Features In Power Spectrum Research Articles

Effect of random frequency modulation on ECG power spectra.

Electrocardiographic signals are an example of biological signals whose fundamental characteristics may be altered by environmental factors. Frequency modulation due to the influence of respiration and other factors may alter the signals and affect the characteristic power spectrum. Here the effects of random frequency modulation on ECG power spectra are examined in two cases: when there is random phase modulation in the periodic Fourier series model and when the consecutive cycles for a given sequence of beats deviate from periodicity due to random variation in the fundamental frequency about its mean. It is concluded that the differential effects of random frequency modulation on the power spectrum can be detected and its contribution defined. Thus, the underlying power spectrum of identical signals with and without random phase and frequency modulations can be defined.

Some characteristics of the development process of the wind-wave spectrum

The development process of wind-waves of which spectral peak distributes from 0.6 cps to 9.3 cps will be discussed on the basis of the wind tunnel experiments and of the field observations performed at Lake Biwa. The characteristics of power and slope spectra are here presented. The development process of these wind-waves is characterized by three stages;i.e. “initial-wavelets”, “transition stage” and “sea-waves”. In the wind tunnel experiments, the transition from the stage of the “initial-wavelets” to the “transition stage” occurs when the wave spectral peak arrives at the line 6.40×10−4k−2cm2·sec (wherek is wave number) or when the slope spectral density at the frequencyfmax becomes larger than 6.40×10−4 sec. In the stage of sea-waves, the component wave of a wave-spectral peak is steepest in the component waves. And the wave spectral peak develops along the line 1.02×102f−6 cm2·sec (wheref is the frequency corresponding to the wave numberk) untill it reaches the line 33.3f−4cm2·sec, and thereafter develops along the latter line, which indicates the constant density of slope spectrum. It is suggested that the nonlinearity of wind-waves must become stronger as wind-waves develop. The effective momentum fluxΤ ws from the air flow to wind-waves in this stage is evaluated to be about 4∼9% of the total stressΤ0.

A FOURIER METHOD FOR THE REGIONAL‐RESIDUAL PROBLEM OF POTENTIAL FIELDS*

AbstractAn equivalent stratum model is used to provide an explanation for the power spectrum characteristics of potential fields.The power spectrum of observed magnetic fields is found to consist of effects due to sources which can be represented by apparent monopoles and dipoles. Similarly, the power spectrum of observed gravity fields is found to consist of two groups of effects. A matched filter is proposed whereby the effects due to the two components in the potential field can be separated. As a consequence of the established theoretical expressions a scheme is suggested whereby the aliasing power of sampled data can be estimated. Also, the concepts of downward continuation, reduction to the pole, and reduction to pseudo‐gravity of magnetic fields are re‐examined in light of the theoretical expressions due to the equivalent stratum model.

8. High Frequency Stellar Oscillations: IV: Photoelectric Monitoring of Southern White Dwarfs

Time-series data for 14 stars in the list of Eggen and Greenstein have been used to compute their power spectra, which confirm previously found quiescency in the 4 to 700 sec period range. Additionally, characteristics of the continuous power spectra are considered.

STATISTICAL MODELS FOR INTERPRETING AEROMAGNETIC DATA

A mathematical basis for the application of power spectrum analysis to aeromagnetic map interpretation is developed. An ensemble of blocks of varying depth, width, thickness, and magnetization is considered as a statistical model. With the use of the fundamental postulate of statistical mechanics, a formula which can be used to analyze the power spectrum of an aeromagnetic map is developed. The influences of horizontal size, depth, thickness, and depth extent of the blocks on the shape of the power spectrum are assessed. Examples which include power spectra of maps from Canada and Central America demonstrate the application of the approach. In the cases studied a double ensemble of blocks appears to best explain the observed power spectrum characteristics.

Noise Power Spectrum Characteristics for an He–Ne Laser Operating under Various Discharge Conditions

This paper describes experiments performed to measure the noise characteristics of a helium-neon laser as a function of discharge characteristics. The discharge properties were varied by changing the pressure and current over a wide range of values. The effect of cold vs hot cathodes on the noise characteristics was also measured. Optimum regions are obtained where the laser has low noise characteristics. A large amount of data is included on noise bandwidth, laser power output as a function of pressure and current, discharge impedance characteristics, etc.

Optimum Damping in a Randomly Excited Non-Linear Suspension

Analogue computer studies of an automobile suspension on a simulated random road show that optimum ride and road holding can be achieved with linear damping for all magnitudes of road roughness. Unsymmetrical damping, however, provides better isolation from large bumps and obstacles at the expense of only very moderate increases in the mean-squared values for random inputs. Optimum values for the ratio of bump to rebound damping rates are obtained by use of an integral-square criterion. For a linear system the effects of the seat dynamics and road power spectrum characteristics are illustrated using the results of a digital computer program. The influence of the non-linearities on mean-square values is analysed theoretically and the statistical describing function method applied.

Power spectra measurements for spacecraft vibration data.

Time varying power spectrum characteristics of spacecraft launch vibration data noting error reduction in measurements

Electropneumatic Acoustic Generator

Research and development leading to an electropneumatic acoustic generator capable of producing in excess of 6 KW of acoustic power is reviewed. The modulation of pneumatic power through an electrodynamically controlled valve provides a transducer capable of producing either random or complex periodic signals. The development of this valve required considerable investigation of materials to establish the necessary compliance and optimized moving valve structure to obtain a reliable sound generator. The efficiency of this generator with respect to the pneumatic power supply is as high as 17%. The evaluation of the power spectrum, amplitude distribution, and nonlinear characteristics of the transducer is performed in a progressive-wave tube. Current developments include units that produce considerably more acoustic power or extend the useful frequency range of the sound generator.

Electropneumatic Acoustic Generator

Research and development leading to an electropneumatic acoustic generator capable of producing in excess of 6 kW of acoustic power is reviewed. The modulation of pneumatic power by the use of an electrodynamically controlled valve provides a transducer capable of producing either random or complex periodic signals. The development of this valve required considerable investigation of materials and fabrication techniques to establish the necessary compliance and optimized moving-valve structure to obtain a reliable sound generator. The efficiency of this generator with respect to the pneumatic power supply is as high as 17%. The evaluation of the power spectrum, amplitude distribution, and nonlinear characteristics of the transducer is performed in a progressive-wave tube. Current developments include units that produce considerably more acoustic power or extend the useful frequency range of the sound generator.

On the Spectrum of Optical Waves Propagated through the Atmosphere

It is well known from our day-to-day experience that distant objects appear to shimmer, especially on still, hot hays when relatiively large temperature and humidity gradients may exits. It is ture, however, that refractive index gradients are in the air to some degree at all times; that is, they may be present at night and also when the air is disturbed by the winds. It is not suprising therefore that the power received at some distance from an optical source fluctuates randomly and possesses a characteristic low-frequency power spectrum.

Study of nonindependent random pulse trains, with application to the barkhausen noise

This work covers the calculation of the correlation function and power spectrum of a train of pulses of exponential shape and random amplitude, distributed according to a law of probability experimentally found byH. Sawada in a research on the time interval distribution in the Barkhausen effect. All the features of the experimental power spectra of the Barkhausen noise, found by many authors to be contradictory with the interpretation of statistical independence of pulses, can thus be completely explained. The remarkable physical significance of the introduced distribution gives the results a more general interest.

Some statistical properties of random noise

A considerable volume of knowledge is now available on random fluctuations (noise) as regards the behaviour in amplitude. Familiar names in this field are those of Uhlenbeck and Ornstein(7), Fürth(1) and Rice (4), although very many others have made valuable contributions. A particular class of problem, of considerable practical importance, exists when the frequency spectrum is limited to a relatively narrow range. The resulting noise has then the character of a more or less regular oscillation modulated randomly in amplitude and phase. In this case, if we write the fluctuation in the form(where R(t) and θ(t) are variables changing slowly in comparison with sinω0t), it is clear that the magnitude of the envelope R(t) and the phase θ(t) are now the significant quantities. Rice (4), among others, has made a study of the statistical properties of R, deriving in particular the correlation function R(t) R(t + τ) in terms of the characteristics of the (power) spectrum ω(f). Fürth and the writer (2) have extended this work and carried out a collateral experimental investigation.