Measuring three-dimensional behavior of early reflections in a sound field has been an important issue in auditorium acoustics, since they are found to be strongly correlated with the subjective experience of spatial impression in rooms. A conventional way of detecting the direction and relative amplitude of reflections is to utilize a four-channel microphone system. This system, however, has the inherent possibility of missing some important reflections while exploring the early part of reflections. A new measurement system, which has five microphones on the apex of a tetrahedron and at the center of gravity, is proposed in the present work. The proposed system enables us to find more early reflections accurately, which previous four-channel microphone systems might miss. A peak detecting pair matching algorithm, which gives dominant peaks of impulse response automatically by typical iteration, was adopted, since we should find exact peak positions of measured impulse response in order to calculate source positions accurately. In the present paper, the theoretical backgrounds and features of the five-channel microphone system are presented. Also, some results from experiments using the system are discussed. Experimental results clearly show the advantage of the five-channel microphone system over the conventional four-channel microphone system. [Work supported by Korean Research Foundation Grant KRF-1999-1-310-004-3.]
A ramped voltage stress (RVS) method to screen defective magnetic tunnel junctions (MTJs) is investigated in order to improve screen accuracy and shorten test time. Approximately 1500 MTJs with 1.25 nm thick tunnel barriers were fabricated for this evaluation, and normal MTJs show a 189% tunnel magnetoresistance ratio, a 365 Ω μm2 resistance-area product, and a 1.8 V breakdown voltage, which is enough for applying reliable screen tests. We successfully classified MTJs as normal MTJs having good characteristics or defective MTJs having insufficient endurance and showing resistance degradation after only short-term cycling. Using the RVS screen test with low ramp speed, it is demonstrated that remarkable screening performance and little dependence on temperature are obtained for short test time.
This study investigated the effects of ZrAl under layers and capping layers on the thermal stability and structural stability behavior of MTJs with ZrAl-oxide barrier at elevated temperatures. The results were compared with conventional Ta-based MTJs with Al-oxide barrier. Magnetron sputtering was used to prepare an MTJ consisting of Si/SiO/sub 2//ZrAl 2 nm(or Ta 5 nm)/NiFe 4 nm/IrMn 7.5 nm/CoFe 3 nm/ZrAl 1.6 nm(or Al)-oxidation/CoFe 3 nm/ZrAl 2 nm(or Ta 5 nm). AFM and TEM analysis showed that the ZrAl under layer improved interface uniformity, developed fine and dense grain structure, and retarded the development of texture of upper layers due to their noncrystallinity. Also, the ZrAl-oxide barrier is found to be not only a structurally stable, high-quality barrier, but is also thermally stable. TMR ratio variation were measured as a function of annealing temperatures.
Time-dependent dielectric breakdown (TDDB), which is used to measure reliability, depends on both the thickness of the tunnel barrier and bias voltage. In addition, the heat generated by self-heating in a magnetic tunneling junction (MTJ) affects TDDB. Therefore, we investigated TDDB with the self-heating effect for a MgO tunnel barrier with thicknesses of 1.1 and 1.2 nm by the constant voltage stress (CVS) method. Using the results of this experiment, we predicted a TDDB of 1.0 nm for the tunnel barrier. Also, we suggested the use of not only the CVS method, which is a common way of determining TDDB, but also the constant current stress (CCS) method, which compensates for the disadvantages of the CVS method.
We experience the coloring of reproduced sound in an automotive space in a different way than in a large space such as a concert hall. It comes from the well-separated acoustic modes in the low frequency range up to a relatively high crossover frequency. The unwanted sound coloring can be reduced by equalization. However, this is not a simple matter because the binaural response is different in each person and drivers are likely to move their heads while driving. We introduce a novel approach, based on minimum phase inversion, for the equalization of the low frequency response to compensate the coloring. We compare the proposed approach with the conventional least square based inversion and show the superiority of our approach by experiment. This is confirmed by the results of listening tests.
The strong early reflections and short delay times have been known to improve the intelligibility of speech heard in rooms. D50 and C80, the most frequently used physical parameters, were developed taking this fact into consideration. However, these monaural parameters have limited applications for the practical design of rooms because of their lack of spatial information. The present work investigates how temporal changes in three-dimensional distribution of early reflections influence speech intelligibility in rooms. A new measurement method, using a five microphone array and an omnidirectional source setup, is employed, and a series of post-processing procedures are involved, for getting different early reflections in their spatial distributions. The changes were made for the impulse responses obtained through a five microphone array in the arrival times of early reflections from all, and the horizontal and vertical directions, respectively. Anechoic samples of the Korean language were convolved binaurally with the reproduced impulses by applying a head-related transfer function. A series of speech intelligibility tests, conducted for 22 university students, found that the percentage of correct responses significantly deteriorated by increasing delay times of early reflections from the vertical direction. The result suggests that vertical components of early reflections play a significant role in improving speech intelligibility. [Work supported by Korean Research Foundation Grant KRF-1999-1-310-004-3.]
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