Hyu-Sang Kwon

Moving frame technique for planar acoustic holography

Acoustic holography is one of the best methods to visualize sound fields. The quality of the visualized sound is primarily determined by the size of the hologram, its microphone spacing, and the number of microphones. This paper describes a way to virtually increase the hologram size and the spatial resolution of the holograph. For a stationary sound field, the method continuously sweeps the sound field by a line array of microphones. For moving sound sources, radiating sound is measured by using a line array of microphones fixed in space. In both cases, the measured signals have Doppler effects. The theoretical formulation has been systematically addressed by employing a moving coordinate which has relative motion between the measurement coordinate and the hologram coordinate. Simulations and experiments support the proposed theory. The drawback is that the method is only applicable to discrete frequencies.

Partial sound field decomposition in multireference near-field acoustical holography by using optimally located virtual references

It has been shown previously that the multiple reference and field signals recorded during a scanning acoustical holography measurement can be used to decompose the sound field radiated by a composite sound source into mutually incoherent partial fields. To obtain physically meaningful partial fields, i.e., fields closely related to particular component sources, the reference microphones should be positioned as close as possible to the component physical sources that together comprise the complete source. However, it is not always possible either to identify the optimal reference microphone locations prior to performing a holographic measurement, or to place reference microphones at those optimal locations, even if known, owing to physical constraints. Here, post-processing procedures are described that make it possible both to identify the optimal reference microphone locations and to place virtual references at those locations after performing a holographic measurement. The optimal reference microphone locations are defined to be those at which the MUSIC power is maximized in a three-dimensional space reconstructed by holographic projection. The acoustic pressure signals at the locations thus identified can then be used as optimal “virtual” reference signals. It is shown through an experiment and numerical simulation that the optimal virtual reference signals can be successfully used to identify physically meaningful partial sound fields, particularly when used in conjunction with partial coherence decomposition procedures.

Minimization of bias error due to windows in planar acoustic holography using a minimum error window

Bias errors in the prediction of a sound field using planar acoustic holography are due to aliasing and window effects. It is noteworthy that aliasing is negligible in the forward predictions if the sampling space is less than a quarter wavelength. However, bias error induced by a window is the major concern for accurate holographic prediction because of the small number of measurement points in the planar acoustic holography. For the reduction of this error, a new class of window, the MEW, is proposed and compared with Hann, Gaussian, and Kaiser–Bessel windows. It is built by modifying the method that Papoulis proposed by minimizing the second‐order moment of the window spectrum. The characteristics of the MEW vary with the number of weighting values and the number of higher‐order moment terms to be eliminated. The applicability of the MEW on planar acoustic holography is demonstrated.

Localization accuracy of single current dipoles from tangential components of auditory evoked fields

We investigated the localization accuracy of single current dipoles from the tangential components of auditory evoked fields. The tangential fields were measured using planar gradiometers arranged in a way so as to detect two orthogonal field components parallel to a flat plane. Field responses to 1 kHz pure tones were recorded and equivalent current dipoles (ECDs) of N1m peak were estimated based on a locally fitted spherical conductor model. As a measure of localization accuracy, the standard deviation of the coordinates of the ECDs of N1m was obtained from repeated measurements for one subject. The estimated ECDs had a standard deviation of 5.5 mm and their mean location was at the supratemporal plane in the sylvian fissure on the MR image of the subject. In order to investigate the contribution of various errors to the localization accuracy, simulations using a sphere model and experiments using a realistically shaped skull phantom were performed. It was found that the background noise, which consisted of instrumental noise and spontaneous brain fields, was the main source of the errors that could explain the observed standard deviation. Further, the amount of systematic error was much less than the standard deviation due to the background noise. These results suggest that the volume currents in a non-spherical conductor shape such as the temporal region do not produce substantial errors in the localization of current dipoles from tangential components of auditory evoked fields.

Planar nearfield acoustical holography in moving fluid medium at subsonic and uniform velocitya)

Nearfield acoustical holography (NAH) data measured by using a microphone array attached to a high-speed aircraft or ground vehicle include significant airflow effects. For the purpose of processing the measured NAH data, an improved nearfield acoustical holography procedure is introduced that includes the effects of a fluid medium moving at a subsonic and uniform velocity. The convective wave equation along with the convective Eulers equation is used to develop the proposed NAH procedure. A mapping function between static and moving fluid medium cases is derived from the convective wave equation. Then, a conventional wave number filter designed for static fluid media is modified to be applicable to the moving fluid cases by applying the mapping function to the static wave number filter. In order to validate the proposed NAH procedure, a monopole simulation at the airflow speed of Mach=-0.6 is conducted. The reconstructed acoustic fields obtained by applying the proposed NAH procedure to the simulation data agree well with directly-calculated acoustic fields. Through an experiment with two loudspeakers performed in a wind tunnel operating at Mach=-0.12, it is shown that the proposed NAH procedure can be also used to reconstruct the sound fields radiated from the two loudspeakers.

A high-sensitivity magnetocardiography system with a divided gradiometer array inside a low boil-off Dewar

We fabricated a low-noise 64-channel first-order axial gradiometer system for measuring magnetocardiography (MCG) signals. The key technical features of the system are the compact structure of the gradiometer, division of the sensor array plate, direct mounting of the sensor plates into the Dewar bottom, reduced neck diameter of the liquid He Dewar, and compact readout electronics. To make the refill interval of liquid He longer, the distance between the compensation coil of the gradiometer and the input coil pads of the superconducting quantum interference device (SQUID) was reduced to 20 mm. By using direct ultrasonic bonding of Nb wires between the pickup coil wires and input coil pads, the superconductive connection structure became simple. The baseline of the first-order gradiometer is 70 mm, a little longer than for typical conventional axial gradiometers, to provide a larger signal amplitude for deep sources. The 64-channel gradiometer array consists of four blocks, and each block is fixed separately onto the bottom of the Dewar. The neck diameter of the He Dewar (192 mm) is smaller than the bottom diameter (280 mm) in which the gradiometers are distributed. The average boil-off rate of the Dewar is 3 l per day when the 64-channel system is in operation every day. Double relaxation oscillation SQUIDs (DROSs) having large flux-to-voltage transfer coefficients were used to operate SQUIDs via compact electronics. The magnetically shielded room (MSR) has a wall thickness of 80 mm, and consists of two layers of permalloy and one layer of aluminum. When the 64-channel system was installed inside the MSR, the field noise level of the system was about 3.5 fTrms Hz−1/2 at 100 Hz. MCG measurements with high signal quality were done successfully using the developed system. In addition to the parameter analysis method, we developed software for the three-dimensional imaging of the myocardial current on a realistic image of the heart based on the anatomical image of the torso.

Noise characteristics of double relaxation oscillation superconducting quantum interference devices with reference junction

We fabricated double relaxation oscillation SQUIDs (DROSs) with a reference junction and investigated their noise characteristics. Because of the large flux-to-voltage transfer of 1-3 mV/0, the room-temperature dc preamplifier could readout SQUID output voltage directly and the contribution of the preamplifier input noise to the total system noise was negligible. The flux noise of the DROS planar gradiometer is about 4 µ0 Hz-1/2 at 100 Hz, corresponding to a field gradient noise of 1 fT cm-1 Hz-1/2, when operated inside a moderately shielded room. Because of the large modulation voltage of about 100 µV, stable flux-locked-loop operation was possible against drift in the input offset voltage.

Beam pattern improvement by compensating array nonuniformities in a guided wave phased array

This paper presents a simple data processing algorithm which can improve the performance of a uniform circular array based on guided wave transducers. The algorithm, being intended to be used with the delay-and-sum beamformer, effectively eliminates the effects of nonuniformities that can significantly degrade the beam pattern. Nonuniformities can arise intrinsically from the array geometry when the circular array is transformed to a linear array for beam steering and extrinsically from unequal conditions of transducers such as element-to-element variations of sensitivity and directivity. The effects of nonuniformities are compensated by appropriately imposing weight factors on the elements in the projected linear array. Different cases are simulated, where the improvements of the beam pattern, especially the level of the highest sidelobe, are clearly seen, and related issues are discussed. An experiment is performed which uses A0 mode Lamb waves in a steel plate, to demonstrate the usefulness of the proposed method. The discrepancy between theoretical and experimental beam patterns is explained by accounting for near-field effects.

An integrated planar gradiometer based on a double relaxation oscillation SQUID

The design and performance of an integrated planar gradiometer based on a double relaxation oscillation SQUID (DROS) are presented. The DROS was made from hysteretic junctions and the devices were fabricated by a simple four-level process. The signal SQUID loop is a gradiometric type with two square holes connected in parallel and a reference junction is used instead of the reference SQUID. The high flux-to-voltage transfer coefficient of typically enabled direct readout by a simple electronics with a modest voltage noise. The pickup coil integrated on the same wafer as the SQUID consists of two planar coils connected in series and has a baseline of 30 mm. The overall size of the device is . The field gradient noise is in the white region and at 1 Hz.

Guided Wave Phased Array for Inspection of Plate Structures

This paper describes a general approach for processing data from an omni-directional guided wave transducer array for the rapid inspection of large plate structures. A basic phased array algorithm is presented that can be applied to any array geometry. For guided waves on plate, beam steering algorithm is derived and the corresponding beam pattern is analyzed. The algorithms are applied to simulation and experimental data. The results show well its usefulness in structural applications.