The Inverse Grating Problem: Efficient Design of Anomalous Flexural Wave Reflectors and Refractors

We present an extensive formulation of the inverse grating problem for exural waves, in which the energy of each diffracted mode is selected and the grating configuration is then obtained by solving a linear system of equations. The grating is designed as a lineal periodic repetition of a unit cell comprising a cluster of resonators attached at points whose physical properties are directly derived by inversion of a given matrix. Although both active and passive attachments can be required in the most general case, it is possible to find configurations with only passive, i.e. damped, solutions. This inverse design approach presents an alternative to the design of metasurfaces for exural waves overcoming the limitations of gradient phase metasurfaces, which require a continuous variation of the surface's impedance. When the grating is designed in such a way that all the energy is channeled to a single diffracted mode, it behaves as an anomalous refractor or re ector. The negative refractor is analyzed in depth, and it is shown that with only three scatterers per unit cell is it possible to build such a device with unitary efficiency

Loss Compensation in Time-Dependent Elastic Metamaterials

Materials with properties that are modulated in time are known to display wave phenomena showing energy increasing with time, with the rate mediated by the modulation. Until now there has been no accounting for material dissipation, which clearly counteracts energy growth. This paper provides an exact expression for the amplitude of elastic or acoustic waves propagating in lossy materials with properties that are periodically modulated in time. It is found that these materials can support a special propagation regime in which waves travel at constant amplitude, with temporal modulation compensating for the normal energy dissipation. We derive a general condition under which amplification due to time-dependent properties offsets the material dissipation. This identity relates band-gap properties associated with the temporal modulation and the average of the viscosity coefficient, thereby providing a simple recipe for the design of loss-compensated mechanical metamaterials

Chicago Music City

Chicago Music City compares the strength and vitality of music industries and scenes across the United States. Sociologists, urban planners, and real-estate developers point to quality of life and availability of cultural amenities as important indicators of the health and future success of urban areas. Economic impact studies show the importance of music to local economies. This publication compares Chicago's musical strength with the 50 largest metropolitan areas in the U.S., focusing on 11 comparison cities: Chicago and its demographic peers, New York and Los Angeles, and eight other cities with strong musical reputations -- Atlanta, Austin, Boston, Las Vegas, Memphis, Nashville, New Orleans and Seattle

Bryoecology of the Appalachian Spruce-Fir Zone

Introduction: Ecological investigation of an area ordinarily follows a certain course. First explorations attempt to discover the nature of the flora, and ecological information is incidental to the habitat notes of collected species. Later, general qualitative observations appear regarding he vegetation of particular habitats. Only after this is a quantitative study using quadrats or other sampling techniques begun, and this quantitative work lays the foundation for future autecological, ecosystem and productivity work. Traditionally the above course is followed first by workers in vascular plant ecology, and studies of cryptograms are seldom as far advanced as that of the spermatophytes. The vascular vegetation of the southern Appalachians is well known, due to the important works of Cain (1935), Whittaker (1956), and others. Studies in the spruce-fir zone of the Great Smoky Mountains National Park are exceptionally complete (Cain, 1935; Crandall, 1958; Schofield, 1960). The knowledge of the bryoecology of the boreal coniferous zone is not, however, so adequate. Sharp (1939) has provided an excellent bryophyte flora of eastern Tennessee, and Cain and Sharp (1938) did important exploratory work in which many of the bryophyte communities recognized in this study were described. The present report is based upon researches made on the bryophyte communities of the major substrates in the spruce-fir zone of the southern Appalachians. It attempts to deal now only with the vegetational compostion of each community but also with a description of the frequency of that bryophyte community in the plant association as a whole. The studies were made from 1958 to 1963 during all months of the year, but primarily in summer and fall. Most of the work was done in the Great Smoky Mountains National Park, but excursion were also made to most of the other mountainous areas in North Carolina and Tennessee which have a forest dominated by boreal conifers. The spruce-fir forest of the Adirondacks of New York was also visited for comparative studies. In the Smokies every area of spruce and fir accessible by trail was seen at least once. The region on Forney Ridge between Mt. Kephart and Siler Bald, and the Mt. LeConte area were most intensively studied because of their easy accessibility. Figure 1, adapted from Crandall (1957), illustrates the location of spruce and fir in the Great Smoky Mountains National Park and maps the location of some of the places mentioned in this paper. The data obtained in this study have laid a foundation for more intensive examination of particular communities, using objective methods of classifying and quantifying vegetation instead of this very subjective ones here used

The Influence Of Vocational Agriculture In Secondary Schools Upon Choice Of Courses By Students Entering Prairie View A&M College

In 1917-18 four departments of vocational agriculture for Negroes were established. By 1939-40 the number of departments had increased to 146. With an enrollment of 4,700 all-day pupils, other enrollments for the year include 722 in day-\u27unit classes; 944 in part-time classes; and 5,511 in evening classes. The enrollments were included in the total enrollment figures of Texas. One of the most significant developments in the Negro program was the organization of the state into districts for supervisory purposes. The teachers were brought together by areas about four times a year. Some group meetings, however, within the districts are held which frequently take the form of skill training schools. The New Farmers of America, a national organization of Negro farm boys studying vocational agriculture, has been developed, in Texas to a high degree. Ordinarily the N. F. A. Chapters sponsor exhibits, at local and district fairs, of projects grown by members. The State Pig Feeding Contest has its termination at these fairs and. has been instrumental in stimulating the live at home program for Negro families in areas served by departments of vocational agriculture. The Prairie View A & M College which is the official training center for Negro teachers of vocational agriculture in Texas was established as a branch of the Texas Agricultural and Mechanical College of Texas in 1879. In 1882 Agricultural and. Industrial courses were added to the college offerings. Although a state conference of teachers of vocational agriculture •was held, at Prairie View in 1918, the teacher training course in vocational agriculture was not organized until 1919-20. The bachelor of science degree in agriculture was awarded in 1921-22. This formed the basis for the organization of the department of vocational agriculture for students who had completed, a seventh-grade education and. who was fourteen years of age or older

Beyond Offsets: Innovative Approaches to Achieving and Exceeding Our Climate Commitments

Climate change is a defining issue of our time, and colleges and university are leading the way by setting ambitious greenhouse gas reduction targets. Many of us are even striving for carbon neutrality under the President’s Climate Commitment. To meet these targets we take measures to reduce our footprints, or do “less bad”. In this situation, we often reach a point where no more reductions are possible, and to meet our targets we need to consider purchasing offsets or renewable energy credits. But instead of spending money in these abstract ways that leave very little room for student interaction, could we focus on doing “more good” – identifying ways we can improve the environment and the community around us? Could we use the money spent on traditional offset programs to support these projects? And, can we somehow quantify these impacts and include them as part of a holistic “less bad and more good” sustainability strategy? These are the questions this workshop will explore. Two thought leaders in this space will give a short presentation on the ways they are thinking beyond offsets, from calculating and flourishing your handprint in addition to reducing your footprint to onsetting instead of offsetting travel emissions to have real impacts in local communities. Then we will spend the remainder of the workshop brainstorming and discussing how we might apply these concepts to rethink our own approaches to offsetting campus emissions. The goal will be to use this as a seed conversation to begin a larger dialogue

Peer Gender and Schooling:Evidence from Ethiopia

In this paper, we study how classmate gender composition matters for students in Ethiopia. We base our results on a unique survey of students across classrooms and schools and among those randomly assigned to class. We find a strong asymmetry: males do not and females do benefit from exposure to more female classmates with less school absence and improvement on math test scores. We further find that exposure to more female classmates improves motivation and participation in class, and in general, that the effects of classmate gender composition are consistent with social interaction effects