“招鹤回鸣”:布拉格共振声学景观

自然界存在各种各样的声学现象,譬如著名的天坛回音壁、三音石。这些现象一般与当地的自然声学环境密切相关,形成独特的声学景观。为解开青岛即墨鹤山声学景观"招鹤回鸣"之谜,进行了三次现场试验和测试。测试数据谱分析结果表明:当在景点北面47级石阶正前方16~20 m区域击掌或发射一定频率的CW脉冲声波时,石阶所构成的准周期性结构形成布拉格声反(散)射体回波,在某些频率附近相长干涉,明显加强,另一些频率则相消干涉而削弱,回波各频率叠加后形成宛如"鹤鸣"声。采用布拉格散射原理分析该石阶结构的回波,数值计算所获得的接收点回波增强频带和回波削弱"带隙"频带区,和上述测试分析结果完全一致。声学景观"招鹤回鸣"源于准周期结构回波特性,不同于天坛回音壁的声学机理。文章所述方法可为类似声景观的设计及分析提供借鉴。

Calling crane echo:acoustic landscape caused by Bragg resonance

There are all kinds of acoustic phenomena in nature, such as the famous Tiantan Echo wall and Three Sound Stone. Generally speaking, these phenomena are closely related to the local natural acoustic environment, which results in the unique acoustic landscape. In order to explain the cause of the famous landscape called "Calling Cranes Echo" occurred at Crane Mountain in Qingdao, three experiments were performed. The analysis results of the experimental data show that, the 47-stone steps in the north of the sight spot forms a scatter with quasi-periodical structure, which satisfies the condition of Bragg Scattering; at some frequencies which are related to the width of each step, the echo in the horizontal direction at the clapping-hand site will form constructive interference, which is strengthened; however, the low-frequency echo caused by stone steps at the clapping-hand site will be weakened for the sound diffusing to all directions, all the frequency components compose the echo like "crane ring". The echoes of the stone step structure are analyzed based on Bragg scattering principle, the echo enhanced bands and echo weakened band (called "band gap") at receiving point by numerical calculation are entirely consistent with the analysis results by measurement and test. Acoustic landscape "Calling Crane Echo" originating from the echo characteristics of quasi periodic structure is different from the acoustic mechanism of Tiantan echo wall. This method can provide reference for similar sound landscape design and analysis.