Transfer Function Measurements Research Articles

Assessment of transparent alternative listening devices' effects on localization cues using head-related transfer function measurements

The past few years have seen a meteoric rise in technological advancements in the hearing health industry. New alternative listening devices, often referred to as "hearables", aim to become real "bionic ears" offering hearing protection, amplification, monitoring and even biosensing functionalities. With the recent approval of the "Over-the-Counter Hearing Aid Act" in the United States and the introduction of so-called "OTC" hearing aids, these alternative listening devices now have the potential to revolutionize the world of traditional auditory amplification. Since these devices may be used by individuals with hearing loss, it is important to consider how they might influence spatial auditory awareness. Therefore, this paper studies the effect of alternative listening devices on localization cues using the Head Related Transfer Function (HRTF). In this purpose, we measured the HRTFs on a Head and Torso Simulator (HATS) with and without digital listening devices. To evaluate the acoustic performance of these devices, the devices were set into their "acoustically transparent" mode. These measures enable a preliminary characterization of the effect of hearables and OTC devices on localization cues.

Towards validating the diffraction-enhanced image source method

Accurately simulating room acoustics between directional sources and receivers in reverberant environments can be challenging. Wave-based numerical models can be used at low frequencies, but at higher frequencies, the cost of solving these models is often too prohibitive to permit practical use. Geometrical models can be used to simulate sound fields in rooms at higher frequencies but with limited accuracy due to the inherent assumptions made when deriving the models. Therefore, there are ongoing efforts within the room acoustics community to improve the accuracy of geometrical models. Recently, an extension to the image source method called the diffraction-enhanced image source method, which uses spherical harmonic descriptions to include directional sources and receivers, has been proposed. The proposed method can be used to efficiently model sound fields in shoebox-shaped rooms with directional sources and receivers at high frequencies. In this paper, predictions of the proposed method are compared to data measured in a shoebox-shaped room. It is shown that the proposed method captures many of the pertinent features of the measured transfer functions and impulse responses.

A feasibility study on active sound reduction across an acoustic plenum window by cancelling source clusters on internal periphery of the window cavity.

The possibility of applying active control to reduce sound transmission across a practical plenum window is examined experimentally in the present study using measured transfer functions of all related sound transmission paths. As a result of the limited space within the window, the error microphones are located at the indoor window opening while the secondary cancelling sources are mounted along the periphery of the window void. Results show that the cancelling sources near the outdoor window opening corners and within the overlapping region of the window play more useful roles in the control. Also, the highest sound reduction is around 6 dB with six error microphones positioned either at the central region or along the periphery of the indoor window opening. However, the results with the central error microphones suggest the possibility of adopting a dual control system to enhance the low frequency performance. Control systems with fewer error microphones result in lower sound reduction. Besides, it is found that four cancelling sources, located around the outdoor opening of the window, will be enough to achieve meaningful active sound transmission reduction between 100 and 1000 Hz. Involving more cancelling sources does not result in better performance despite the added complexity.

Forced response of transverse reacting jet in vitiated crossflow to harmonic velocity disturbances

The orthogonally oriented injector configuration typically used in axial fuel staging-capable gas turbine combustors is known to produce complicated thermoacoustic interactions between two distinct reaction zones. Despite the fundamental importance of transfer functions of transverse reacting jets in vitiated crossflow, however, their distinctive properties remain unknown, and associated low-order modeling combined with two different flame transfer functions remains entirely undemonstrated. From detailed measurements of transfer functions of lean-premixed primary and secondary flames in response to harmonic velocity disturbances, here we show that the transfer function of a transverse reacting jet in high-temperature crossflow is described as having relatively larger gain without undulating patterns, near-linear slow decay in magnitude with respect to the forcing frequency, and remarkably shorter duration response time. By leveraging a reduced-order modeling approach to account for the finite time delay between the two distinct transfer functions and by validating the calculation results against velocity and pressure measurement data, we demonstrate that self-induced instabilities in an axial fuel-staged system can be simultaneously driven by the dynamics of primary and secondary flames, and that the triggering of second-stage-induced instability is characteristically connected to higher-order acoustic modes. Such non-axisymmetric intra-combustor interactions generate a cascade of spectral peaks, including the first and third longitudinal modes and their respective higher harmonics, and additional peaks at the sum and difference of two individual frequencies. While the primary flame's dynamics are capable of selectively exciting the first mode, the secondary jet flame exhibits more complicated modal dynamics, manifested as the L3 mode-coupled jet merging-related flame surface annihilation and the L1 mode-coupled lateral movements of the transverse jet column.

Surface construction vibration impacts on underground tunnels: Approaches for using measurements and finite-element modeling to predict vibration in the frequency and time domain

Quantitative analyses of vibration impacts on underground tunnel structures were carried out on two projects. The first project was a new facility situated above an existing physics laboratory accelerator tunnel embedded in soil where the concern was vibration interference with sensitive research. Published and in-house data for construction equipment source levels were combined with measured transfer functions, between the ground surface and the tunnel, to estimate vibration transmitted to the tunnel in the frequency domain. This case study presents an approach for site-specific evaluations of construction vibration where spectral data are needed for evaluating vibration-sensitive equipment/receivers. The second project was a new facility situated above a storm relief tunnel embedded in rock where the concern was for potential vibration damage to the tunnel. A 3-D finite-element model of the rock and tunnel structure was developed to calculate the peak particle velocity and strain response of the tunnel walls in the time domain. This case study presents an approach for site-specific evaluations of construction vibration where peak velocity amplitudes and strain data are needed for evaluating structural damage potential.

Use of Electron Beam-Induced Current Technique to Characterize Transport Properties of Narrow-Gap-Energy Materials for IR Detection

The electron beam-induced current technique (EBIC) developed at CEA-Leti has been a valuable tool for studying the transport properties of minority carriers in HdCdTe. Indeed, previous work has demonstrated the use of this technique for estimating diffusion length from the exponential decay of EBIC as a function of junction distance, as well as direct measurement of the modulation transfer function (MTF) of small pixel pitch diodes. In this work, a modulated electron beam and a lock-in amplifier are used to measure the variation in current and phase shift with the scan distance to the junction. This work is focused on the estimation of the minority carrier lifetime from the linear evolution of the phase shift as a function of junction distance.

Comparison of transfer function measurement methods for AIMD MRI safety evaluation

Abstract The transfer function is an important method in ISO/TS 10974 used to evaluate the safety of active implantable medical devices (AIMD). To measure the transfer function, there are two methods used during research and testing, which are the piecewise excitation method and the reciprocity method. In theory, they are equivalent. And for measurement, they have their own advantages and disadvantages. Simulations and experiments using both methods were performed and it is proved that they have good agreement in simulations and experiments.

An infrared and visible light video fusion method based on chaos theory and PID control

Differences in the imaging mechanisms of infrared and visible light images lead to differences in the way their visually meaningful gradients are formed. Existing fusion methods use the same feature extractor to extract features from the source video frames, which ignores the differences in the gradients of video frames from different modalities. In this paper, we propose an infrared and visible light video fusion method based on chaos theory and proportional integral differential (PID) control. Firstly, for the Lorentz chaotic system, we give it initial values and parameters, and iterate it to obtain three scrambling sequences, through which the source video frames are scrambled in the rows, columns, and diagonal directions respectively to eliminate their visually meaningful gradients, so that the features extracted are of comparable scales in the same layer, and the fusion process can be carried out in the scale-consistent space. Second, we propose a structure-aware relative total variation feature extraction method (saRTV) for the two-scale decomposition of source video frames, which can transfer more features of source video frames to the detail layer. Then, based on our previous work, this paper introduces PID control to construct a closed-loop control system through transfer function design, controller design and measurement function design. This control system is used to fuse the detail layer, so as to realize the real-time guidance of the source video frames to the fusion process. Experiments on public datasets demonstrate that our method has better performance compared to some state-of-the-art methods.

Integrating eye rotation and contrast sensitivity into image quality evaluation of virtual reality head-mounted displays.

Visual perception on virtual reality head-mounted displays (VR HMDs) involves human vision in the imaging pipeline. Image quality evaluation of VR HMDs may need to be expanded from optical bench testing by incorporating human visual perception. In this study, we implement a 5-degree-of-freedom (5DoF) experimental setup that simulates the human eye geometry and rotation mechanism. Optical modulation transfer function (MTF) measurements are performed using various camera rotation configurations namely pupil rotation, eye rotation, and eye rotation with angle kappa of the human visual system. The measured MTFs of the VR HMD are inserted into a human eye contrast sensitivity model to predict the perceptual contrast sensitivity function (CSF) on a VR HMD. At the same time, we develop a WebXR test platform to perform human observer experiments. Monocular CSFs of human subjects with different interpupillary distance (IPD) are extracted and compared with those calculated from optical MTF measurements. The result shows that image quality, measured as MTF and CSF, degrades at the periphery of display field of view, especially for subjects with an IPD different than that of the HMD. We observed that both the shift of visual point on the HMD eyepiece and the angle between the optical axes of the eye and eyepiece degrade image quality due to optical aberration. The computed CSFs from optical measurement correlates with those of the human observer experiment, with the optimal correlation achieved using the eye rotation with angle kappa setup. The finding demonstrates that more precise image quality assessment can be achieved by integrating eye rotation and human eye contrast sensitivity into optical bench testing.

PSF and MTF from a bar pattern in digital mammography

Background. The MTF has difficulties being determined (according to the provisions of the IEC standards) in the hospital setting due to the lack of resources. Purpose. The objective of this work is to propose a quantitative method for obtaining the point spread function (PSF) and the modulation transfer function (MTF) of a digital mammography system from an image of a bar pattern. Methods. The method is based on the measurement of the contrast transfer function (CTF) of the system over the image of the bar pattern. In addition, a theoretical model for the PSF is proposed, from which the theoretical CTF of the system is obtained by means of convolution with a square wave (mathematical simulation of the bar pattern). Through an iterative process, the free parameters of the PSF model are varied until the experimental CTF coincides with the one calculated by convolution. Once the PSF of the system is obtained, we calculate the MTF by means of its Fourier transform. The MTF calculated from the model PSF have been compared with those calculated from an image of a 65 μm diameter gold wire using an oversampling process. Results. The CTF has been calculated for three digital mammographic systems (DMS 1, DMS 2 and DMS 3), no differences of more than 5 % were found with the CTF obtained with the PSF model. The comparison of the MTF shows us the goodness of the PSF model. Conclusions. The proposed method for obtaining PSF and MTF is a simple and accessible method, which does not require a complex configuration or the use of phantoms that are difficult to access in the hospital world. In addition, it can be used to calculate other magnitudes of interest such as the normalized noise power spectrum (NNPS) and the detection quantum efficiency (DQE).

Modelling of railway ballast as a poro-elastic medium and its effects on sleeper sound radiation

Conventional railway track is laid in a layer of coarse stones known as ballast. Due to the gaps between the stones, the railway ballast behaves as a porous acoustic material, with absorptive properties that are important for the noise produced by the railway system. Sound radiated by the bottom of the sleepers may also be transmitted through the ballast. Moreover, during a train pass-by the ballast can vibrate, which may contribute to the radiated noise. To consider all three effects simultaneously, the ballast is treated here as a poro-elastic medium. To obtain the properties of the ballast as a porous medium, transfer function measurements are performed in a vertical impedance tube to determine the surface impedance and absorption coefficient of different depths of reduced scale ballast. The complex wavenumber is also determined through transfer function measurements within the ballast in the vertical tube. Porosity and flow resistivity are determined by non-acoustic measurements, and the Johnson-Champoux-Allard model for porous materials is then fitted to the measured wavenumbers, allowing the remaining parameters to be determined. Comparison is made with the measured impedance and normal incidence absorption coefficient. The model is then used to predict the diffuse field absorption coefficients of both the reduced scale ballast and full-size ballast, which are compared with measurements, showing acceptable agreement. Finally, the effects on the sleeper radiation of introducing the poro-elastic ballast model are estimated and the sensitivity of the results to a practical range of ballast properties is assessed. The ballast vibration increases the sleeper radiation at low frequency, and especially between 120 and 280 Hz. This is caused by the structural vibration of the ballast driven by the sleepers, which has a resonance around 250 Hz. The acoustic velocity within the ballast is 180° out of phase with the structural velocity, and this leads to a small reduction in the sound radiation around 100 Hz. These effects could not be predicted using a surface impedance model for the ballast. Compared with the effect of the ballast vibration and sound transmission, the absorption of the ballast has a much smaller influence on the sleeper radiation.

An improved method to measure transfer functions using MRI.

A previously published method for MRI-based transfer function assessment makes use of the so-called transceive phase assumption (TPA). This limits its applicability to shorter leads and/or lower field strengths. A new method is presented where the background electric field is determined from both - and -field distributions, avoiding the TPA and making it more generally applicable. These -distributions are determined from a spoiled gradient echo multiflip angle acquisition. From the separated -components the background electrical field and the induced current are computed. Further improvement is achieved by recasting the -field model as a "magnitude squared least squares" problem. The proposed reconstruction method is used to determine transfer functions of various copper wire lengths up to 40 cm inside an elliptical ASTM phantom. The method is first tested on EM-simulated data and subsequently phantom and bench measurements are used to determine transfer functions experimentally. In silica reconstructions demonstrate the validity of the proposed -field model resulting in highly accurate reconstructed -fields, currents, incident electric fields and transfer functions. The experimental results show slight deviations in the field model, however, resulting transfer functions are accurately determined with high similarity to simulations and comparable to bench measurements. A more generally applicable method for MRI-based transfer function assessment is presented. The proposed method circumvents phase assumptions making it applicable for longer objects and/or higher field strengths. Additional improvements are implemented in the -mapping method and the solution algorithm.

Modulation transfer function of implantable phakic intraocular contact lens (IPCL) for myopia and presbyopia.

This study aimed to assess the optical quality of myopic and presbyopic IPCLs with different additional powers, and to investigate the effects of pupil size on the optical quality of these IPCLs using an in-vitro modulation transfer function (MTF) measurement system. Linear scatter functions (LSFs) were recorded using the OPAL Vector system and an eye phantom consisting of wet cells filled with a balanced salt solution. A myopic IPCL or a presbyopic IPCL was placed in the posterior chamber of this model. The MTF was calculated from the LSF using the fast Fourier transform techniques. The effective apertures were set at 2.0 to 5.0mm in 1.0mm steps. The in-focus MTF values of the myopic IPCL and presbyopic IPCL with additional powers of + 2.0 and + 4.0 diopters at 100 cycles/mm for an effective aperture of 3.0mm were 43%, 27%, and 24%, respectively. The in-focus MTF value of both myopic and presbyopic IPCLs was the highest when the effective aperture was set at 3.0mm, and it gradually worsened when the effective aperture became larger than 3.0mm at 20, 60, and 100 cycles/mm. Both myopic and presbyopic IPCLs provided excellent MTF values, but the additional power profile can deteriorate optical performance in presbyopic IPCL-implanted eyes, even with a low additional power. Pupil size can influence visual quality in IPCL-implanted eyes for both myopia and presbyopia.

58‐4: Late‐News Paper: Dynamic MTF Measurements of Gaming Monitors

Recent gaming monitors claim high refresh rates of ≥240 Hz with response times ≤1 ms. However, these temporal metrics do not clearly reveal their true spatiotemporal resolution, which depends on the velocity at which the images are displayed. This is the first study to demonstrate the line‐based dynamic modulation transfer function (MTF) measurement method for comparing the performances of different gaming monitors. A single line was scrolled on the monitors, and a small region of the screen was captured with a high‐speed camera during one display refresh period. The dynamic MTF results elucidate the dependence of spatiotemporal resolution characteristics on the scroll speed, refresh rate, and response time of the monitors.

Measurement of Modulation Transfer Functions in Computed Tomography

Measurement of Modulation Transfer Functions in Computed Tomography

Random depolarization film design based on modulation transfer function measurements of displays with different pixel pitches

Polarized light emitted from most displays causes blackout and color change problems when viewed through polarized sunglasses. These problems are addressed by random depolarization films (RDFs) doped with birefringent particles. These dopant particles, however, scatter light and degrade the sharpness of the displayed images. To maintain the image sharpness, we designed an RDF based on modulation transfer function measurements of displays with different pixel pitches. RDFs doped with larger particles achieved higher modulations; however, particles larger than a specific size caused undesirable sparkles on the displayed image. A display with a larger pixel pitch achieved higher modulations, suggesting that the RDF is particularly suitable for large-screen displays.

Dynamic Viscosity Measurement Method Based on the Stokes Drag of Prolate Ellipsoidal Mass

Viscometer plays an important role in the field of tribology. One way to measure viscosity is to use the Stokes drag principle in the underdamped harmonic oscillation phenomenon. This paper proposes a dynamic viscosity measurement method based on the related physical laws. Our experimental model involves a prolate ellipsoidal mass that experiences underdamped harmonic oscillation within viscous liquid samples. We observed the oscillations of the prolate ellipsoid to obtain the viscous damping coefficient of each sample and substituted it to the theoretical formula of dynamic viscosity. Experimental data suggest that the mathematical model has failed to predict the viscosity values of the samples. In addition, the regression curve of the reference viscosity and the measured viscous damping coefficient shows that the two quantities have an exponential relation instead of linear relation as explained in the theoretical model. We considered the regression formula as the empirical measurement transfer function and used it to measure the viscosity of an ISO VG 150 industrial oil sample. This measurement resulted in a 2.40 % of relative error percentage. Lastly, this measurement method is only valid for measuring samples with viscosities ranging from 0.0400 Pa s to 0.256 Pa s.

Real-time assessment of spectrometer alignment using modulation transfer function (MTF) measurement

The effective spectral resolution of a spectrometer that utilizes dispersive optics is normally limited by either the size of the sensor pixels or the optical resolution of the focusing optics. Therefore, achieving optimal optical alignment is crucial to maximize the spectrometer's effective spectral resolution. In this paper, we introduce a laboratory method for real-time assessment of spectrometer alignment by measuring the modulation transfer function (MTF). The modulation on spectral signal is generated using an interferometer, allowing control of the modulation frequency by adjusting the optical path difference. The results show that the proposed technique can provide useful information about the alignment of the spectrometer, which can be used to obtain the optimum alignment of the spectrometer. The accuracy of the proposed method was confirmed by comparing the MTF measurement, and hence the resolution evaluation, with the specifications of a commercial spectrometer. In addition, we demonstrate the use case of the proposed method by performing a through-focus MTF measurement on a custom-built spectrometer with known misalignments.

Translational Excitation Strategy for Scaled Transfer Function Measurement of Medical Implants

Translational Excitation Strategy for Scaled Transfer Function Measurement of Medical Implants

High-resolution terrain imager development and performance evaluation for lunar exploration.

The Lunar Terrain Imager (LUTI) is a crucial scientific instrument aboard the Korea Pathfinder Lunar Orbiter. We present an analysis of the optical design, assembly, and performance of the LUTI camera system. We outline the key components and technical specifications of the cameras and detail the assembly and alignment process, including collimator calibration and the knife-edge scanning technique for modulation transfer function measurements. Furthermore, the thermal vacuum optical performance of the LUTI is evaluated by simulating space conditions. The optical design, assembly, and thermal vacuum performance testing make the system valuable for high-resolution lunar surface imaging and future space exploration missions.