Low Frequency Sound Field Reproduction within a Cylindrical Cavity Using Higher Order Ambisonics

基于HOA的圆柱腔低频声场重构

School of Marine Science and Technology, Northwestern Polytechnic University, Xi’an 710072

Received: 6 May 2017

Abstract

Sound field reproduction of the aircraft and submarine within a cabin mock-up using a loudspeaker array is of great importance to the active noise control technology.The conventional method is to calculate the driving functions of the secondary sources by solving an acoustic inverse problem in a least square sense, which requires a large number of microphones and only the sound field near the microphone array can be reproduced accurately.In order to overcome these drawbacks, higher order ambisonics (HOA) method which is widely used in spatial sound field synthesis for a large room is introduced to reproduce a low frequency sound field within a cylindrical cavity.Due to the different sound propagation characteristics within the cavity compared with a free field and a diffuse field, the Green function spectrum in spherical harmonics domain which is modeled as a superposition of the acoustic modes and the reproduction formulas are deduced.Reproduction characteristics are investigated by numerical simulations.Results show that for a small, the Green function spectrum in spherical harmonics domain is mainly concentrated on low orders and contributed by the low order acoustic modes, with the increase of, high order components of the Green function arise and the contributions of high order acoustic modes increase.In the reproduction process, the high order components of the pressure spectrum over the sphere in harmonics domain will be greatly amplified by the reproduction filter.Finally, HOA method is compared with the acoustic inversion method in terms of the microphone array system, the impact factors on the reproductions and the reproduction accuracy, and validated through experiments.Results show that HOA can better reproduce the entire sound field within the cylindrical cavity and the reproduction accuracy is improved.

摘要

利用扬声器重构声场的传统方法是求解声学逆问题, 计算最小二乘意义下次级声源驱动函数, 缺点是测量时需要布置大量传感器且只能有效重构声阵列区域附近的声场。为了克服该缺陷, 提出利用高阶Ambisonics(higher order ambisonics, HOA)技术重构封闭空间低频声场。针对腔内基于声模态的传播特性, 推导了相关重构公式并分析其重构特性。结果表明, 当重构参数kr较小时, 波数域内格林函数谱分量主要集中于低阶成分且主要由低阶声模态贡献, 随着kr的增大, 其高阶成分变大且高阶声模态的贡献增大。在重构时, 重构滤波器对球阵以外区域的高阶谱分量有明显放大, 意味着对测量噪声也有明显放大, 而对球阵内部有明显抑制。在此基础上, 从声场采集、影响因素及重构效果等方面对HOA与声学逆问题方法进行对比。仿真结果表明, HOA优于声学逆问题方法, 对整个声腔都有较好的重构, 其重构精度与区域范围取决于球谐函数的截断阶数。最后, 在圆柱形舱室模型内进行了实验验证, HOA方法取得了较好的重构结果, 展示出良好的工程应用前景。