Frequency-dependent Sensitivity Research Articles

Broad band and absolute measurement of transient dynamic normal velocity of surface

Broad band direct sensing of the transient dynamic normal velocity of an object surface and evaluating its absolute value were realized by using a sensor fabricated with a piezoelectric polyvinylidene fluoride film and a polyvinyl chloride (PVC) back load and a PVC wear plate. The transient output signal from the sensor obtained by a test employing a well-defined steplike force, at the epicenter of a steel plate, showed very good agreement with the dynamic normal velocity calculated by using a Green’s function and a simulated source function. The output was proportional to the dynamic normal velocity of the surface, and the frequency-dependent sensitivity for the velocity was flat within a deviation of ±3.8 dB for the average in the frequency range up to 2 MHz. The transient dynamic normal velocity of the surface could be absolutely determined by using a sensor calibrated by theoretical one.

A resonant, dielectric micro-motor driven by low ac-voltages (<6V)

An asynchronous, dielectric micro-motor consisting of an Al-SiO2 rotor of 50 to 200 μm diameter was driven with 4 or 8 circularly arranged electrodes in water. The motor elements were fabricated by micromachining. Each electrode was designed to be an oscillatory circuit with a sharp resonance frequency in the upper kHz-range. The resonances at all electrode tips increase the voltage in the stator-rotor gap by a factor of up to 15. As a result, the dielectric rotor operates in a sharp frequency window at 5 Vpp excitation as if driven with more than 75 Vpp. With square wave pulses, the higher order Fourier-components induce several changes in rotor spin direction within two frequency decades. The micro-motor has been driven for hours without noticeable wear. It developed high starting torques and was useful for circular water pumping. The system has the advantage of very sensitive frequency dependence and the low drive voltages (below 10 V) can be produced by most electronic circuits.

Stochastic Resonance with Sensitive Frequency Dependence in Globally Coupled Continuous Systems

A model of globally coupled bistable continuous systems with two series of cells, modeling competing processes between activators and suppressors, subject to periodic and stochastic forces is investigated. In the large system size limit the exact solution of linear response theory is obtained, which shows typical stochastic resonance (SR) behavior. In particular, we find a resonance, sensitively depending not only on noise intensity and coupling strength, but also on the frequency of the periodic forcing. The SR occurs at a noise-induced Hopf bifurcation point, and is associated with the divergence of linear response.

Dipole logging in cased boreholes

Shear wave logging through casing can be challenging, especially when the surrounding formation is unconsolidated. This study identifies two possible causes of the problem using synthetic microseismograms, dispersion curves, signal processing scheme based on Prony’s method, and frequency-dependent sensitivity coefficients with respect to the layers’ parameters. The first possible cause is eccentering of the tool when the cased hole is well bonded. The second possible cause is poor bonding, which is modeled by the presence of fluid annulus. An off-center dipole source located in a well-bonded cased hole generates a Stoneley wave that is poorly coupled to the surrounding formation and propagates at a velocity close to the compressional velocity of the borefluid. This Stoneley wave can obscure the formation shear wave signal associated with a slow (unconsolidated) formation. In a vertical well-bonded cased hole, shear wave logging slow (unconsolidated) formations can be reliably performed despite the leaky character of the flexural mode if the tool is well centralized, and a low-source frequency (around 2 kHz) is used. In a highly deviated or horizontal well, where the tool cannot be efficiently centralized, reliable measurements can still be obtained provided the directivities of the source and receivers are perpendicular to the direction of eccentricity. In poorly bonded cased holes, a dipole tool, regardless of its position, excites supplementary low-velocity modes that are associated with vibrations of the steel casing and the presence of an external fluid annulus. The maximum excitation of these modes is located in the low-frequency region, similar to that of unconsolidated formation signals. When the cement is not bonded to the casing, depending on the thickness of the fluid annulus, an unconsolidated formation signal may be corrupted, regardless of the tool’s position within the borehole. The Stoneley wave generated by the off-center source is overpowered by the annulus mode. Even when no interference occurs, the supplementary mode exhibits high, low-frequency energy due to a strong low-frequency coupling with the casing which causes the formation signal to be weak, regardless of the type of formation (fast or slow). As a consequence, the source must be powerful and the recording system must have a large dynamic range for the formation signal to be reliably detected. When the cement is well bonded to the casing but not to the formation, the effects of the presence of the annulus mode are less drastic.

Components of the dynamic response of mammalian muscle spindles that originate in the sensory terminals

One component of the dynamic response of muscle spindles is characterized by a phase lead and frequency dependent sensitivity in response to sinusoidal stretches at frequencies around 1 Hz. Possible mechanisms producing this component, designated the "mid-frequency" dynamics, were investigated by testing the hypotheses that they arise from the mechanical behavior of the intrafusal muscle and alternatively from within the sensory terminals. Destruction of the myofibrillar structure of the intrafusal muscle fibers did not alter the mid-frequency dynamics, indicating that they do not arise from viscoelastic properties of the intrafusal muscle. An Arrhenius plot of the temperature dependence of the mid-frequency dynamics yielded an equivalent activation energy of 6.5 Kcal/M in the temperature range 23-42 degrees C and a 3-fold higher activation energy at lower temperatures. These observations are consistent with a dynamic process associated with a membrane-bound biochemical process. The addition of Ca++ and Ca(++)-activated-K+ (K(Ca] channel blockers (ZnCl2, Apamin and TEA) to the bathing solution altered the response dynamics by reducing the mid-frequency phase lead. The results suggest a negative feedback on the membrane potential generated by K+ efflux following a Ca++ influx that opens K(Ca) channels. A quantitative model fit to the experimental data yields a time constant of about 80 ms representing the limiting process associated with activation of the K(Ca) channels in this system. The results indicate that the mechanism underlying the mid-frequency dynamics includes at least two processes: one, not identified in this study, generates the phase lead and another, involving Ca++ and K(Ca) channels, provides a negative feedback that modifies the phase lead.

Acoustic multipole logging in transversely isotropic poroelastic formations

The properties of the modes generated by multipole sources in a fluid-filled borehole embedded in (radially layered) formations that include transversely isotropic poroelastic layers are investigated. These layers are modeled following Biot’s and homogenization theories, under the assumption that the principal axes of symmetry of both the transversely isotropic skeleton and the complex permeability tensor are parallel to the vertical axis of the borehole. A general formulation based on the Thomson–Haskell method accounts for any combination of fluid, elastic, and poroelastic layers, either isotropic or transversely isotropic. The investigation is achieved through the computation of dispersion and attenuation curves, and frequency-dependent sensitivity coefficients with respect to the parameters. In the presence of a fast or slow radially semi-infinite transversely isotropic poroelastic formation with an impermeable borehole wall, the shear-wave transverse isotropy of the skeleton can be determined using the low-frequency parts of the fundamental modes generated by a monopole source and one of higher order. With a permeable interface, the fluid flow effects refer to the horizontal mobility (horizontal permeability/saturant fluid viscosity) and disables the determination of the shear-wave transverse isotropy. Without this information, any horizontal mobility determination is incorrect. Whatever the nature of any fluid–poroelastic interface, the presence of radial layering decreases the reliability of any estimation of parameter related to the virgin formation except for the velocity and attenuation of the vertically propagating SV wave. Whatever the configuration, the transverse isotropy of the compressional wave velocity cannot be determined nor can that of the complex permeability tensor or the anelastic attenuation due to the polarization of the (quasi) body waves involved. The results of this work also show that, in contrast to the pure elastic situation, a transverse isotropy of the complex permeability tensor leads to slightly different quasi-SV-wave velocities along the principal directions of propagation at ultrasonic frequencies.

An Investigation into the Microstructure of the Low Frequency Auditory Threshold and of the Loudness Function in the near Threshold Region

The minimum audible field (MAF) is the sound pressure level at the threshold of audibility. The MAF threshold contour clearly demonstrates the variations in sensitivity of the auditory system with frequency. This frequency dependent sensitivity is also apparent at higher intensities, the importance of which is demonstrated by the extensive use of the dB(A) weighting in noise measurements. The sparse data available on low frequency auditory thresholds, and on the subjective effects of low level low frequency noise in the threshold region, indicated that a study of low frequency thresholds and near threshold equal loudness contours would fill a significant gap in the understanding of sound perception in this frequency range. This investigation demonstrates the existence of wide variations in individual sensitivity to low frequency sound. The diversity in auditory response to low frequencies between individuals should therefore be a prime consideration in low frequency noise control.

Nonlinear Optical Materials Based on Benzobisthiazole. Electronic Structure/Molecular Architecture/Polarizability/Hyperpolarizability Relationships Derived from π-Electron Theory

ABSTRACTFactors contributing to the polarizability (αij) and frequencydoubling hyperpolarizability (βijk) of 2-(p-dimethylaminophenyl)-6-(pnitrophenyl) benzo(l,2-d:4,5-d′ )blsthiazole (DNBT) are analyzed via perturbation theory and the PPP-SCF-MECI π-electron model Hamiltonian. While the observable part of β (βvec) is clearly identifiable with a small number of charge transfer excitations along the molecular dipole direction, a (and by inference, the second-order hyperpolarizability γ) is more closely related to the overall size (volume) of the π-electron cloud. As a consequence, βvec is far more sensitive to molecular distortions which affect donor-acceptor charge transfer interactions than is π. The more sensitive frequency dependence of βvec can be understood in terms of the three-photon character of this nonlinearity.

Directional hearing in the grassfrog (Rana temporaria L.). II. Acoustics and modelling of the auditory periphery.

In an earlier paper (Vlaming et al., 1984) we reported on optical measurements (laser-doppler interferometry) of the vibrations characteristics of the grassfrog's tympanic membrane. In the present paper these measurements were extended to include acoustic measurements concerning the functional role of the mouth cavity in frog hearing. Based on these measurements a model of the frog's acoustic periphery, consisting of three coupled linear oscillators with three entrance ports for sound, was developed and analyzed mathematically to give the various relevant transfer functions. The model is characterized by six parameters, all of which could be estimated from the available experimental data. For frequencies up to some 1500 Hz the model adequately describes the experimental data, both our own and earlier, seemingly conflicting data in the literature. For higher frequencies deviations occur, possibly due to nonuniform vibrations of the membranes. The model was used to evaluate the monaural directional sensitivity of the frog under free-field stimulation. Essentially it behaves as a combined pressure-gradient receiver, with highly frequency-dependent directional sensitivity. Directional sensitivity of the tympanic membrane could be modulated drastically by changing the resonance properties of the mouth cavity, without affecting the intrinsic membrane properties. This, theoretically, allows the frog to manipulate its direction sensitivity by actively tuning the volume of its mouth cavity. In order to account for discrepancies with known properties of low-frequency auditory nerve fibers an additional, extra-tympanic channel was included into the model. The extended model, the second-channel possibly involving the opercularis complex, provides a good quantitative fit to the available data on tympanic membrane movement as well as auditory nerve activity. Finally, the model enables to simulate a (moving) sound source in space, while stimulating the frog via closed couplers.

Investigating the Steady State Visually Evoked Cortical Response

Steady state visually evoked cortical responses were obtained by sinusoidally modulating fluorescent lights. Sums of sine waves with a frequency range of 5.5 Hz to 49 Hz were presented simultaneously to subjects to allow a control theoretic examination of the describing function of the EEG response. Subjects observed the stimulus in a “lights only” condition and were also tested viewing the flickering lights while doing a video decision task presented in the same field. Results indicate that reconstruction of the describing function across several frequencies was consistent for replications of each condition. Differences between the decision condition and lights only were observed around 9.5 Hz to 11.5 Hz. Frequency dependent sensitivity to the evoking stimulus was observed, and individual differences were indicated in frequency sensitivity to the evoking stimuli.

The development of some peripheral and central auditory responses in the neonatal cat

The development of cochlear nerve action potential thresholds at different frequencies (AP audiograms) and inferior colliculus (IC) single unit thresholds and tuning was examined in barbiturate-anaesthetized kittens. AP thresholds decreased over the whole frequency spectrum during the first 5 weeks of life. Threshold to high-frequency stimulation remained higher in 7-week-old animals than in adults. These results are in contrast to previous reports which have suggested that the AP response and gross cochlear anatomy are mature by the end of the second week. These differences may be due to the fact that the AP audiogram technique provides a measure of the activity of discrete regions along the cochlear partition. IC units in animals younger than 3.5 weeks had significantly elevated thresholds and broader tuning than those of the adult cat. Comparison of AP audiogram and IC unit thresholds in the adult revealed that these indices show similar frequency-dependent sensitivity. The slower maturation revealed by the AP audiogram may be due to the greater number and/or synchrony of cochlear nerve discharges needed to produce the gross AP. If this were the case, perception of suprathreshold sounds might not develop as quickly as thresholds for sound detection.

Influence of end conduction on the sensitivity to stream temperature fluctuations of a hot-wire anemometer

The influence of end conduction on the sensitivity to stream temperature fluctuations of a constant current hot-wire anemometer operated at low overheat has been investigated using a newly developed dynamic, cross-correlation calibration technique. A lumped parameter heat-transfer model of the wire and support system consistent with the experimental data is presented. Results show that for commonly used tungsten and platinum wires, a frequency dependent sensitivity to stream temperature fluctuations is obtained in air flows at low subsonic velocities. For measurements with an accuracy of approximately 5% under non-isothermal flow conditions, use of a wire of low thermal conductivity such as Pt 10% Rh with a length-to-diameter ratio of at least 400, irrespective of wire diameter, is recommended compared with 800 for equivalent accuracy with tungsten wire. The relevance of the results to velocity measurements with constant temperature hot-wire anemometers in non-isothermal flows is also discussed.

Signal-to-Noise Ratio of a Video Signal Transmitted by a Fiber-Optic System Using Pulse-Frequency Modulation

In this paper, we evaluate the weighted and unweighted signal-to-noise ratio of a camera-head signal which is transmitted by a fiber-optic system using pulse-frequency modulation (PFM). We take into account the camera noise characteristic determined by the camera preamplifier (FET-feedback type), the thermal and shot noise of the avalanche photodiode, and the thermal noise of the following preamplifier (FET-feedback type). All noise sources are superpositioned and then weighted according to the frequency-dependent noise sensitivity of the human eye.

An approach to the sensitivity and statistical variability of biquadratic filters

In attempting to predict the behavior of a filter during and at the end of its life, one is led to the study of sensitivity and then one must compare worst-case and expected results. This paper shows that sensitivity can be expressed as a sum of terms, where each term is the product of two sensitivity functions. One is the frequency-dependent sensitivity of the gain to the transfer function coefficients (the gain-to-coefficient sensitivity) and the other is the well-known coefficient-to-component sensitivity. The gain-to-coefficient sensitivity clearly shows that the gain of a biquadratic function is far more sensitive to changes in the resonant frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_0</tex> than to changes in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</tex> only near the 3-dB frequencies. The gain is actually less sensitive to changes in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_0</tex> near <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_0</tex> . It is also shown that coefficient-to-component sensitivities for resistors and capacitors have no effect on the mean value of the change in the gain, but have marked effects on the standard deviation.

The deflection sensitivity of traveling-wave electron-beam deflection structures

A combination experimental-analytical method for easily determining the frequency-dependent deflection sensitivity of traveling-wave deflection structures is described. Experimental measurements have been made to verify the accuracy of the proposed method.

Responses to Acoustic Stimuli from Single Units in the Eighth Nerve of the Bullfrog

Single unit recordings from the eighth nerve of the bullfrog reveal two strikingly different kinds of auditory units. Both kinds of units exhibit sharply frequency dependent sensitivity (tuning curves). “Simple” units are maximally sensitive to tone bursts of frequency between 1000 and 1500 cps. Their frequency sensitivity and their capacity to “follow” pulse stimuli are well-matched to the spectral composition and pitch period, respectively, of the bullfrog's croak. Simple units cannot be inhibited by acoustic stimuli. “Complex” units are inhibited by acoustic signals in the frequency range from 300 to 1000 cps. Some complex units are excited by acoustic stimuli, some are excited by both acoustic and vibratory stimuli, and some are excited by vibratory stimuli alone. Complex units that can be excited by sound are most sensitive to frequencies between 200 and 700 cps. Simple and complex units probably derive from separate sense organs (the basilar and amphibian papillae, respectively) within the otic capsule. There is evidence that all of these responses are from primary neurons; moreover, it has been shown that the inhibition is not under efferent control. Extremely sensitive units of both types are found with thresholds comparable to those of human listeners. Responses to tone bursts, noise bursts, click trains, and frog croaks are compared. The roles of spectral and temporal pattern of the stimulus in determining the response of a unit are explored.

Evidence for Two Auditory Receptor Organs in the Inner Ear of the Bulffrog

Exceedingly sensitive structures for detecting airborne sound are found within the labyrinth of many vertebrates. The inner ear of the amphibian contains two sense organs that have long been regarded as auditory transducers because of their structures and locations, although there has been no supporting physiological evidence. One of these, the basilar papilla, is a phylogenetic precursor of the cochlea. The other, the amphibian papilla, is found only in amphibia. Single-unit recordings from the branches of the eighth nerve of the bullfrog (R. catesbeiana) demonstrate that both the basilar and the amphibian papillae are sensitive to auditory stimuli. Units arising from each organ exhibit sharply frequency-dependent sensitivity (tuning curves), but the frequency of maximum sensitivity (best frequency) for units from each papilla occurs in a different range. Basilar papilla units (termed “simple”) have best frequencies between 1000 and 1500 cps. It is noteworthy that these units cannot be inhibited by acoustic stimuli. Amphibian papilla units (termed “complex”) are more varied in type: among them are auditory units, with best frequencies below 700 cps; traits that respond both to low-frequency auditory and to vibratory stimuli; and units that respond only to vibration. Activity from all of these units can be totally inhibited by sufficiently intense tone bursts of frequency from about 300–1000 cps. It has been shown that this inhibitory interaction is not under efferent control.