Journal of The Audio Engineering Society, Oct 22, 2020
A formula for low-frequency interaural level difference (LF-ILD) as a function of source distance... more A formula for low-frequency interaural level difference (LF-ILD) as a function of source distance and direction is derived from rigid-sphere head-related transfer function (RS-HRTF) theory. Since ILD at low frequencies (typically f < 700 Hz) is a dominant cue for near-field distance perception in the free-field, a simple formula akin to the Woodworth formula for high-frequency interaural time difference, may be beneficial to easily and efficiently implement distance-and direction-dependent LF-ILDs for real-time spatial audio applications on devices with limited computational resources. By evaluating the limit as f → 0 of the infinite series representation of the RS-HRTF for finite source distances, an exact, closed-form expression for the "DC gain" as a function of source distance and direction is derived. For a given source location, using this expression to compute the DC gain at the left and right "ears," and then taking the ratio of the two quantities gives the desired LF-ILD. As an example, it is shown that the derived formula may be used to extrapolate far-field ILD spectra of a rigid sphere to the near-field exactly for low frequencies and with sufficiently high accuracy for higher frequencies. Furthermore, the derived formula, like the Woodworth formula, is well-suited to individualization.
Journal of The Audio Engineering Society, May 26, 2016
It is often desired that a transducer have a polar radiation pattern that is invariant with frequ... more It is often desired that a transducer have a polar radiation pattern that is invariant with frequency, but there is currently no way of quantifying the extent to which a transducer possesses this quality (often called “constant directivity” or “controlled directivity”). To address this problem, commonly-accepted criteria are used to propose two definitions of constant directivity. The first, stricter definition, is that the polar radiation pattern of a transducer must be invariant over a specified frequency range, whereas the second is that the directivity index (i.e., the difference, in dB, between the on-axis frequency response and the average response over all directions), must be invariant with frequency. Furthermore, to quantify each criterion, five metrics are derived: 1) Fourier analysis of contour lines (i.e., lines of constant sensitivity over frequency and angle), 2) directional average of frequency response distortions, 3) distortion thresholding of polar responses, 4) standard deviation of directivity index, and 5) cross-correlation of polar responses. These five metrics are computed using measured polar radiation data for four loudspeakers. The loudspeakers are then ranked, from most constant-directive to least, according to each metric, and the rankings are used to evaluate, by comparison with each other and with a predetermined “correct” ranking, each metric’s ability to quantify constant directivity. Results show that all five metrics are able to quantify constant directivity according to the criterion on which each is based, while only two of them, metrics 4 and 5, are able to adequately quantify both proposed definitions of constant directivity.
The dominant unstable electrostatic wave modes of an electromagnetically accelerated plasma are i... more The dominant unstable electrostatic wave modes of an electromagnetically accelerated plasma are investigated. The study is the first part of a three-phase program aimed at characterizing the current-driven turbulent dissipation degrading the efficiency of Lorentz force plasma accelerators such as the MPD thruster. The analysis uses a kinetic theory that includes magnetic and thermal effects as well as those of an electron current transverse to the magnetic field and collisions, thus combining all the features of previous models. Analytical and numerical solutions allow a detailed description of threshold criteria, finite growth behavior, destabilization mechanisms and maximized-growth characteristics of the dominant unstable modes. The lower hybrid current-driven instability is implicated as dominant and was found to preserve its character in the collisional plasma regime. 22 refs.
A systems-level trade-off study is presented comparing the propulsion requirements and associated... more A systems-level trade-off study is presented comparing the propulsion requirements and associated final masses for different architectural implementations of the Terrestrial Planet Finder (TPF) mission. The study focuses on the mN-level propulsion chores associated with rotation and repointing. Three interferometer configurations; free-flying, monolithic and tethered; lead to estimates of power requirements and spacecraft masses associated with different plasma propulsion systems required to maneuver the interferometer throughout its lifetime. The parametric study includes the following plasma propulsion options: Hall thruster, Field Emission Electric Propulsion (FEEP), Ablative Pulsed Plasma Thruster (APPT), Ablative Z-pinch Pulsed Plasma Thruster (AZPPT) and Gas-Fed Pulsed Plasma Thruster (GF-PPT). For the different thruster and architecture combinations, it is found that the initial mass for a system falls between 3200 and 4200 kg. Also, in general, for a given architecture, the tether has the lowest initial mass followed by the free flyer and the monolith. Finally, the initial mass was found not to be particularly sensitive to the type of plasma propulsion system so the choice should be made based on technological readiness, systems integration considerations and spacecraft contamination issues associated with the chosen system. 'Graduate Student, Electric Propulsion & Plasma Dynamics Lab (EPPDyL). Applied Physics Group. Member AIAA.
45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Aug 2, 2009
The effects of inductive coil geometry on current sheet velocity in a conical theta pinch acceler... more The effects of inductive coil geometry on current sheet velocity in a conical theta pinch acceleration stage were explored through experiments and calculations in order to obtain insight into optimizing the coil geometry for the theta pinch Faraday accelerator with radio frequency assisted discharge (CΘP-FARAD). The experiment consisted of carrying out static measurements of the mutual inductance between a set of conical inductive coils of varying lengths and half-cone angles and their associated currents sheet analogs made out of metallic foil. The measured dependences of mutual inductance on the axial distance between the coil and the sheet were used as input to an analytical circuit model of inductive current sheet acceleration. This allowed the inference of some of the effects of coil geometry on the performance of the accelerator (the final sheet velocity). It was found that shorter driving coils, and to a lesser extent larger half-cone angles (whose effects are less pronounced for longer coils), are more favorable for vigorous current sheet acceleration. The trends were explained by considering that the uniformity of the magnetic field within the volume of a coil increases for longer, more solenoidal coils, and therefore would present a reduced magnetic field gradient to a current sheet translating away from the coil, thus reducing the rate of the change of inductance with distance and consequently decreasing the final sheet velocity. Nomenclature C capacitor capacitance, µF v z axial velocity, km/s I 1 , I 2 driving coil current, plasma current, Amps z 0 decoupling distance, m L 0 , L C initial, coil inductance, nH z axial displacement, m l coil length, cm α cone half angle, degrees M mutual inductance, nH η efficiency m mass, kg φ flux, weber R p plasma resistance, ohms ρ A linear mass density, kg/m
Journal of The Audio Engineering Society, Oct 8, 2017
A method is developed for efficiently calculating head-related transfer functions (HRTFs) directl... more A method is developed for efficiently calculating head-related transfer functions (HRTFs) directly from head scan point clouds of a subject using a database of HRTFs, and corresponding head scans, of many subjects. Consumer applications require HRTFs be estimated accurately and efficiently, but existing methods do not simultaneously meet these requirements. The presented method uses efficient matrix multiplications to compute HRTFs from spherical harmonic representations of head scan point clouds that may be obtained from consumer-grade cameras. The method was applied to a database of only 23 subjects, and while calculated interaural time difference errors are found to be above estimated perceptual thresholds for some spatial directions, HRTF spectral distortions up to 6 kHz fall below perceptual thresholds for most directions.
Journal of The Audio Engineering Society, Oct 23, 2015
Anechoic directivity data for a variety of loudspeakers have been measured and compiled into a fr... more Anechoic directivity data for a variety of loudspeakers have been measured and compiled into a freely available online database, which may be used to evaluate these loudspeakers based on their directivities. The measurements are illustrated through four types of plots (frequency response, polar, contour, and waterfall) and are also given as raw impulse responses. Two sets of directivity metrics are defined and are used to rank the loudspeakers. The first set consists of full and partial directivity indices that isolate sections of the loudspeaker's radiation pattern (e.g., forward radiation alone) and quantify its directivity over those sections. The second set quantifies the extent to which the loudspeaker exhibits constant directivity. Measurements are taken, in an anechoic chamber, along horizontal and vertical orbits with a (nominal) radius of 1.6 m and an angular resolution of five degrees.
Quantitative gas composition analysis method for a wide pressure range up to atmospheric pressure... more Quantitative gas composition analysis method for a wide pressure range up to atmospheric pressure-CO 2 plasma case study
Journal of The Audio Engineering Society, Nov 22, 2019
Suitable domains are established for the practical application of two state-of-the-art parametric... more Suitable domains are established for the practical application of two state-of-the-art parametric interpolation methods for virtual navigation of ambisonics-encoded sound fields. Although several navigational methods have been developed, existing studies rarely include comparisons between methods and, significantly, practical assessments of such methods have been limited. To that end, the errors introduced by both methods are objectively evaluated, in terms of metrics for sound level, spectral coloration, source localization, and diffuseness, through numerical simulations. Various practical domains are subsequently identified, and guidelines are established with which to choose between these methods based on their intended application. Results show that the first method, which entails a time-frequency analysis of the sound field, is preferable for large-area recordings and when spatial localization accuracy is critical, as this method achieves superior localization performance (compared to the second method) with sparsely distributed microphones. However, the second method, which parametrically excludes from the interpolation any microphones that are farther from the listening position than is any source, is shown to be more suitable for applications in which sound quality attributes such as coloration and diffuseness are critical, since this method achieves smaller spectral errors with sparsely distributed microphones and smaller diffuseness errors under all conditions.
Journal of the Acoustical Society of America, May 1, 2017
Soundfields that have been decomposed into spherical harmonics (i.e., encoded into higher-order a... more Soundfields that have been decomposed into spherical harmonics (i.e., encoded into higher-order ambisonics-HOA) can be rendered binaurally for off-center listening positions, but doing so requires additional processing to translate the listener and necessarily leads to increased reproduction errors as the listener navigates further away from the original expansion center. Three techniques for performing this navigation (simulating HOA playback and listener movement within a virtual loudspeaker array, computing and translating along plane-waves, and re-expanding the soundfield about the listener) are compared through numerical simulations of simple incident soundfields and evaluated in terms of both overall soundfield reconstruction accuracy and predicted localization. Results show that soundfield re-expansion achieves arbitrarily low reconstruction errors (relative to the original expansion) in the vicinity of the listener, whereas errors generated by virtual-HOA and plane-wave techniques necessarily impose additional restrictions on the navigable range. Results also suggest that soundfield re-expansion is the only technique capable of accurately generating high-frequency localization cues for off-center listening positions, although the frequencies and translation distances over which this is possible are strictly limited by the original expansion order.
Arcjet Device that heats a propellant stream by passing a high-current electrical arc through it,... more Arcjet Device that heats a propellant stream by passing a high-current electrical arc through it, before the propellant is expanded through a downstream nozzle. Hall effect Conduction of electric current perpendicular to an applied electric field in a superimposed magnetic field. Inductive thruster Device that heats a propellant stream by means of an inductive discharge before the propellant is expanded through a downstream nozzle. Ion thruster Device that accelerates propellant ions by an electrostatic field. Magnetoplasmadynamic thruster Device that accelerates a propellant plasma by an internal or external magnetic field acting on an internal arc current. Plasma Heavily ionized state of matter, usually gaseous, composed of ions, electrons, and neutral atoms or molecules, that has sufficient electrical conductivity to carry substantial current and to react to electric and magnetic body forces. Resistojet Device that heats a propellant stream by passing it through a resistively heated chamber before the propellant is expanded through a downstream nozzle. Thrust Unbalanced internal force exerted on a rocket during expulsion of its propellant mass. THE SCIENCE AND TECHNOLOGY of electric propulsion (EP) encompass a broad variety of strategies for achieving very high exhaust velocities in order to reduce the total propellant burden and corresponding launch mass of present and future space transportation systems. These techniques group broadly into three categories: electrothermal propulsion, wherein the propellant is electrically heated, then expanded thermodynamically through a nozzle; electrostatic propulsion, wherein ionized propellant particles are accelerated through an electric field; and electromagnetic propulsion, wherein current driven through a propellant plasma interacts with an internal or external magnetic field to provide a
Electron demagnetization in a magnetically expanding plasma, a fundamental process for plasma flo... more Electron demagnetization in a magnetically expanding plasma, a fundamental process for plasma flow and detachment in magnetic nozzles, is experimentally investigated using a rf plasma source and magnetic nozzle (MN). Measurements of the plasma potential spatial profile reveal an ion-confining potential surface, indicative of the edge of a magnetized plasma, that extends along the outermost magnetic flux surface. The downstream extent of the potential surface scales inversely with a characteristic electron Larmor radius, which agrees with an existing theory [E. Ahedo and M. Merino, Phys. Plasmas 19, 083501 (2012)] for electron demagnetization via finite electron Larmor radius (FELR) effects. These results represent the first experimental evidence of FELR demagnetization, and provide an empirical metric for the significance of FELR effects based on the degree of separation between electron and magnetic flux surfaces. With this metric, a critical magnetic field strength is found that ensures electrons remain magnetized through the MN turning point, thus avoiding the rapid plume divergence associated with premature demagnetization.
Binaural rendering of Ambisonic signals by head-related impulse response time alignment and a dif... more Binaural rendering of Ambisonic signals by head-related impulse response time alignment and a diffuseness constraint
The fundamental difference between the two variants of Hall thrusters, the stationary plasma thru... more The fundamental difference between the two variants of Hall thrusters, the stationary plasma thruster (SPT) and the thruster with anode layer (TAL), is illustrated quantitatively using an analytical model that accounts for the effects of secondary electron emission (SEE) from the walls. The model includes a prescription for the quenching of the temperature of the electrons on their way to the anode which results from the enhanced electron energy losses to the wall that occur at, and upstream of, an axial location where the wall potential reverses from electronattracting to electron-repellent. For the higher SEE coefficient of an insulator wall (compared to that of a metallic one) this sign reversal occurs at a lower electron temperature and is shown to lead to a more extended acceleration region due to the resulting relaxation of the potential gradient required to balance the electron pressure gradient. By replacing the boron nitride walls (SPT) of an idealized Hall thruster with stainless steel (TAL), the acceleration zone is shown analytically to collapse to a region near the anode having an extent that is about eight times smaller than that for the SPT. The results are used to construct a detailed phenomenological picture of the fundamental difference between the two Hall thruster variants.
Plasma Sources Science and Technology, Feb 1, 2022
A first-principles approach to obtain the attachment length within a hollow cathode with a constr... more A first-principles approach to obtain the attachment length within a hollow cathode with a constrictive orifice, and its scaling with internal cathode pressure, is developed. This parameter, defined herein as the plasma density decay length scale upstream of (away from) the cathode orifice, is critical because it controls the utilization of the hollow cathode insert and influences cathode life. A two-dimensional framework is developed from the ambipolar diffusion equation for the insert-region plasma. A closed-form solution for the plasma density is obtained using standard partial differential equation techniques by applying an approximate boundary condition at the cathode orifice plane. This approach also yields the attachment length and electron temperature without reliance on measured plasma property data or complex computational models. The predicted plasma density profile is validated against measurements from the NSTAR discharge cathode, and calculated electron temperatures and attachment lengths agree with published values. Nondimensionalization of the governing equations reveals that the solution depends almost exclusively on the neutral pressure-diameter product in the insert plasma region. Evaluation of analytical results over a wide range of input parameters yields scaling relations for the variation of the attachment length and electron temperature with the pressurediameter product. For the range of orifice-to-insert diameter ratio studied, the influence of orifice size is shown to be small except through its effect on insert pressure, and the attachment length is shown to be proportional to the insert inner radius, suggesting high-pressure cathodes should be constructed with largerdiameter inserts.
Journal of The Audio Engineering Society, May 4, 2021
A formula is derived for computing the order at which the infinite series representation of the r... more A formula is derived for computing the order at which the infinite series representation of the rigid-sphere head-related transfer function (RS-HRTF) must be truncated to minimize the time required to compute the HRTF to a sufficiently high accuracy based on binaural perception metrics. Quick and accurate computation of this HRTF may be useful for implementing spatial audio in computationally limited and portable devices. Using a brute-force approach, the lowest truncation order, N min , that yields the RS-HRTF that differs from the benchmark (i.e., the RS-HRTF computed with the highest possible accuracy) by less than just-noticeable difference thresholds in interaural time and level differences is approximately computed for a wide range of source distances. By fitting power and rational functions to these computed values, a formula that approximates N min as a function of frequency and source distance is derived. It is shown that truncation order varies significantly with source distance and that the proposed formula, unlike a previous one, accurately captures this variation. Consequently, using the proposed formula instead of the previous one results in a more accurate RS-HRTF that is also computed 48% faster on average.
Journal of The Audio Engineering Society, Oct 22, 2020
A formula for low-frequency interaural level difference (LF-ILD) as a function of source distance... more A formula for low-frequency interaural level difference (LF-ILD) as a function of source distance and direction is derived from rigid-sphere head-related transfer function (RS-HRTF) theory. Since ILD at low frequencies (typically f < 700 Hz) is a dominant cue for near-field distance perception in the free-field, a simple formula akin to the Woodworth formula for high-frequency interaural time difference, may be beneficial to easily and efficiently implement distance-and direction-dependent LF-ILDs for real-time spatial audio applications on devices with limited computational resources. By evaluating the limit as f → 0 of the infinite series representation of the RS-HRTF for finite source distances, an exact, closed-form expression for the "DC gain" as a function of source distance and direction is derived. For a given source location, using this expression to compute the DC gain at the left and right "ears," and then taking the ratio of the two quantities gives the desired LF-ILD. As an example, it is shown that the derived formula may be used to extrapolate far-field ILD spectra of a rigid sphere to the near-field exactly for low frequencies and with sufficiently high accuracy for higher frequencies. Furthermore, the derived formula, like the Woodworth formula, is well-suited to individualization.
Journal of The Audio Engineering Society, May 26, 2016
It is often desired that a transducer have a polar radiation pattern that is invariant with frequ... more It is often desired that a transducer have a polar radiation pattern that is invariant with frequency, but there is currently no way of quantifying the extent to which a transducer possesses this quality (often called “constant directivity” or “controlled directivity”). To address this problem, commonly-accepted criteria are used to propose two definitions of constant directivity. The first, stricter definition, is that the polar radiation pattern of a transducer must be invariant over a specified frequency range, whereas the second is that the directivity index (i.e., the difference, in dB, between the on-axis frequency response and the average response over all directions), must be invariant with frequency. Furthermore, to quantify each criterion, five metrics are derived: 1) Fourier analysis of contour lines (i.e., lines of constant sensitivity over frequency and angle), 2) directional average of frequency response distortions, 3) distortion thresholding of polar responses, 4) standard deviation of directivity index, and 5) cross-correlation of polar responses. These five metrics are computed using measured polar radiation data for four loudspeakers. The loudspeakers are then ranked, from most constant-directive to least, according to each metric, and the rankings are used to evaluate, by comparison with each other and with a predetermined “correct” ranking, each metric’s ability to quantify constant directivity. Results show that all five metrics are able to quantify constant directivity according to the criterion on which each is based, while only two of them, metrics 4 and 5, are able to adequately quantify both proposed definitions of constant directivity.
The dominant unstable electrostatic wave modes of an electromagnetically accelerated plasma are i... more The dominant unstable electrostatic wave modes of an electromagnetically accelerated plasma are investigated. The study is the first part of a three-phase program aimed at characterizing the current-driven turbulent dissipation degrading the efficiency of Lorentz force plasma accelerators such as the MPD thruster. The analysis uses a kinetic theory that includes magnetic and thermal effects as well as those of an electron current transverse to the magnetic field and collisions, thus combining all the features of previous models. Analytical and numerical solutions allow a detailed description of threshold criteria, finite growth behavior, destabilization mechanisms and maximized-growth characteristics of the dominant unstable modes. The lower hybrid current-driven instability is implicated as dominant and was found to preserve its character in the collisional plasma regime. 22 refs.
A systems-level trade-off study is presented comparing the propulsion requirements and associated... more A systems-level trade-off study is presented comparing the propulsion requirements and associated final masses for different architectural implementations of the Terrestrial Planet Finder (TPF) mission. The study focuses on the mN-level propulsion chores associated with rotation and repointing. Three interferometer configurations; free-flying, monolithic and tethered; lead to estimates of power requirements and spacecraft masses associated with different plasma propulsion systems required to maneuver the interferometer throughout its lifetime. The parametric study includes the following plasma propulsion options: Hall thruster, Field Emission Electric Propulsion (FEEP), Ablative Pulsed Plasma Thruster (APPT), Ablative Z-pinch Pulsed Plasma Thruster (AZPPT) and Gas-Fed Pulsed Plasma Thruster (GF-PPT). For the different thruster and architecture combinations, it is found that the initial mass for a system falls between 3200 and 4200 kg. Also, in general, for a given architecture, the tether has the lowest initial mass followed by the free flyer and the monolith. Finally, the initial mass was found not to be particularly sensitive to the type of plasma propulsion system so the choice should be made based on technological readiness, systems integration considerations and spacecraft contamination issues associated with the chosen system. 'Graduate Student, Electric Propulsion & Plasma Dynamics Lab (EPPDyL). Applied Physics Group. Member AIAA.
45th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Aug 2, 2009
The effects of inductive coil geometry on current sheet velocity in a conical theta pinch acceler... more The effects of inductive coil geometry on current sheet velocity in a conical theta pinch acceleration stage were explored through experiments and calculations in order to obtain insight into optimizing the coil geometry for the theta pinch Faraday accelerator with radio frequency assisted discharge (CΘP-FARAD). The experiment consisted of carrying out static measurements of the mutual inductance between a set of conical inductive coils of varying lengths and half-cone angles and their associated currents sheet analogs made out of metallic foil. The measured dependences of mutual inductance on the axial distance between the coil and the sheet were used as input to an analytical circuit model of inductive current sheet acceleration. This allowed the inference of some of the effects of coil geometry on the performance of the accelerator (the final sheet velocity). It was found that shorter driving coils, and to a lesser extent larger half-cone angles (whose effects are less pronounced for longer coils), are more favorable for vigorous current sheet acceleration. The trends were explained by considering that the uniformity of the magnetic field within the volume of a coil increases for longer, more solenoidal coils, and therefore would present a reduced magnetic field gradient to a current sheet translating away from the coil, thus reducing the rate of the change of inductance with distance and consequently decreasing the final sheet velocity. Nomenclature C capacitor capacitance, µF v z axial velocity, km/s I 1 , I 2 driving coil current, plasma current, Amps z 0 decoupling distance, m L 0 , L C initial, coil inductance, nH z axial displacement, m l coil length, cm α cone half angle, degrees M mutual inductance, nH η efficiency m mass, kg φ flux, weber R p plasma resistance, ohms ρ A linear mass density, kg/m
Journal of The Audio Engineering Society, Oct 8, 2017
A method is developed for efficiently calculating head-related transfer functions (HRTFs) directl... more A method is developed for efficiently calculating head-related transfer functions (HRTFs) directly from head scan point clouds of a subject using a database of HRTFs, and corresponding head scans, of many subjects. Consumer applications require HRTFs be estimated accurately and efficiently, but existing methods do not simultaneously meet these requirements. The presented method uses efficient matrix multiplications to compute HRTFs from spherical harmonic representations of head scan point clouds that may be obtained from consumer-grade cameras. The method was applied to a database of only 23 subjects, and while calculated interaural time difference errors are found to be above estimated perceptual thresholds for some spatial directions, HRTF spectral distortions up to 6 kHz fall below perceptual thresholds for most directions.
Journal of The Audio Engineering Society, Oct 23, 2015
Anechoic directivity data for a variety of loudspeakers have been measured and compiled into a fr... more Anechoic directivity data for a variety of loudspeakers have been measured and compiled into a freely available online database, which may be used to evaluate these loudspeakers based on their directivities. The measurements are illustrated through four types of plots (frequency response, polar, contour, and waterfall) and are also given as raw impulse responses. Two sets of directivity metrics are defined and are used to rank the loudspeakers. The first set consists of full and partial directivity indices that isolate sections of the loudspeaker's radiation pattern (e.g., forward radiation alone) and quantify its directivity over those sections. The second set quantifies the extent to which the loudspeaker exhibits constant directivity. Measurements are taken, in an anechoic chamber, along horizontal and vertical orbits with a (nominal) radius of 1.6 m and an angular resolution of five degrees.
Quantitative gas composition analysis method for a wide pressure range up to atmospheric pressure... more Quantitative gas composition analysis method for a wide pressure range up to atmospheric pressure-CO 2 plasma case study
Journal of The Audio Engineering Society, Nov 22, 2019
Suitable domains are established for the practical application of two state-of-the-art parametric... more Suitable domains are established for the practical application of two state-of-the-art parametric interpolation methods for virtual navigation of ambisonics-encoded sound fields. Although several navigational methods have been developed, existing studies rarely include comparisons between methods and, significantly, practical assessments of such methods have been limited. To that end, the errors introduced by both methods are objectively evaluated, in terms of metrics for sound level, spectral coloration, source localization, and diffuseness, through numerical simulations. Various practical domains are subsequently identified, and guidelines are established with which to choose between these methods based on their intended application. Results show that the first method, which entails a time-frequency analysis of the sound field, is preferable for large-area recordings and when spatial localization accuracy is critical, as this method achieves superior localization performance (compared to the second method) with sparsely distributed microphones. However, the second method, which parametrically excludes from the interpolation any microphones that are farther from the listening position than is any source, is shown to be more suitable for applications in which sound quality attributes such as coloration and diffuseness are critical, since this method achieves smaller spectral errors with sparsely distributed microphones and smaller diffuseness errors under all conditions.
Journal of the Acoustical Society of America, May 1, 2017
Soundfields that have been decomposed into spherical harmonics (i.e., encoded into higher-order a... more Soundfields that have been decomposed into spherical harmonics (i.e., encoded into higher-order ambisonics-HOA) can be rendered binaurally for off-center listening positions, but doing so requires additional processing to translate the listener and necessarily leads to increased reproduction errors as the listener navigates further away from the original expansion center. Three techniques for performing this navigation (simulating HOA playback and listener movement within a virtual loudspeaker array, computing and translating along plane-waves, and re-expanding the soundfield about the listener) are compared through numerical simulations of simple incident soundfields and evaluated in terms of both overall soundfield reconstruction accuracy and predicted localization. Results show that soundfield re-expansion achieves arbitrarily low reconstruction errors (relative to the original expansion) in the vicinity of the listener, whereas errors generated by virtual-HOA and plane-wave techniques necessarily impose additional restrictions on the navigable range. Results also suggest that soundfield re-expansion is the only technique capable of accurately generating high-frequency localization cues for off-center listening positions, although the frequencies and translation distances over which this is possible are strictly limited by the original expansion order.
Arcjet Device that heats a propellant stream by passing a high-current electrical arc through it,... more Arcjet Device that heats a propellant stream by passing a high-current electrical arc through it, before the propellant is expanded through a downstream nozzle. Hall effect Conduction of electric current perpendicular to an applied electric field in a superimposed magnetic field. Inductive thruster Device that heats a propellant stream by means of an inductive discharge before the propellant is expanded through a downstream nozzle. Ion thruster Device that accelerates propellant ions by an electrostatic field. Magnetoplasmadynamic thruster Device that accelerates a propellant plasma by an internal or external magnetic field acting on an internal arc current. Plasma Heavily ionized state of matter, usually gaseous, composed of ions, electrons, and neutral atoms or molecules, that has sufficient electrical conductivity to carry substantial current and to react to electric and magnetic body forces. Resistojet Device that heats a propellant stream by passing it through a resistively heated chamber before the propellant is expanded through a downstream nozzle. Thrust Unbalanced internal force exerted on a rocket during expulsion of its propellant mass. THE SCIENCE AND TECHNOLOGY of electric propulsion (EP) encompass a broad variety of strategies for achieving very high exhaust velocities in order to reduce the total propellant burden and corresponding launch mass of present and future space transportation systems. These techniques group broadly into three categories: electrothermal propulsion, wherein the propellant is electrically heated, then expanded thermodynamically through a nozzle; electrostatic propulsion, wherein ionized propellant particles are accelerated through an electric field; and electromagnetic propulsion, wherein current driven through a propellant plasma interacts with an internal or external magnetic field to provide a
Electron demagnetization in a magnetically expanding plasma, a fundamental process for plasma flo... more Electron demagnetization in a magnetically expanding plasma, a fundamental process for plasma flow and detachment in magnetic nozzles, is experimentally investigated using a rf plasma source and magnetic nozzle (MN). Measurements of the plasma potential spatial profile reveal an ion-confining potential surface, indicative of the edge of a magnetized plasma, that extends along the outermost magnetic flux surface. The downstream extent of the potential surface scales inversely with a characteristic electron Larmor radius, which agrees with an existing theory [E. Ahedo and M. Merino, Phys. Plasmas 19, 083501 (2012)] for electron demagnetization via finite electron Larmor radius (FELR) effects. These results represent the first experimental evidence of FELR demagnetization, and provide an empirical metric for the significance of FELR effects based on the degree of separation between electron and magnetic flux surfaces. With this metric, a critical magnetic field strength is found that ensures electrons remain magnetized through the MN turning point, thus avoiding the rapid plume divergence associated with premature demagnetization.
Binaural rendering of Ambisonic signals by head-related impulse response time alignment and a dif... more Binaural rendering of Ambisonic signals by head-related impulse response time alignment and a diffuseness constraint
The fundamental difference between the two variants of Hall thrusters, the stationary plasma thru... more The fundamental difference between the two variants of Hall thrusters, the stationary plasma thruster (SPT) and the thruster with anode layer (TAL), is illustrated quantitatively using an analytical model that accounts for the effects of secondary electron emission (SEE) from the walls. The model includes a prescription for the quenching of the temperature of the electrons on their way to the anode which results from the enhanced electron energy losses to the wall that occur at, and upstream of, an axial location where the wall potential reverses from electronattracting to electron-repellent. For the higher SEE coefficient of an insulator wall (compared to that of a metallic one) this sign reversal occurs at a lower electron temperature and is shown to lead to a more extended acceleration region due to the resulting relaxation of the potential gradient required to balance the electron pressure gradient. By replacing the boron nitride walls (SPT) of an idealized Hall thruster with stainless steel (TAL), the acceleration zone is shown analytically to collapse to a region near the anode having an extent that is about eight times smaller than that for the SPT. The results are used to construct a detailed phenomenological picture of the fundamental difference between the two Hall thruster variants.
Plasma Sources Science and Technology, Feb 1, 2022
A first-principles approach to obtain the attachment length within a hollow cathode with a constr... more A first-principles approach to obtain the attachment length within a hollow cathode with a constrictive orifice, and its scaling with internal cathode pressure, is developed. This parameter, defined herein as the plasma density decay length scale upstream of (away from) the cathode orifice, is critical because it controls the utilization of the hollow cathode insert and influences cathode life. A two-dimensional framework is developed from the ambipolar diffusion equation for the insert-region plasma. A closed-form solution for the plasma density is obtained using standard partial differential equation techniques by applying an approximate boundary condition at the cathode orifice plane. This approach also yields the attachment length and electron temperature without reliance on measured plasma property data or complex computational models. The predicted plasma density profile is validated against measurements from the NSTAR discharge cathode, and calculated electron temperatures and attachment lengths agree with published values. Nondimensionalization of the governing equations reveals that the solution depends almost exclusively on the neutral pressure-diameter product in the insert plasma region. Evaluation of analytical results over a wide range of input parameters yields scaling relations for the variation of the attachment length and electron temperature with the pressurediameter product. For the range of orifice-to-insert diameter ratio studied, the influence of orifice size is shown to be small except through its effect on insert pressure, and the attachment length is shown to be proportional to the insert inner radius, suggesting high-pressure cathodes should be constructed with largerdiameter inserts.
Journal of The Audio Engineering Society, May 4, 2021
A formula is derived for computing the order at which the infinite series representation of the r... more A formula is derived for computing the order at which the infinite series representation of the rigid-sphere head-related transfer function (RS-HRTF) must be truncated to minimize the time required to compute the HRTF to a sufficiently high accuracy based on binaural perception metrics. Quick and accurate computation of this HRTF may be useful for implementing spatial audio in computationally limited and portable devices. Using a brute-force approach, the lowest truncation order, N min , that yields the RS-HRTF that differs from the benchmark (i.e., the RS-HRTF computed with the highest possible accuracy) by less than just-noticeable difference thresholds in interaural time and level differences is approximately computed for a wide range of source distances. By fitting power and rational functions to these computed values, a formula that approximates N min as a function of frequency and source distance is derived. It is shown that truncation order varies significantly with source distance and that the proposed formula, unlike a previous one, accurately captures this variation. Consequently, using the proposed formula instead of the previous one results in a more accurate RS-HRTF that is also computed 48% faster on average.
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Papers by Edgar Choueiri