浅海小掠射角的海底界面声反向散射模型的简化

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.     

浅海中的混响衰减

本文定义混响衰减作为描述浅海混响的基本物理量。对不同的声速剖面和海底散射模型，分别用射线方法和ＷＫＢＺ简正波方法（考虑和不考虑复本征值的影响）计算了混响衰减随时间（距离）的变化。数值模拟结果表明复本征值的影响不能忽略，在中等距离上按射线方法计算的混响衰减与考虑了复本征值影响的ＷＫＢＺ方法计算结果相一致。负跃层浅海中的实验结果表明混响衰减与源和接收器的深度有关，但与带宽无关。理论预示的混响强度的几何平均规律也从实验得到了证实。     

双基地冰-水界面混响强度的理论预报

针对北极冰层冰水界面具有小尺度粗糙界面的情况,给出由冰层造成的混响平均强度的理论预报公式。首先将北极冰层等效为具有粗糙界面的弹性介质,并且采用小粗糙度微扰理论在精细拟合粗糙度谱的基础上,建立粗糙冰层的三维散射强度模型,然后建立三维双基地声呐几何模型确定有效散射区域,最后给出双基地冰下混响平均强度的估算公式,并在所拟合的粗糙度谱的情况下,利用该预报公式计算双基地声呐配置参数和海冰物理及声学特性对冰下混响强度的影响算例。数值仿真表明该公式能够估算出北极冰下双基地声呐产生的混响平均强度,并且分析出了时延、基线长度等声呐配置参数和冰层声速比、密度比等海冰物理参数会对混响强度造成的影响。声呐的配置参数主要影响散射的有效面积,海冰的物理参数则影响着冰层的散射强度。其中,冰层声速比不仅影响混响强度的大小,还会影响混响强度随时间的衰减速度。      

浅海海底界面混响平均强度特性仿真

海底粗糙界面是产生混响的主要因素之一。本文通过理论分析和数值仿真的方法,根据浅海全波动混响模型对不同海底粗糙界面所引起浅海混响平均强度特性进行研究,主要考虑Goff-Jordan谱、Gauss谱和指数谱三种不同粗糙界面条件下的海底反向散射强度和混响平均强度特性。计算结果表明:海底粗糙界面会引起海底反向散射强度的频率特性的差异,进而导致海底平均混响强度的频率特性的差异,但随入射角度的变化不大。即使界面起伏的方差和相关长度相同,不同的粗糙度谱也会引起平均混响强度的差异。     

浅海平均混响强度的数值模拟

海中混响是回声探测的严重干扰。利用本文给出的混响模型可较快地计算分层海洋中平均混响强度的衰减规律及深度结构。 对于近程混响,由于海水非均匀性的影响较小,使用均匀层的射线方法计算。对于远程混响,使用简正波方法计算,它保持了平滑平均后的深度结构。在中等距离上两者能很好地衔接起来,由于考虑了复本征值对简正波入射场的影响,提高了计算精度,通过比较不同散射模型对混响强度计算的影响,提出以分离型二元散射模型作为混响数值模拟的基础,使能由界面反向散射系数估算二元后向散射系数,并大大简化混响强度的数值计算。     

浅海平均混响强度的简正波理论

本文用简正波方法导出了浅海远程混响强度的一般表式,该式对较广泛的一类声速剖面、海底反射与散射系数皆适用。由于考虑了复本征值对简正波入射场的影响,所得结果比以往的混响场简正波理论有较大的改进。所得出的平均混响强度的表式物理意义清楚,形式简单,便于进行数值计算与解析讨论。作为例子,文中分析了负跃层浅海中平均混响强度的衰减规律。计算表明,当声源与接收器皆位于跃变层之上时,远程混响强度与r~-5成正比。     

非线性内波存在下的浅海远程混响模型

研究了浅海非线性内波对远程混响场的影响。通过分析非线性内波活动引起声源到海底散射元以及散射元到接收点之间的声传播变化,给出了非线性内波引起远程混响强度变化的表示,建立了非线性内波存在下的浅海远程混响模型,数值计算了非线性内波运动引起远程混响强度的变化。理论和数值计算表明,非线性内波的活动能够引起远程混响强度的变化,在某些情况下会导致远程混响强度增强。通过讨论非线性内波引声简正波的耦合效应,给出了其引起远程混响强度增强的原因。      

斜程大气湍流中漫射目标的散射特性

根据推广的惠更斯-菲涅尔原理及ITU-R大气结构常数模型(2001年国际电信联盟提出的随高度变化的大气结构常数模型),考虑了大气湍流对激光从发射机到目标和从目标到接收机双程路径的影响,研究了大气湍流中漫射目标的激光波束散射问题,导出了考虑对数振幅起伏和相位占优情况下在斜程大气湍流中传输时激光波束散射场的互相关函数、强度协方差和强度方差的计算公式,并给出了数值计算结果.数值分析了波长、接收机高度和传播距离对强度协方差和归一化强度方差的影响. 关键词： 激光散射 斜程湍流 强度方差 强度协方差     

低频混响信号中的简正波衰减系数提取方法

提出一种从低频混响信号中提取简正波衰减系数的方法。利用简正波过滤技术对垂直阵混响信号进行分析,获得单阶简正波混响声场。假设海底反向散射矩阵可分离,从单阶简正波平均混响强度中提取出有效海底反向散射矩阵元素,最后利用不同距离上的有效海底反向散射矩阵元素计算出简正波的衰减系数。利用该方法从混响信号中提取出的简正波衰减系数预报的声传播损失和相同海域实测声传播损失一致。该简正波衰减系数提取方法有效避免了海底散射衰减和简正波传播衰减耦合的问题,同时对海底参数反演和水声环境的快速评估也具有重要的意义。      

南海北部海域声速剖面水平变化环境声学层析

为获取水平变化的海水声速分布,在等效平均海水声速反演基础上,提出了一种声速剖面水平变化环境声学层析方法。首先利用经验正交函数结合匹配场处理方法反演不同距离下的等效平均声速剖面,进而通过逐步迭代重构出随水平距离变化的声速场分布,并通过数值仿真及海上实验两方面验证了声速剖面水平变化环境声学层析方法的可行性及其准确性。结果表明:声速剖面水平变化环境声学层析结果与实验测量声速剖面均方根误差小于0.8 m/s,利用声传播信号基本能较为准确地重构出随距离水平变化的海水声速分布,为进一步层析中尺度三维结构及声呐水声环境保障应用奠定了基础。      

Reverberation of rapid and slow trills: implications for signal adaptations to long-range communication

Many acoustic signals in animals include trills, i.e., rapid repetitions of similar elements. Elements within these trills usually are frequency modulated and are degraded by reverberation during long-range transmission. Reverberation primarily affects consecutive elements with the same frequency characteristics and thus imposes a major constraint in the evolution of design and perception of long-range signals containing trills. Here transmission of frequency-unmodulated trills with different element repetition rates was studied. Trills were generated at different frequencies to assess frequency dependence of reverberation and then broadcast under three acoustic conditions--an open field and to assess seasonal changes in transmission properties, a deciduous forest before and after foliage had emerged. Reverberation was quantified at different positions within trills. The results show strong effects of vegetation density (season), transmission distance, frequency, element repetition rate, and element position within the trill on effects of reverberation. The experiments indicate that fast trills transmit less well than slow trills and thus are less effective in long-range communication. They show in particular that selection on trills should not act only on element repetition rate within trills but also on the trill duration as effects of reverberation increased with trill duration.     

浅海海底反射系数幅值参数的反演

理论分析了一种通过混响强度衰减特性获取海底反射系数的幅值参数的方法.将海底反射系数的幅值参数和相位参数引入到全波动混响模型中,为海底反射系数的反演提供理论基础。理论分析和数值仿真表明,在小掠射角条件下,利用混响强度衰减特性反演海底反射系数幅值参数的可行性和准确性。该反演方法只需要输入4个变量:本地混响强度的衰减特性,反射系数的相位参数,海深以及海深处的声速,同时要求混响数据具有一定的混响噪声比(大于6 dB)才能够使反演结果准确可信。根据本地静态混响实验数据成功反演得到海底反射系数的幅值参数.      

界面混响的耦合简正波理论研究

利用耦合简正波理论分析了粗糙界面散射引起的混响。引入了粗糙海底以及海面引起的各个模态间的耦合系数表述粗糙界面对声场的散射过程。结合风浪谱Pierson模型,仿真了不同海况下的海面混响,重点在于分析不同海况下海面起伏对海底混响的影响。利用耦合系数的求解从粗糙海面引起模态间能量耦合的角度对这种影响进行了机理性的探究。结果表明,尽管在浅海中海底混响占支配性地位,但随着海况等级的增加,海面散射对海底混响是有影响的,而这种影响可以从粗糙界面对声波模态间的耦合进行机理分析。      

Effects of reverberation and masking on speech intelligibility in cochlear implant simulations

Two experiments investigated the impact of reverberation and masking on speech understanding using cochlear implant (CI) simulations. Experiment 1 tested sentence recognition in quiet. Stimuli were processed with reverberation simulation (T=0.425, 0.266, 0.152, and 0.0 s) and then either processed with vocoding (6, 12, or 24 channels) or were subjected to no further processing. Reverberation alone had only a small impact on perception when as few as 12 channels of information were available. However, when the processing was limited to 6 channels, perception was extremely vulnerable to the effects of reverberation. In experiment 2, subjects listened to reverberated sentences, through 6- and 12-channel processors, in the presence of either speech-spectrum noise (SSN) or two-talker babble (TTB) at various target-to-masker ratios. The combined impact of reverberation and masking was profound, although there was no interaction between the two effects. This differs from results obtained in subjects listening to unprocessed speech where interactions between reverberation and masking have been shown to exist. A speech transmission index (STI) analysis indicated a reasonably good prediction of speech recognition performance. Unlike previous investigations, the SSN and TTB maskers produced equivalent results, raising questions about the role of informational masking in CI processed speech.     

Quantifying seismic survey reverberation off the Alaskan North Slope

Shallow-water airgun survey activities off the North Slope of Alaska generate impulsive sounds that are the focus of much regulatory attention. Reverberation from repetitive airgun shots, however, can also increase background noise levels, which can decrease the detection range of nearby passive acoustic monitoring (PAM) systems. Typical acoustic metrics for impulsive signals provide no quantitative information about reverberation or its relative effect on the ambient acoustic environment. Here, two conservative metrics are defined for quantifying reverberation: a minimum level metric measures reverberation levels that exist between airgun pulse arrivals, while a reverberation metric estimates the relative magnitude of reverberation vs expected ambient levels in the hypothetical absence of airgun activity, using satellite-measured wind data. The metrics are applied to acoustic data measured by autonomous recorders in the Alaskan Beaufort Sea in 2008 and demonstrate how seismic surveys can increase the background noise over natural ambient levels by 30-45 dB within 1 km of the activity, by 10-25 dB within 15 km of the activity, and by a few dB at 128 km range. These results suggest that shallow-water reverberation would reduce the performance of nearby PAM systems when monitoring for marine mammals within a few kilometers of shallow-water seismic surveys.     

Speech segregation in rooms: monaural, binaural, and interacting effects of reverberation on target and interferer

Speech reception thresholds were measured in virtual rooms to investigate the influence of reverberation on speech intelligibility for spatially separated targets and interferers. The measurements were realized under headphones, using target sentences and noise or two-voice interferers. The room simulation allowed variation of the absorption coefficient of the room surfaces independently for target and interferer. The direct-to-reverberant ratio and interaural coherence of sources were also varied independently by considering binaural and diotic listening. The main effect of reverberation on the interferer was binaural and mediated by the coherence, in agreement with binaural unmasking theories. It appeared at lower reverberation levels than the effect of reverberation on the target, which was mainly monaural and associated with the direct-to-reverberant ratio, and could be explained by the loss of amplitude modulation in the reverberant speech signals. This effect was slightly smaller when listening binaurally. Reverberation might also be responsible for a disruption of the mechanism by which the auditory system exploits fundamental frequency differences to segregate competing voices, and a disruption of the "listening in the gaps" associated with speech interferers. These disruptions may explain an interaction observed between the effects of reverberation on the targets and two-voice interferers.     

A small-scale multi-purpose reverberation room

A one-fourth scale model of the large (425 m³) National Bureau of Standards reverberation room is described. This facility was constructed to carry out acoustical research at relatively low cost in a frequency range two octaves higher than that used in the larger facility. Initial experimental measurements carried out in this facility concern sound power emitted by small sources. The pure tone qualification procedure specified in American National Standard S1.21-1972 ‘Metods for the Determination of Sound Power Levels of Small Sources in Reverberation Rooms’ was carried out using computer control of the experiment. This standard is of particular interest to the international acoustical community since it is technically equivalent to ISO Documents DIS 3741 and DIS 3742. The effect of increased low frequency absorption upon room qualification was tested in the model room. Future research plans to make use of the small-scale reverberation rooms are described.     

早晚期混响划分对理想比值掩蔽在语音识别性能上的影响

真实环境中存在的噪声和混响会降低语音识别系统的性能。封闭空间中的混响包括直达声、早期反射和后期混响3部分,它们对语音识别系统具有不同的影响.我们研究了早期反射和后期混响的不同划分方法,以其中的早期反射为目标语音,计算出了不同的理想比值掩蔽并研究了它们对语音识别系统性能的影响;在此基础上,利用双向长短时记忆网络(BLSTM)估计理想比值掩蔽,测试它们对语音识别系统性能的影响.实验结果表明,基于Abel早期反射和后期混响的划分方法,理想比值掩蔽能够降低词错误率约2.8%;基于BLSTM的估计方法过低估计了理想比值掩蔽,未能有效提高语音识别系统的性能。