Talmadge Carrick L, Waxler Roger, Di Xiao, Gilbert Kenneth E, Kulichkov Sergey
National Center for Physical Acoustics, University of Mississippi, University, Mississippi 38677, USA.
J Acoust Soc Am. 2008 Oct;124(4):1956-62. doi: 10.1121/1.2967474.
A natural terrain surface, because of its porosity, can support an acoustic surface wave that is a mechanical analog of the familiar vertically polarized surface wave in AM radio transmission. At frequencies of several hundred hertz, the acoustic surface wave is attenuated over distances of a few hundred meters. At lower frequencies (e.g., below approximately 200 Hz) the attenuation is much less, allowing surface waves to propagate thousands of meters. At night, a low-frequency surface wave is generally present at long ranges even when downward refraction is weak. Thus, surface waves represent a ubiquitous nighttime transmission mode that exists even when other transmission modes are weak or absent. Data from recent nighttime field experiments and theoretical calculations are presented, demonstrating the persistence of the surface wave under different meteorological conditions. The low-frequency surface wave described here is the "quasiharmonical" tail observed previously in nighttime measurements but not identified by S. Kulichkov and his colleagues (Chunchuzov, I. P. et al. 1990. "On acoustical impulse propagation in a moving inhomogeneous atmospheric layer," J. Acoust. Soc. Am. 88, 455-461).
由于其孔隙率，自然地形表面能够支持一种声表面波，它是调幅无线电传输中常见的垂直极化表面波的机械类似物。在几百赫兹的频率下，声表面波在几百米的距离上会衰减。在较低频率（例如，低于约200赫兹）时，衰减要小得多，使得表面波能够传播数千米。在夜间，即使向下折射较弱，低频表面波通常也会在远距离存在。因此，表面波代表了一种普遍存在的夜间传输模式，即使在其他传输模式较弱或不存在时也依然存在。本文给出了近期夜间野外实验的数据和理论计算结果，证明了表面波在不同气象条件下的持续性。这里描述的低频表面波是先前在夜间测量中观察到的“准谐波”尾部，但未被S. 库利奇科夫及其同事识别出来（丘恩丘佐夫，I. P. 等人，1990年。“关于在移动的非均匀大气层中声脉冲的传播”，《美国声学学会杂志》88，455 - 461）。
