Acoustic detection of manatee vocalizations

The West Indian manatee (trichechus manatus latirostris) has become endangered partly because of a growing number of collisions with boats. A system to warn boaters of the presence of manatees, that can signal to boaters that manatees are present in the immediate vicinity, could potentially reduce these boat collisions. In order to identify the presence of manatees, acoustic methods are employed. Within this paper, three different detection algorithms are used to detect the calls of the West Indian manatee. The detection systems are tested in the laboratory using simulated manatee vocalizations from an audio compact disk. The detection method that provides the best overall performance is able to correctly identify approximately 96% of the manatee vocalizations. However, the system also results in a false alarm rate of approximately 16%. The results of this work may ultimately lead to the development of a manatee warning system that can warn boaters of the presence of manatees.     

膨胀腔类超材料的传递损失研究*

声学超材料及结构已被广泛研究,其超结构通常利用3D打印技术实现,当结构刚度较小或者面积较大时,由声固耦合所导致的声学效果与设计不符的情况广泛存在。本文针对含有膨胀腔类的超材料,研究了声固耦合对其声学性能的影响,采用有限元计算结合阻抗管实验的方法,得到其传递损失,分析了声固耦合现象对传递损失的影响。结果表明：薄壁膨胀腔结构的作用频率范围与只考虑声学理论计算的设计不符,声固耦合现象对传递损失产生显著影响;采用增加膨胀腔壁厚、减少膨胀腔内径或选择金属材料的方式,都可以使得声固耦合现象对传递损失的影响减小;仿真结果与实验结果基本吻合。该研究结果说明：对于膨胀腔类超材料,当刚度较小或者面积较大时,对其进行声固耦合分析是完全必要的。     

Transmission loss and dispersion in plastic terahertz photonic band-gap fibers

We present an investigation into the transmission loss and dispersion of terahertz waves in plastic photonic band-gap fibers having a cladding with a finite number of air hole rings. The leakage loss and absorption loss caused by background material are analyzed by a full-vectorial two-dimensional finite difference frequency domain method and the lowest power transmission loss of 6.126 dB/m at 1.75 THz is realized. Numerical results show that a larger diameter-to-pitch ratio is suitable for lower transmission loss and lower group-velocity dispersion in plastic terahertz photonic band-gap fibers. PACS 41.20.Jb; 41.20.Cv; 42.81.Dp     

Determination of West Indian manatee vocalization levels and rate

The West Indian manatee (Trichechus manatus latirostris) has become endangered partly because of a growing number of collisions with boats. A system to warn boaters of the presence of manatees, based upon the vocalizations of manatees, could potentially reduce these boat collisions. The feasibility of this warning system would depend mainly upon two factors: the rate at which manatees vocalize and the distance in which the manatees can be detected. The research presented in this paper verifies that the average vocalization rate of the West Indian manatee is approximately one to two times per 5-min period. Several different manatee vocalization recordings were broadcast to the manatees and their response was observed. It was found that during the broadcast periods, the vocalization rates for the manatees increased substantially when compared with the average vocalization rates during nonbroadcast periods. An array of four hydrophones was used while recording the manatees. This allowed for position estimation techniques to be used to determine the location of the vocalizing manatee. Knowing the position of the manatee, the source level was determined and it was found that the mean source level of the manatee vocalizations is approximately 112 dB (re 1 microPa) @ 1 m.     

Theoretical detection ranges for acoustic based manatee avoidance technology

The West Indian manatee (Trichechus manatus latirostris) has become endangered partly because of watercraft collisions in Florida's coastal waterways. To reduce the number of collisions, warning systems based upon detecting manatee vocalizations have been proposed. One aspect of the feasibility of an acoustically based warning system relies upon the distance at which a manatee vocalization is detectable. Assuming a mixed spreading model, this paper presents a theoretical analysis of the system detection capabilities operating within various background and watercraft noise conditions. This study combines measured source levels of manatee vocalizations with the modeled acoustic properties of manatee habitats to develop a method for determining the detection range and hydrophone spacing requirements for acoustic based manatee avoidance technologies. In quiet environments (background noise approximately 70 dB) it was estimated that manatee vocalizations are detectable at approximately 250 m, with a 6 dB detection threshold, In louder environments (background noise approximately 100dB) the detection range drops to 2.5 m. In a habitat with 90 dB of background noise, a passing boat with a maximum noise floor of 120 dB would be the limiting factor when it is within approximately 100 m of a hydrophone. The detection range was also found to be strongly dependent on the manatee vocalization source level.     

Transmission loss of honeycomb sandwich structures with attached gas layers

Layering gasses of differing acoustic impedances on a panel substantially reduced the amount of sound energy transmitted through the panel with respect to the panel alone or an equivalent-thickness single species gas layer. The additional transmission loss derives from successive impedance mismatches at the interfaces between gas layers and the resulting inefficient energy transfer. Attachment of additional gas layers increased the transmission loss by as much as 17 dB at certain frequencies. The location and ordering of the gasses with respect to the panel were important factors in determining the magnitude of the total transmission loss. Theoretical analysis using a transfer matrix method was used to calculate the frequency dependence of sound transmission for the different configurations tested. The method accurately predicted the relative increases in transmission loss observed with the addition of different gas layer configurations.     

Transmission loss of orthogonally rib-stiffened double-panel structures with cavity absorption

The transmission loss of sound through infinite orthogonally rib-stiffened double-panel structures having cavity-filling fibrous sound absorptive materials is theoretically investigated. The propagation of sound across the fibrous material is characterized using an equivalent fluid model, and the motions of the rib-stiffeners are described by including all possible vibrations, i.e., flexural displacements, bending, and torsional rotations. The effects of fluid-structure coupling are account for by enforcing velocity continuity conditions at fluid-panel interfaces. By taking full advantage of the periodic nature of the double-panel, the space-harmonic approach and virtual work principle are applied to solve the sets of resultant governing equations, which are eventually truncated as a finite system of simultaneous algebraic equations and numerically solved insofar as the solution converges. To validate the proposed model, a comparison between the present model predictions and existing numerical and experimental results for a simplified version of the double-panel structure is carried out, with overall agreement achieved. The model is subsequently employed to explore the influence of the fluid-structure coupling between fluid in the cavity and the two panels on sound transmission across the orthogonally rib-stiffened double-panel structure. Obtained results demonstrate that this fluid-structure coupling affects significantly sound transmission loss (STL) at low frequencies and cannot be ignored when the rib-stiffeners are sparsely distributed. As a highlight of this research, an integrated optimal algorithm toward lightweight, high-stiffness and superior sound insulation capability is proposed, based on which a preliminary optimal design of the double-panel structure is performed.     

Transmission loss of damped asymmetric sandwich panels with orthotropic cores

A theoretical model of the acoustic performance of asymmetric sandwich panels is developed and verified by comparison with experimental data. The panel models consist of unequal elastic isotropic skins sandwiching an elastic orthotropic core. Damping is incorporated in both the skins and the core. The roles of various structural and material properties are determined via a parametric study. The importance of phase wave speeds and panel impedances for physically symmetric panels is discussed, as are implications for transmission loss characterization. For asymmetric panels it is seen that a relatively thick skin on one side produces some change in the transmission loss.     

Background noise cancellation for improved acoustic detection of manatee vocalizations

The West Indian manatee (Trichechus manatus latirostris) has become endangered partly because of an increase in the number of collisions with boats. A device to alert boaters of the presence of manatees, so that a collision can be avoided, is desired. A practical implementation of the technology is dependent on the hydrophone spacing and range of detection. These parameters are primarily dependent on the manatee vocalization strength, the decay of the signal's strength with distance, and the background noise levels. An efficient method to extend the detection range by using background noise cancellation is proposed in this paper. An adaptive line enhancer (ALE) that can detect and track narrow band signals buried in broadband noise is implemented to cancel the background noise. The results indicate that the ALE algorithm can efficiently extract the manatee calls from the background noise. The improved signal-to-noise ratio of the signal can be used to extend the range of detection of manatee vocalizations and reduce the false alarm and missing detection rate in their natural habitat.     

A wavelet packet adaptive filtering algorithm for enhancing manatee vocalizations

Approximately a quarter of all West Indian manatee (Trichechus manatus latirostris) mortalities are attributed to collisions with watercraft. A boater warning system based on the passive acoustic detection of manatee vocalizations is one possible solution to reduce manatee-watercraft collisions. The success of such a warning system depends on effective enhancement of the vocalization signals in the presence of high levels of background noise, in particular, noise emitted from watercraft. Recent research has indicated that wavelet domain pre-processing of the noisy vocalizations is capable of significantly improving the detection ranges of passive acoustic vocalization detectors. In this paper, an adaptive denoising procedure, implemented on the wavelet packet transform coefficients obtained from the noisy vocalization signals, is investigated. The proposed denoising algorithm is shown to improve the manatee detection ranges by a factor ranging from two (minimum) to sixteen (maximum) compared to high-pass filtering alone, when evaluated using real manatee vocalization and background noise signals of varying signal-to-noise ratios (SNR). Furthermore, the proposed method is also shown to outperform a previously suggested feedback adaptive line enhancer (FALE) filter on average 3.4 dB in terms of noise suppression and 0.6 dB in terms of waveform preservation.     

Background noise cancellation of manatee vocalizations using an adaptive line enhancer

The West Indian manatee (Trichechus manatus latirostris) has become an endangered species partly because of an increase in the number of collisions with boats. A device to alert boaters of the presence of manatees is desired. Previous research has shown that background noise limits the manatee vocalization detection range (which is critical for practical implementation). By improving the signal-to-noise ratio of the measured manatee vocalization signal, it is possible to extend the detection range. The finite impulse response (FIR) structure of the adaptive line enhancer (ALE) can detect and track narrow-band signals buried in broadband noise. In this paper, a constrained infinite impulse response (IIR) ALE, called a feedback ALE (FALE), is implemented to reduce the background noise. In addition, a bandpass filter is used as a baseline for comparison. A library consisting of 100 manatee calls spanning ten different signal categories is used to evaluate the performance of the bandpass filter, FIR-ALE, and FALE. The results show that the FALE is capable of reducing background noise by about 6.0 and 21.4 dB better than that of the FIR-ALE and bandpass filter, respectively, when the signal-to-noise ratio (SNR) of the original manatee call is -5 dB.     

Transmission enhancement through a metallic slit assisted by low scattering loss corrugations

A method to obtain large transmission of light through a nano-metallic slit bordered by nano trenches is demonstrated theoretically. The model is based on the composite diffraction of evanescent waves (CDEW) theory and utilizes a single trench design to effectively convert free-space light into surface plasma waves (SPW). The effectiveness is confirmed by the finite difference time domain (FDTD) method. The SPW loss due to scattering by the trench is also analyzed. It is found that when the width of the trench is slightly larger than λ_spp/2, the SPW becomes localized and resonates at the bottom edges performing as a dipole-like radiation source. At this time, the propagating SPW suffers from high scattering loss. Trenches can be used to enhance the transmission of a nano-scaled slit while simultaneously preserving high conversion efficiency and low scattering loss. A nano-scaled slit bordered by 10 pairs of trenches can be enhanced by a factor as high as 16. It is determined that a device designed to have a large clear aperture of 15 μm would have an advantage in terms of alignment.     

The prediction of sound fields inside non-diffuse spaces: Transmission loss considerations

The prediction of sound levels inside internally complex enclosures is extended to include transmission through walls. The sound field is estimated from the spatial variation of phonon density which is calculated by using a random walk technique. This statistical geometrical method also includes consideration of air attenuation and specular reflection of sound from walls whose individual random incidence absorption coefficients are individually known. Solutions agree with experimental results obtained in models, to within, on average, 4 dB.     

The underwater audiogram of the West Indian manatee (Trichechus manatus).

The hearing thresholds of two adult manatees were measured using a forced-choice two alternative paradigm and an up/down staircase psychometric method. This is the first behavioral audiogram measured for any Sirenian, as well as the first underwater infrasonic psychometric test with a marine mammal. Auditory thresholds were obtained from 0.4 to 46 kHz, and detection thresholds of possible vibrotactile origin were measured at 0.015-0.2 kHz. The U-shaped audiogram demonstrates an upper limit of functional hearing at 46 kHz with peak frequency sensitivity at 16 and 18 kHz (50 dB re: 1 microPa). The range of best hearing is 6-20 kHz (approximately 9 dB down from maximum sensitivity). Sensitivity falls 20 dB per octave below 0.8 kHz and approximately 40 dB per octave above 26 kHz. The audiogram demonstrates a wider range of hearing and greater sensitivity than was suggested from evoked potential and anatomical studies. High frequency sensitivity may be an adaptation to shallow water, where the propagation of low frequency sound is limited by physical boundary effects. Hearing abilities of manatees and other marine mammals may have also been shaped by ambient and thermal noise curves in the sea. Inadequate hearing sensitivity at low frequencies may be a contributing factor to the manatees' inability to effectively detect boat noise and avoid collisions with boats.     

Autocorrelation based denoising of manatee vocalizations using the undecimated discrete wavelet transform

Recent interest in the West Indian manatee (Trichechus manatus latirostris) vocalizations has been primarily induced by an effort to reduce manatee mortality rates due to watercraft collisions. A warning system based on passive acoustic detection of manatee vocalizations is desired. The success and feasibility of such a system depends on effective denoising of the vocalizations in the presence of high levels of background noise. In the last decade, simple and effective wavelet domain nonlinear denoising methods have emerged as an alternative to linear estimation methods. However, the denoising performances of these methods degrades considerably with decreasing signal-to-noise ratio (SNR) and therefore are not suited for denoising manatee vocalizations in which the typical SNR is below 0 dB. Manatee vocalizations possess a strong harmonic content and a slow decaying autocorrelation function. In this paper, an efficient denoising scheme that exploits both the autocorrelation function of manatee vocalizations and effectiveness of the nonlinear wavelet transform based denoising algorithms is introduced. The suggested wavelet-based denoising algorithm is shown to outperform linear filtering methods, extending the detection range of vocalizations.     

Transmission loss measurement of acoustic material using time-domain pulse-separation method (L)

An alternative method for measuring the normal incidence sound transmission loss (nSTL) is presented in this paper based on the time-domain separation of so-called Butterworth pulse with a short-duration time about 1 ms in a standing wave tube. During the generation process of the pulse, inverse filter principle was adopted to compensate the loudspeaker response, besides this, the effect of the characteristics of tube termination can be eliminated through the generation process of the pulse so as to obtain a single plane pulse wave in the standing wave tube which makes the nSTL measurement very simple. A polyurethane foam material with low transmission loss and a kind of rubber material with relatively high transmission loss are used to verify the proposed method. When compared with the traditional two-load method, a relatively good agreement between these two methods can be observed. The main error of this method results from the measuring accuracy of the amplitude of transmission coefficient.     

Transmission loss predictions for dissipative silencers of arbitrary cross section in the presence of mean flow

A numerical technique is developed for the analysis of dissipative silencers of arbitrary, but axially uniform, cross section. Mean gas flow is included in a central airway that is separated from a bulk reacting porous material by a concentric perforate screen. The analysis begins by employing the finite element method to extract the eigenvalues and associated eigenvectors for a silencer of infinite length. Point collocation is then used to match the expanded acoustic pressure and velocity fields in the silencer chamber to those in the inlet and outlet pipes. Transmission loss predictions are compared with experimental measurements taken for two automotive dissipative silencers with elliptical cross sections. Good agreement between prediction and experiment is observed both without mean flow and for a mean flow Mach number of 0.15. It is demonstrated also that the technique presented offers a considerable reduction in the computational expenditure when compared to a three-dimensional finite element analysis.     

水介质管路弹性背腔微穿孔消声器的传递损失特性

为抑制水介质管路系统低频噪声,兼顾结构的紧凑性,提出弹性背腔微穿孔管路消声结构,弹性管壁为橡胶帘线复合材料,并推导了传递损失的数值解法。首先,基于Biot-Allard多孔弹性理论,将弹性微穿孔板等效为弹性多孔材料;然后,利用双尺度法建立帘布的周期性代表单元,求得其刚度矩阵;接着,基于分层理论,建立弹性管壁的多层复合材料模型,并与内部声场耦合计算,得到弹性背腔微穿孔管路消声器的传递损失。在水介质驻波管中,利用双声源法测量弹性背腔微穿孔管路消声器样机的传递损失曲线,并与扩张式管路消声器和刚性背腔微穿孔管路消声器进行对比,理论结果与试验结果吻合良好。研究表明,弹性背腔微穿孔管路消声器属于反射耗散复合式消声器,具有低频域、宽频带的消声特性。样机B2在40~300 Hz和40~1200 Hz频段内的传递损失分别为36 dB和30 dB,而相同尺寸扩张式消声器在对应频段的传递损失分别为7 dB和11 dB。