator which captured each emitted click and retransmitted the signal back to the animal after an appropriate delay time. The use of a “phantom” target allowed for precise control of the number of highlights, the time separation between highlights and the relative amplitudes of highlights in the simulated echoes. An echolocating dolphin was trained to perform a target detection task in the presence of masking noise using these phantom echoes. The properties of simulated echoes were systematically varied, and corresponding shifts in the dolphin's detection threshold were observed, allowing the inference of how the dolphin perceived the echoes. The dolphin performed like an energy detector with an integration time of approximately 290 μs.

Neural representation of bisonar information in the auditory cortex of the mustached bat

For echolocation, the mustached bat (Pteronotus parnellii rubiginosus) emits orientation sounds containing up to four harmonics, each of which consists of a long constant-frequency (CF) component followed by a short frequency-modulated (FM) component. The auditory system of this species extracts different types of bisonar information from complex orientation sound and echo pairs and represents these systematically in the separate areas of the cerebral cortex. Nine clusters of combination-sensitive neurons have thus far been found in the auditory cortex. Two of these clusters making up the CF/CF area consist of CF1/CF2 and CF1/CF3 facilitation neurons which are tuned to particular Doppler shifts, i.e., target velocities from 8 to −2 m/s. The remaining seven clusters consist of different types of FM-FM neurons which are tuned to particular echo delays, i.e., target distances. Three of these clusters, making up the FM-FM area, consist of FM1-FM2, FM1-FM3, and FM1-FM4 facilitation neurons. The FM-FM area projects bilaterally to the dorsal fringe (DF) and ventrolateral (VL) areas of the nonprimary auditory cortex. The DF area consists of three clusters of FM-FM facilitation neurons. The VL area contains a small cluster of facilitation neurons that are tuned only to the FM1-FM2 combination. The FM-FM area represents target distances from 7 to 310 era, while the DF area represents these from 14 to 140 cm. [Work supported by 1-R01-NS 17333.]

Adrenal carcinoma and adenoma in children: a review of 17 patients.

Seventeen children with adrenocortical neoplasms (13 carcinomas: four adenomas) are reviewed and attention is focused on the value of the newer imaging modalities in the management of these children. All the lesions were functioning tumors. CT is the single most important modality in assessing primary and metastatic disease at diagnosis and during follow-up. In children suspected of having an adrenal lesion, high resolution CT will promptly localize the lesion to an adrenal. Since the only curative treatment is complete surgical removal CT plays an important role in defining the extent of the primary lesion pre-operatively. Large carcinomas have an inhomogeneous density on CT and a complex echo pattern on ultrasound reflecting the areas of hemorrhage and necrosis seen macroscopically in these lesions. Smaller lesions have a more homogeneous density on CT but benign and malignant disease could not be differentiated by this modality. Ultrasound is useful in screening the adrenal area in those patients in whom there is a low clinical index of suspicion for an adrenal tumor and also in the post-operative period.

Qualitative Distortion at Fluid-Air Interfaces During Echography of Simulated Pleural Effusions

Beams of diagnostic ultrasound passing through pleural effusions produce an irregular band of complex echoes, when such beams strike aerated lung. To simulate pleural effusions and study this fluid-gas interface, we scanned latex bags of water and air which lay in a water bath beneath a uniform portion of veal rib cage. The general shape of an air-containing object could be determined under these conditions. The display of the upper surface of the gas-filled object was broad and heterogeneous but this zone of distortion was thin in relation to the overlying fluid layer. Fluid thicknesses exceeding 1 cm could be detected under these conditions even when they abut gas-filled structures. These preliminary data suggest that complex artefacts occurring at fluid-gas interfaces during echography of laboratory models simulating pleural effusions would not preclude useful volumentric estimations of overlying fluid layers.

Reverberations in echocardiograms

Different types of reverberations which may cause confusing artifacts in echocardiograms are described. Reverations between the targets may result in identifiable complex echo patterns. An echocardiogram with reverberating echoes is presented. Several experimental studies were done to explain the reverbating echoes. The reverberating echoes may be more prominently displayed than the primary echoes. This raises the possibility of obtaining useful information from the reverberating echoes.

Doppler velocity extraction from atmospheric acoustic echoes using a level crossing technique

The scattering of a narrow-band acoustic signal from atmospheric turbulence generates an echo of rapidly varying amplitude and frequency. The mean frequency of the echo power spectrum is related by a Doppler shift to the mean motion of the scatterers. In the level crossing technique this shift is deduced from measurements of the “instantaneous” envelope amplitude and rate of zero crossing of one component of the complex echo waveform. Data interpretation is based on an analysis of the statistics of instantaneous echo frequency and power which shows the probability density function of frequency to depend only on the first two moments of the power spectrum [R.C. Srivastava and R. E. Carbone, Radio Sci. 4, 381–393 (1969)]. These moments may be determined from averages in the time domain without the need for Fourier transformation. The technique has been tested on an electronically simulated echo signal and found to yield statistics in good agreement with the theoretical predictions. Similar tests on atmospheric echoes show good agreement for the frequency statistics but not for those of power.

Pulsed-Doppler Velocity Isotach Displays of Storm Winds in Real Time

First results derived from an NSSL effort to develop a real-time Plan Position Indicator (PPI) display of Doppler isotachs are described. The mean Doppler velocity is estimated for multiple range locations by measuring the phase change of the complex echo envelope over time intervals equal to the pulse repetition period. This technique, phase change per pulse pair, provides velocity estimates directly from samples of the Doppler time record to circumvent spectrum computation.

Hegel On Consent and Social-Contract Theory: Does He "Cancel and Preserve" the Will?

N MANY ways it is difficult to think of two philosophers more different than Hegel and Rousseau: who can imagine Rousseau lecturing regularly and punctually perhaps on the destructive effects of the arts and sciences in a university? Who can imagine the still more inconceivable existence of Hegel's Confessions? And yet it is instructive to compare them, for in respect to one restricted but significant problem that of relating individuals to a social whole by means of their wills, in a way such that means only rational and social will, and not arbitrariness or caprice or natural will they have a difficulty in common. Their dilemma, that is, is much the same insofar as both writers at once value the will, and fear it, as the source of freedom on the one hand, and of mere wilfulness on the other. (What Rousseau says about the barrenness and destructiveness of the egoistic will in the 1conomie Politiquel surely finds a complex echo in Hegel's Phenomenology.) Comparison of the two from other points of view would, of course, yield only negative relationships: Hegel was the chief defender of the rationality of the large modern states which Rousseau hated, and his universal civil-service comes close to the government by clerks that Rousseau detested equally.2 Still, it is worth noting that, however critical of Rousseau Hegel often was, he rarely failed to point out that Rousseau had been right in basing his theory of the state on the idea of will, that he had, however, not succeeded in finding a proper content for the will, or a proper means of arriving at such a content.3 The problem of the content of the will is always critical in Hegel, and there is throughout his political writings an unrelenting attack on the merely abstract or one-sided view of the will in its numerous forms: as mere independence or differentiation of the ego from the outside world, as Stoic indifference to everything but

Some current developments in radio systems for sounding ionospheric structure and motions

Two new developments in radio pulse sounding of the ionosphere are described with emphasis on some advantages available from modern digital techniques. The current stage of development of a digital ionosonde (dynasonde) is illustrated, showing the effectiveness of a simple logical process for distinguishing echoes from noise. Another digital radio-sounding system, including instrumentation (the kinesonde) and data analysis methods, has been developed to provide a statistical description of ionospheric motion and microstructure. From digital recordings of the time variation of complex echo amplitude at four antennas and six frequencies, a multidimensional correlation (correloid) analysis provides a profile of the three-dimensional mean motion, random (characteristic) velocity, shape, size, anisotropy, and lifetime of irregularities in the radio echo pattern; these irregularities can be related to their counterparts in the ionosphere.

The heuristics of cepstrum analysis of a stationary complex echoed Gaussian signal in stationary Gaussian noise

We consider cepstrum analysis of a model consisting of a detector receiving a Gaussian single complex echoed signal in Gaussian noise of the form x(t) + \beta[\cos \theta x(t- \tau) + \sin \theta x_{H}(t- \tau)] + n(t), where \beta is the echo amplitude, \theta an arbitrary constant, and x_{H}(t - \tau) is the Hilbert transform of x(t - \tau) . We assume no a priori knowledge of the spectra of the signal and noise except that they are smooth. The cepstrum is obtained as follows. We compute the log power spectral density estimate Â(f) of the received echoed signal and noise. The estimate Â(f) equals the true log spectral density Â(f) plus the sampling fluctuation of the log spectral density estimates. The echo produces a ripple of frequency \tau in the log spectral density. Â(f) is filtered to remove the slowly varying spectrum components and the result is analyzed by power spectrum estimation procedures to yield the cepstrum in which there is a peak due to the echo. To assess performance, we compare the cepstrum due to the log spectrum sampling fluctuation with the cepstrum peak due to the echo. The echo detectability for a given value of \tau can then be found by assuming the cepstrum estimates due to the log spectrum sampling fluctuations alone to he distributed as a scaled chi-square, and when an echo is present the distribution is assumed to be a scaled noncentral chi-square. We compare a special case of these results to an adaptation of a maximum likelihood procedure discussed by Whittle, for which \theta = 0 , and find that the cepstrum is only 1.8 dB worse for the echo amplitude and data length assumed. This degradation is not surprising since the cepstrum is equally effective for a complex echo in which \theta is not known a priori. Results of computer experiments using irregular spectra are presented which indicate the superiority of the cepstrum over autocovariance.