Department of Power Mechanical Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu 30013, Taiwan.
J Acoust Soc Am. 2013 Apr;133(4):2036-45. doi: 10.1121/1.4792147.
A shotgun microphone is a highly directional pickup device widely used in noisy environments. The key element that leads to its superior directivity is a tube with multiple slot openings along its length. One traditional way to model the directional response of a shotgun is to assume plane waves traveling in the tube as if it is in the free field. However, the frequency response and directivity predicted by this traveling wave model can differ drastically from practical measurements. In this paper, an in-depth electroacoustic analysis was conducted to examine the problem by considering the standing waves inside the tube with an analogous circuit containing phased pressure sources and T-networks of tube segments. A further refinement is to model the housing diffraction effect with the aid of the equivalent source method (ESM). The on-axis frequency response and directivity pattern predicted by the proposed model are in close agreement with the measurements. From the results, a peculiar bifurcation phenomenon of directivity pattern at the Helmholtz frequency was also noted. While the shotgun behaves like an endfire array above the Helmholtz frequency, it becomes a broadside array below the Helmholtz frequency. The standing wave effect can be mitigated by covering the slot openings with mesh screen, which was found to alter the shotgun response to be closer to that of the traveling wave model above a critical frequency predicted by the half-wavelength rule. A mode-switching model was developed to predict the directional responses of mesh-treated shotguns.
shotgun 麦克风是一种高度指向性的拾音器，广泛应用于嘈杂环境中。导致其具有优越指向性的关键元件是一根沿长度方向开有多条狭缝的管子。一种传统的建模 shotgun 指向性响应的方法是假设沿管子传播的平面波，就好像它处于自由场中一样。然而，这种行波模型预测的频率响应和指向性与实际测量结果有很大的差异。在本文中，通过考虑管子内的驻波，并使用包含相位压力源和管子段 T 网络的类似电路，对该问题进行了深入的电声分析。进一步的改进是借助等效源法 (ESM) 来建模外壳衍射效应。所提出模型预测的轴上频率响应和指向性图案与测量结果非常吻合。从结果中，还注意到指向性图案在亥姆霍兹频率处的特殊分岔现象。当 shotgun 的频率高于亥姆霍兹频率时，它的行为类似于端射阵列，而低于亥姆霍兹频率时，它的行为类似于宽边阵列。通过用网眼覆盖狭缝开口，可以减轻驻波效应，结果发现，在半波长规则预测的临界频率之上，网眼处理 shotgun 的响应更接近行波模型。开发了一种模式切换模型来预测网眼处理 shotgun 的指向性响应。
