全文获取类型
收费全文 | 3758篇 |
免费 | 61篇 |
国内免费 | 17篇 |
专业分类
化学 | 2300篇 |
晶体学 | 26篇 |
力学 | 76篇 |
数学 | 600篇 |
物理学 | 834篇 |
出版年
2022年 | 30篇 |
2021年 | 37篇 |
2020年 | 58篇 |
2019年 | 41篇 |
2018年 | 25篇 |
2017年 | 22篇 |
2016年 | 45篇 |
2015年 | 51篇 |
2014年 | 47篇 |
2013年 | 164篇 |
2012年 | 162篇 |
2011年 | 178篇 |
2010年 | 99篇 |
2009年 | 87篇 |
2008年 | 153篇 |
2007年 | 189篇 |
2006年 | 177篇 |
2005年 | 173篇 |
2004年 | 168篇 |
2003年 | 139篇 |
2002年 | 124篇 |
2001年 | 84篇 |
2000年 | 96篇 |
1999年 | 56篇 |
1998年 | 65篇 |
1997年 | 53篇 |
1996年 | 63篇 |
1995年 | 64篇 |
1994年 | 52篇 |
1993年 | 74篇 |
1992年 | 45篇 |
1991年 | 41篇 |
1990年 | 45篇 |
1989年 | 45篇 |
1988年 | 39篇 |
1987年 | 42篇 |
1986年 | 43篇 |
1985年 | 58篇 |
1984年 | 47篇 |
1983年 | 38篇 |
1982年 | 34篇 |
1981年 | 37篇 |
1980年 | 36篇 |
1979年 | 26篇 |
1978年 | 30篇 |
1977年 | 39篇 |
1976年 | 45篇 |
1975年 | 49篇 |
1974年 | 41篇 |
1973年 | 45篇 |
排序方式: 共有3836条查询结果,搜索用时 15 毫秒
1.
Quasi-wavelets (QWs) are a representation of turbulence consisting of self-similar, eddy-like structures with random orientations and positions in space. They are used in this paper to calculate the scattering, due to turbulent velocity fluctuations, of sound behind noise barriers as a function of the size and spatial location of the eddies. The sound scattering cross-section for QWs of an individual size class (eddy size) is derived and shown to reproduce results for the von Kármán spectrum when the scattered energies from a continuous distribution of QW sizes are combined. A Bragg resonance condition is derived for the eddy size that scatters most strongly for a given acoustic wavenumber and scattering angle. Results for scattering over barriers show that, for typical barrier conditions, most of the scattered energy originates from eddies in the size range of approximately one-half to twice the size of the eddies responsible for maximum scattering. The results also suggest that scattering over the barrier due to eddies with a line of sight to both the source and receiver is generally significant only for frequencies above several kilohertz, for sources and receivers no more than a few meters below the top of the barrier, and for very turbulent atmospheric conditions. 相似文献
2.
3.
It is proved that for finite groups G, the probability thattwo randomly chosen elements of G generate a soluble subgrouptends to zero as the index of the largest soluble normal subgroupof G tends to infinity. 相似文献
4.
5.
6.
7.
A. L'Esprance J. Nicolas D.K. Wilson D.W. Thomson Y. Gabillet G. Daigle 《Applied Acoustics》1993,40(4)
This paper presents a set of acoustical and meteorological data from an outdoor sound propagation experiment. This experiment was done in a farm field near Rock Springs, Pennsylvania, on 7 July 1990. Meteorological and acoustical measurements were recorded simultaneously during six different times in the day. The meteorological measurements permitted determination of the sound speed profiles during each of the measurement sessions, using a method based on surface-layer similarity scaling. The acoustical measurements allowed precise determination of the relative sound pressure levels for a frequency range up to 3150 Hz at six different distances (66, 88, 125, 175, 250 and 350 m). The results show atmospheric conditions have an important effect on sound propagation. At medium and high frequencies, variations of the relative SPL have been measured at distances as short as 62 m. These effects increased with the distances so that variations as great as 30 dB have been measured during that day. Comparisons with the fast field program predictions are also presented, and amply demonstrate the accuracies of this model, especially for the downward refraction cases. 相似文献
8.
9.
10.