Upper Limit Of Frequency Research Articles

Mechanisms of active control of sound radiation from an opening with boundary installed secondary sources.

Previous work has demonstrated that installing secondary sources at the edge of a cavity opening can reduce sound radiation through it, but the mechanisms are not clear, which is investigated in this paper by using the modal decomposition method. It is found that a double layer edge system achieves better performance than a single layer system because secondary sources at the edge of the same layer cannot excite some modes effectively and those at different heights compensate this. There exists an upper limit frequency for the systems with boundary installed secondary sources, which is mainly decided by the length of the short side of the opening. More secondary source layers at the edge will increase the upper limit frequency.

A spectrally efficient equalization technique for optical sources with direct modulation.

In this Letter, a novel equalization method for directly modulated optical sources is introduced. Conventional source equalization methods balance the low- and high-frequency responses of the source by cutting down the low-frequency components in any drive signal. Typically, this ensures a flat frequency and linear signal response up to some predetermined upper-frequency limit. It is conventionally done under a fixed linear dynamic range. However, in this Letter, it is found that the source's dynamic range varies by frequency. We describe a novel method that determines the limit of signal linearity at each frequency and uses this to create the enhanced equalizer response. This leads to an improved source bandwidth and, in practice, allows greater transmitted signal energy. Experimental results for a resonant-cavity LED transmitter show date-rate improvement of ∼40% and, to the best of our knowledge, a record date rate of 8.76Gb/s with a bit-error-rate less than 3.8×10-3.

Vibro-acoustic metamaterial for longitudinal vibration suppression in a low frequency range

This study proposes a theoretical model of a vibro-acoustic metamaterial for longitudinal vibration suppression in a low frequency range and computationally and experimentally demonstrates the vibration attenuation performance of the proposed metamaterial. The vibro-acoustic coupling analysis is performed on a theoretical model in which a discrete vibration system and a short-length duct are periodically repeated. The transfer matrix method and the Bloch–Floquet theorem were developed to calculate the Bloch phase of a unit cell of the proposed vibro-acoustic metamaterial. Its stop band predicted from the Bloch phase commenced at 0 Hz and coincided with the frequency range of low transmissibility (<1). The effects of unit cell parameters on the upper limit frequency of the stop band are discussed, and the dispersion relation and effective mass density curves of the proposed vibro-acoustic metamaterial explain its underlying physics. The developed theoretical approach is extended to vibro-acoustic metamaterials including a continuous vibration system, instead of a discrete vibration system, for actual application. Finite element analysis and experiments on the extended vibro-acoustic metamaterials were performed to validate the vibration attenuation performance of the proposed metamaterial, which can be used to suppress longitudinal vibration waves transmitted between two mechanical parts.

Phase-locking in acoustic hearing: Upper limits, enhancement, and transformations

Phase-locking is a remarkable neural property of various mechanoreceptor-based sensory systems, and the neural coding of cochlear vibrations is the primary model system to study this phenomenon. Transduction and synaptic transmission by cochlear inner hair cells enables but also limits temporal coding in the auditory nerve. I will review phase-locking to sound fine-structure in the auditory nerve of various species, including humans, assessed with different types of sound stimuli and using different metrics. The upper-frequency limit of phase-locking in cell populations in the central nervous system is lower than that in the auditory nerve, and at the same time, the nature of the temporal code is transformed in these neurons: phase-locking is enhanced in that it is more consistent and has decreased jitter, and it has profound effects on the average discharge rate of various populations of neurons, both monaurally and binaurally. Similar phenomena (changing upper limits, enhancement, and rate effects) are observed in the temporal coding of envelopes and click trains. While it is increasingly clear that many cell groups in the auditory brainstem have specialized mechanisms towards temporal coding, the relationship of the resulting response properties to perception remains unclear.

On the Piezoelectric Detection of Guided Ultrasonic Waves.

In order to quantify the wave motion of guided ultrasonic waves, the characteristics of piezoelectric detectors, or ultrasonic transducers and acoustic emission sensors, have been evaluated systematically. Such guided waves are widely used in structural health monitoring and nondestructive evaluation, but methods of calibrating piezoelectric detectors have been inadequate. This study relied on laser interferometry for the base displacement measurement of bar waves, from which eight different guided wave test set-ups are developed with known wave motion using piezoelectric transmitters. Both plates and bars of 12.7 and 6.4 mm thickness were used as wave propagation media. The upper frequency limit was 2 MHz. Output of guided wave detectors were obtained on the test set-ups and their receiving sensitivities were characterized and averaged. While each sensitivity spectrum was noisy for a detector, the averaged spectrum showed a good convergence to a unique receiving sensitivity. Twelve detectors were evaluated and their sensitivity spectra determined in absolute units. Generally, these showed rapidly dropping sensitivity with increasing frequency due to waveform cancellation on their sensing areas. This effect contributed to vastly different sensitivities to guided wave and to normally incident wave for each one of the 12 detectors tested. Various other effects are discussed and recommendations on methods of implementing the approach developed are provided.

Separation of two attractive ferromagnetic ellipsoidal particles by hydrodynamic interactions under alternating magnetic field.

In applications where magnetic particles are used to detect and dose targeted molecules, it is of major importance to prevent particle clustering and aggregation during the capture stage in order to maximize the capture rate. Elongated ferromagnetic particles can be more interesting than spherical ones due to their large magnetic moment, which facilitates their separation by magnets or the detection by optical measurement of their orientation relaxation time. Under alternating magnetic field, the rotational dynamics of elongated ferromagnetic particles results from the balance between magnetic torque that tends to align the particle axis with the field direction and viscous torque. As for their translational motion, it results from a competition between direct magnetic particle-particle interactions and solvent-flow-mediated hydrodynamic interactions. Due to particle anisotropy, this may lead to intricate translation-rotation couplings. Using numerical simulations and theoretical modeling of the system, we show that two ellipsoidal magnetic particles, initially in a head-to-tail attractive configuration resulting from their remnant magnetization, can repel each other due to hydrodynamic interactions when alternating field is operated. The separation takes place in a range of low frequencies f_{c1}<f<f_{c2}. The upper frequency limit f_{c2}τ_{r}≈0.04 (where τ_{r} is the rotation time scale) depends weakly on the ratio of magnetic field to particle magnetization strength, whereas f_{c1} tends to zero when this ratio increases.

Rapid impedance tube determination of fabric transparency

Acousticians are continually being asked to verify fabric transparency for applications with absorptive and diffusive surfaces, as well as in sound reinforcement. Standard reverberation chamber methods can be used, but require large fabric and fiberglass samples. A quick and simple impedance tube method has been developed requiring only a 160 mm x 160 mm sample. Two measurements are made. One with an anechoic wedge termination and another with a rigid termination in which the fabric is applied to a 50 mm 6pcf fiberglass substrate. Four microphones are placed at a quarter and three quarters of the square tube’s width and height and the signal is summed. This unique microphone placement minimizes the 1st, 2nd, and 3rd harmonics resulting in a fourfold increase in the upper frequency limit. Impulse response measurements are made at three distances from the sample to calculate the reflection factor, impedance and normal incidence absorption coefficient from 63 Hz to 4000 Hz in one measurement. The small fabric sample is large enough to minimize non-homogeneous effects. A study of fabrics with a wide range of transparencies reveals how both acoustically transparent and backed fabrics can be used as sonic equalizers depending on the application.

Postprocessing Phase Stabilizer for Wide Frequency Range Photonic-Microwave Signal Distribution

In the field of leading-edge techniques, such as broadband communications and large-scale radio interferometer applications, the need for a highly stable/wide-frequency-range photonic-microwave signal (continuous sine wave) is increasing. The main purpose of this research is to provide a highly stable photonic-microwave signal over a large broadband from microwave to terahertz frequencies using the same optical signal generation/transmission system. While transmitting signals through an optical cable, the transmission delay fluctuates and it suffers from chromatic dispersion that affects the distribution signal coherence. We have developed a postprocessing scheme transmission phase stabilizer. In the scheme, the measured phase of the round-tripped signal is acquired at time intervals at the optical transmitting port (an optical input port), and then the measured phase data are sent to the destination port (an optical output port) for the transmission phase compensation. The postprocessing method has advantages for a frequency dynamic range and compensation range issue. Theoretically, there is no upper frequency limit on the proposed phase stabilizer. We also discuss a high-frequency signal generator based on an optical comb signal generator up to terahertz frequencies.

Special requirements of high frequency current transformers in the on-line detection of partial discharges in power cables

The insulation condition of power cables is closely associated with partial discharge (PD) [1]. Early signs of cable insulation failure such as from electrical treeing are revealed by PD detection [2], and insulation failure will eventually lead to breakdown [3]–[5]. For AC cross-linked polyethylene (XLPE) cables, online PD detection is used to determine potential defects in the cable, and high frequency current transformers (HFCT) are most commonly applied for convenient and noninvasive detection. Conventional PD measuring techniques have been specified in IEC 60270 [6], and cable factory tests are based on this method. When PD detection is conducted on cables in service, some special issues such as noise reduction and dynamic range have to be dealt with [7]. Complex electromagnetic interferences and noise may significantly affect detection results by reducing the detection sensitivity [8]. Some high frequency components of PD signals cannot be covered by the detection bandwidth of a measuring instrument because the upper limit frequency is not high enough [9]. One more challenge is that a long cable can cause significant attenuation of PD signals [10]–[12], which will decrease the signal amplitudes to be detected. Therefore, the PD measuring instruments for on-line detection for cables have to satisfy some additional requirements.

Discrimination of virtual sound fields different in spatial aliasing

There are theoretical techniques for sound field reproduction with a loudspeaker array. However, physical precision of synthesized sound field is limited in frequency domain. When the distance among loudspeakers is below a half of wavelength, spatial aliasing will occur. The ideal distance among loudspeakers was less than 8.5 mm@20 kHz, and it is impossible to arrange loudspeakers. Therefore, we carried out experiments to know an upper-limit frequency condition, which can give subjective experience identical to an ideal sound field. We assumed a virtual spherical boundary around a listener. Numerous virtual loudspeakers were set on the boundary to simulate a WFS system. Room impulse responses and head-related impulse responses were calculated by computer simulation. Then, binaural impulse responses were synthesized as combinations of RIRs and HRIRs. We picked up the boundary points so that upper-limit frequency without the spatial aliasing of BRIRs was systematically controlled. Listening test was conduct...

Relating Click-Evoked Auditory Brainstem Response Waveforms to Hearing Loss in the Bottlenose Dolphin (Tursiops truncatus)

Hearing sensitivity in captive Atlantic bottlenose dolphins was assessed using a portable electrophysiologic data collection system, a transducer attached to the pan region of the mandible, and non-invasive recording electrodes. The auditory steady-state response (ASSR) was evoked using sinusoidal amplitude-modulated tones at half octave steps from 20–160 kHz and utilized to determine the upper frequency limit of hearing (i.e., the frequency at which threshold was ≤ 120 dB re 1 μPa). An auditory brainstem response (ABR) was then recorded to a moderate-amplitude click (peak-equivalent sound pressure level of 122 dB re 1 μPa) and examined to determine if relationships existed between the upper frequency limit of hearing and the waveform characteristics of the click-evoked ABR. The ASSR and click-evoked ABR were measured in 6 bottlenose dolphins with varying hearing sensitivity and frequency range of hearing. A significant relationship existed between click-evoked ABR wave amplitudes and the upper frequency limit of hearing. Test times for assessment using frequency-specific ASSR and click-evoked ABR were ∼45 minutes and 1 minute, respectively. With further definition of normative data, measurement of click-evoked ABRs could form the basis of an expedited electrophysiologic method for hearing screening in marine mammals.

On the Spectrum and Generation Regions of Solar Microbursts in the Decimeter Wave Band

We analyze the nature and physical conditions in the generation regions of decimeter microbursts (MBs), which were discovered with the radiotelescope of the Russian Academy of Sciences (RATAN-600). One of the main peculiarities of MBs is an almost constant upper-frequency limit of about 1.1 GHz, which has not been explained in previously studied generation models. Here it is shown that this spectral peculiarity can be explained by the generation of the upper-hybrid waves at the double plasma resonance (DPR) and a subsequent transformation into low-frequency plasma waves considering free–free and cyclotron absorption. Model calculations show that MBs occur in the active regions where the magnetic-field strength is close to 100 G. MBs are most probably generated in the transition region of the solar atmosphere between main magnetic fields with opposite polarities.

Verification of mathematical formulae applied to overhead stage canopy design

Abstract This paper presents a comparison of the experimental research concerning overhead stage canopies with a numerical approach based on selected mathematical models. The numerical predictions are made using the simplified asymptotic curves suggested by Rindel and modified by Skalevik. For singular cases a prediction with detailed calculations based on the Fresnel–Kirchhoff approximation is also given. The aim of the work is to verify proposed algorithms for designing reflective panels as well as to determine the conditions of conducting such procedures. It is shown that based on Rindel’s approximation one may determine some substantial information about sound reflection from the panels i.e. the value of upper limit frequency as well as the relative sound reflection level. On the other hand, the lower cut-off frequency should be calculated using Skalevik’s model as the value obtained from Rindel’s formula is undervalued. Such an approach could be applied to design reflective structures. However, it has some limitations for example for arrays of perturbed symmetry or sparse arrays as well as in the case of non-perpendicular angles of sound wave incidence. Then it may be necessary to apply more accurate numerical models.

On the Image Model of a Buried Horizontal Wire

It is common practice in the engineering analysis process to use an approximate image method for the computation of the current in buried horizontal conductors (in the literature, this is often referred to as the “modified image” or “reflection coefficient” method). According to this approach, the earth/air interface is replaced by a positive mirror image of the charge and current in the conductor, and its field is multiplied by a suitable reflection coefficient. Different opinions on the validity of this approximation have been expressed in published debates, but more systematic analysis of the error introduced by this approach is not available in the literature. To establish the amount of error, we compare the computation results of the image model with the rigorous Sommerfeld integral method for a wide range of parameters. Contrary to widespread opinion, our results suggest that the modified image (or reflection coefficient) method in EFIE-based solutions (e.g., the Pocklington equation) leads to a large error (larger than 20%) in the low-frequency range for the computation of the current distribution in conductors longer than 10 m. In such a case, MPIE-based methods are preferred for use to achieve a smaller error (approximately 5%). Guidelines for the application of image models related to the conductor, earth and excitation parameters, upper frequency limit, and modeling method are presented.

Maximizing the value of gate capacitance in field-effect devices using an organic interface layer

Past research has confirmed the existence of negative capacitance in organics such as tris (8-Hydroxyquinoline) Aluminum (Alq3). This work explored using such an organic interface layer to enhance the channel voltage in the field-effect transistor (FET) thereby lowering the sub-threshold swing. In particular, if the values of the positive and negative gate capacitances are approximately equal, the composite negative capacitance will increase by orders of magnitude. One concern is the upper frequency limit (∼100Hz) over which negative capacitance has been observed. Nonetheless, this frequency limit can be raised to kHz when the organic layer is subjected to a DC bias.

Determining optimal equivalent source positions in wave superposition method by self-adaptive searching algorithm

The calculation accuracy of wave superposition method is influenced greatly by equivalent sources, their positions, and the frequency. What’s more, nonuniqueness occurs at eigenfrequencies when monopole or dipole is used as the equivalent source. Although tripole can overcome the nonuniqueness, the computational overhead is very large due to its complex expression and the accuracy is still affected greatly by the positions. A method is proposed to reduce the calculation errors. In this method, monopole, which is the simplest source, is used as the equivalent source. The upper limit frequency for a radiator under a certain meshing pattern is predicted by using the fictitious pressure generated via a reference source. Then, the optimal equivalent source positions for each frequency corresponding to the actual vibration velocity boundary condition, in which the average rate of pressure change is minimal, are determined within the upper limit frequency by a searching algorithm. Numerical simulation results of a complex structure show that the calculation errors are significantly reduced. At last, the method is verified by an experiment for a cuboid radiator. Experimental results show that the proposed method is practicable and good at accuracy.

Relating click-evoked auditory brainstem response waveforms to hearing loss in the bottlenose dolphin (Tursiops truncatus)

Hearing sensitivity in captive Atlantic bottlenose dolphins was assessed using a portable electrophysiologic data collection system, a transducer attached to the pan region of the mandible, and non-invasive recording electrodes. The auditory steady-state response (ASSR) was evoked using sinusoidal amplitude-modulated tones at half octave steps from 20–160 kHz and utilized to determine the upper frequency limit of hearing (i.e., the frequency at which threshold was ≤ 120 dB re 1 μPa). An auditory brainstem response (ABR) was then recorded to a moderate-amplitude click (peak-equivalent sound pressure level of 122 dB re 1 μPa) and examined to determine if relationships existed between the upper frequency limit of hearing and the waveform characteristics of the click-evoked ABR. The ASSR and click-evoked ABR were measured in 6 bottlenose dolphins with varying hearing sensitivity and frequency range of hearing. A significant relationship existed between click-evoked ABR wave amplitudes and the upper frequency limit of hearing. Test times for assessment using frequency-specific ASSR and click-evoked ABR were ∼45 minutes and 1 minute, respectively. With further definition of normative data, measurement of click-evoked ABRs could form the basis of an expedited electrophysiologic method for hearing screening in marine mammals.

Source characterization of full-scale jet noise using vector intensity

Vector acoustic intensity has the benefit over pressure measurements in that both the direction and magnitude of energy flow are represented. However, this important quantity has seen little application previously in aeroacoustics. In the present work, an intensity probe captured the radiated field to the sideline and aft of a tethered, full-scale military jet aircraft as one engine was operated at multiple engine conditions. Intensity data from each probe location provide a frequency-dependent map of the sound flow near the aircraft. The vector acoustic intensity is estimated using a recently developed processing technique that extends the traditional upper-frequency limit on estimation accuracy. The intensity vectors are traced back to the jet centerline as a method of approximating the extent and location of the source region as a function of frequency. This method is validated by numerical simulation. As expected for jet mixing noise sources, the resulting source region estimates contract and move upstream with increasing frequency. In addition, the source region at afterburner compared to full-throttle is located about 1 m farther downstream at frequencies above about 300 Hz, and the intensity tends to point more to the sideline by up to 10°. [Work supported by ONR.]

Heat transport by phonons and the generation of heat by fast phonon processes in ferroelastic materials

Thermal conductivity of ferroelastic device materials can be reversibly controlled by strain. The nucleation and growth of twin boundaries reduces thermal conductivity if the heat flow is perpendicular to the twin wall. The twin walls act as phonon barriers whereby the thermal conductivity decreases linearly with the number of such phonon barriers. Ferroelastic materials also show elasto-caloric properties with a high frequency dynamics. The upper frequency limit is determined by heat generation on a time scale, which is some 5 orders of magnitude below the typical bulk phonon times. Some of these nano-structural processes are irreversible under stress release (but remain reversible under temperature cycling), in particular the annihilation of needle domains that are a key indicator for ferroelastic behaviour in multiferroic materials.

The Characteristic of Sound Reflections from Curved Reflective Panels

Abstract The paper presents the verification of a solution to the narrow sound frequency range problem of flat reflective panels. The analytical, numerical and experimental studies concerned flat panels, panels with curved edges and also semicircular elements. There were compared the characteristics of sound reflected from the studied elements in order to verify which panel will provide effective sound reflection and also scattering in the required band of higher frequencies, i.e. above the upper limit frequency. Based on the conducted analyzes, it was found that among some presented solutions to narrow sound frequency range problem, the array composed of panels with curved edges is the most preferred one. Nevertheless, its reflection characteristic does not meet all of the requirements, therefore, it is necessary to search for another solution of canopy which is effective over a wide frequency range.