Rabbitt R D, Friedrich M T
Department of Mechanical Engineering, Washington University, St. Louis, Missouri 63130.
J Acoust Soc Am. 1991 May;89(5):2379-90. doi: 10.1121/1.400926.
Cross-sectional pressure distributions, natural acoustic modes, and associated cutoff frequencies are determined for real ear-canal geometries using an asymptotic theory in combination with a numerical method. The technique is particularly well suited to obtain the higher modes, which are trapped near both ends of the ear canal. Results detail the influence of the canal geometry and frequency on the spatial distribution of the pressure. Adult ear-canal geometries are determined near the concha from ear-mold sections using a light microscope interfaced to a video-data-acquisition system. Computed results compare favorably to the exact solutions for circular and square acoustic waveguides. The cutoff frequency of the two adult ear canals studied averaged 20% less than the cutoff frequency of a circular tube of identical cross-sectional area. Inserting a probe microphone into the canal decreases the rate of decay of circumferential nonplanar modes while increasing the rate of decay of radial modes. Relative to the pressure beyond the tube, insertion increases the plane-wave component of the pressure around the tube by a multiplicative factor approximately equal to the square root of the original area divided by the occluded area. Eccentric placement of the probe tube has a relatively small influence on the cutoff frequency. The transition of the pressure distribution at the entrance to a simple plane wave in the core region of the canal is calculated and shown graphically for the actual geometry of two adult subjects.
利用渐近理论结合数值方法，针对真实耳道几何形状确定了横截面压力分布、自然声学模式及相关截止频率。该技术特别适合于获取被困在耳道两端附近的高阶模式。结果详细说明了耳道几何形状和频率对压力空间分布的影响。使用与视频数据采集系统相连的光学显微镜，从耳模切片中确定了靠近耳甲的成人耳道几何形状。计算结果与圆形和方形声波导管的精确解相比具有优势。所研究的两条成人耳道的截止频率平均比相同横截面积的圆形管的截止频率低20%。将探头麦克风插入耳道会降低圆周非平面模式的衰减率，同时增加径向模式的衰减率。相对于管外的压力，插入会使管周围压力的平面波分量增加一个约等于原始面积除以阻塞面积的平方根的倍增因子。探头管的偏心放置对截止频率的影响相对较小。针对两名成年受试者的实际几何形状，计算并以图形方式展示了耳道核心区域入口处压力分布向简单平面波的转变。
