Traffic noise reduction by means of surface wave exclusion above parallel grooves in the roadside

A new method to reduce traffic noise by means of an ‘invisible’ wall has been investigated both theoretically and experimentally. A formula was derived for the frequency dependent impedance of an infinite structure of parallel ribs on an impedance boundary. From the definition of surface waves it followed that these waves can only exist for certain combinations of frequencies, heights of ribs and phases of the complex reflection coefficient of the underlying surface. Upon making this surface softer, more low frequency sound is absorbed. Outdoor experiments above an array of 16 or 21 low brick walls showed a considerable absorption of sound. Attenuations occurred up to 20 dB in the one-third octave bands from 125 to 400 Hz and amplifications up to 12 dB in the range of 400–1000 Hz. It was possible to explain these measurements qualitatively by the theory of surface waves. The wall structure caused an insertion loss of approximately 4 dB(A) in the total sound pressure level of the A-weighted one-third octave bands from 100 to 12,500 Hz.     

Acoustically assisted removal of nitrogen oxide from high temperature flue gas

The present study focuses on a possibility for improving NO removal efficiency from flue gas by application of powerful sound waves. The sound waves (frequency 6.9-17.2kHz, intensity 144-160dB) are propagated from Hartmann sound generators to a preheated graphite disk inside a vertical reaction tube (height 1.8m, I.D. 105mm). An Ar-NO synthetic mixture (NO 911-934ppm) is blown onto the disk surface to perform reactions in the system C-NO. It is found that the NO reduction rate can be significantly enhanced by the sound waves but the enhancement effect is dependent on the sound frequency, intensity and temperature of disk surface. The better effects are obtained at a temperature of 973K and sound frequencies between 9.6 and 12.4kHz. Under these conditions, the sound application results in 3-5-fold enhancement of NO reduction rate. The obtained effects are explained in terms of gas-phase mass transfer controlling mechanism and of near surface turbulent diffusivity.     

The effects of the ocean surface wave spectrum on low frequency ambient noise in deep water

An ocean surface wave spectrum which is used for low frequency ambient noise in deep water is proposed. It explains the mechanism of low frequency ambient noise from the theoretical relation between the spectrum of sound pressure and wave. Combining the surface wave spectrum and local wind speed in deep water, a theoretical expression of low frequency ambient noise is obtained with wave generated noise theory. Simulation results show that the wave spectrum is crucial to the intensity and the spectral slope of radiated noise spectrum,and the theoretical noise spectrum could be used to predict the ambient noise in deep water.The predicting results axe verified through the experimental data recorded by an ocean bottom seismometer that was deployed on the floor of deep water in April 2016. It is observed that the statistical noise levels from the experimental data for frequencies from 1 Hz to 100 Hz are larger than 70 dB, and the low frequency ambient noise spectrum follows the shape of inverted"N",the valley of noise spectrum is at 3-4 Hz, and the noise intensity is 70 dB. The peak of noise spectrum is at 50 Hz, and the noise intensity is 92 dB. The correlation coefficient is 0.95 between the model spectrum and measured data.     

海面波浪谱对深海低频环境噪声的影响

提出了一种适用于深海低频环境噪声的波浪谱,通过声压谱和波浪谱的理论关系,分析了深海低频噪声在百赫兹以下的谱特征,解释了不同频段噪声谱的主要产生机理。将深海传播条件下海面波浪谱与海面风速相结合,利用波浪发声理论得到一种低频海洋环境噪声理论表示方法。仿真结果表明,波浪谱决定着辐射噪声谱的强度和斜率,本模型得到的理论噪声谱可以对低频海洋环境噪声进行预报。2016年的深海实验观测数据分析显示,统计的环境噪声谱级在1 Hz至100 Hz频段范围内大于70 dB,并且噪声谱在低频段呈倒“N”型,在34 Hz处为噪声谱的谷值,噪声级为70 dB,在50 Hz处为噪声谱的峰值,噪声级为92 dB,通过理论计算和实验对比,相关系数为0.95,理论结果和实验测量对比结果符合较好。      

南海北部浅海声场起伏及统计特性

水下声场的时间积分声强和峰值声强是声呐检测中的重要物理量,而海洋中的内波等动力学过程会造成声场强度的起伏。对南海北部浅海内波环境下定点声起伏实验数据分析,结合数值仿真,总结了试验海域近海底发射和近海底接收条件下内波引起接收信号强度起伏的统计特性.分析结果表明:接收信号的时间积分声强起伏小于峰值声强起伏;时间积分声强与峰值声强起伏的概率分布与对数正态分布较为接近.时间积分声强和峰值声强起伏大小与信号的频率有关,同一收发距离,中心频率650 Hz信号的声强起伏较中心频率310 Hz和375 Hz信号的声强起伏更为剧烈。对于同一频率,当海底较平坦时,声强起伏的闪烁指数随传播距离的增加而增大;当水深随传播距离逐渐变深时,声强起伏规律受内波和水深变化共同影响而更为复杂,闪烁指数先随传播距离增加而增大,之后又随传播距离的增加而逐渐变小.      

Sorption kinetics of aqueous benzene for attached bacteria on sorbents

In this study we investigated the effect of bacteria attached onto the surfaces of two sorbents e.g., activated carbon and alginate bead, on the sorption kinetics of aqueous benzene. Kinetic sorption experiments were performed to obtain time-dependent data of aqueous concentration subjected to sorption at an initial concentration. The kinetic sorption data were fitted with two-site sorption model that consists of fast (equilibrium type) sorption site and limited slow (nonequilibrium or kinetic type) sorption site. Experimental results showed that both fast sorption and biodegradation was associated with removal of benzene for alginate bead, but only fast sorption for activated carbon for either case of bacterial treatment. Much lower sorption for attached case of carbon can be explained by the fact that bacteria inhibited sorption of aqueous benzene by filling them in the cavities of carbon surface. Modeling study revealed that sorption process for activated carbon followed two-site kinetic with unlimited slow sorption but the one for bead was described by two-site kinetic with limited slow sorption model since a significant amount of aqueous benzene was present in aqueous phase due to lower sorbing capacity.     

Sound fluctuations caused by internal waves in a shallow sea

Estimates are presented for the fluctuations of the parameters of low-frequency sound fields in shallow-water regions of the Barents Sea, in the presence of seasonal internal gravity waves. The objective of the experiments is to reveal the main mechanisms that govern the sound fluctuations and their statistical parameters on paths of moderate lengths (50–60 to 100–120 km). Another objective is to determine the features of the sound interaction with internal waves for the sound speed profile of the summer—autumn type for which the water stratification is most pronounced. As the probing signals, continuous tonal ones produced by bottommoored sources at the frequencies about 100 and 300 Hz are used along with the 1/3-octave noise signals with the central frequency 1000 Hz, which are generated by a source deployed from a vessel. For the signal reception, both fixed bottom-moored hydrophones and a vertical chain of hydrophones are used, the chain also being deployed from the vessel. The water temperature, the salinity, and the thermocline displacements are monitored with standard hydrographic sensors. The following main results are presented: the estimate of the degree of correlation between the sound fluctuations and the parameters of the water layer, the comparison of the fluctuations in the signal amplitude envelope with the data obtained in other regions, and the estimate of the statistical parameters of the signal amplitude fluctuations, including their dependence on the path length. One more result consists in the proof of the wave nature of the interaction of sound and internal waves, which manifests itself in a strong dependence of the sound interaction with internal waves of discrete frequencies on the frequency of the probing signal and on the angle at which these wave beams intersect. An attempt is made to explain the observed phenomena by the synchronism in the interacting sound and gravity waves. The data obtained can be used to analyze and compare the fluctuations of the sound fields in the ocean, especially in shallow-water regions.     

AN ACOUSTIC TRANSFORMER COMPOSED OF HORNS AND ITS APPLICATION

A new acoustic transformer was developed by connecting three horns to improve radiation performance in the frequency region below 500 Hz. The proposed acoustic transformer was evaluated by numerical analysis using the commercial computer program SYSNOISE and by experiment. The acoustic transformer is composed of three horns, one of which was used in an inverted form. A design model was developed by use of Webster's horn equation and showed that the transformer can improve radiation efficiency. This was confirmed by numerical calculation using SYSNOISE. An acoustic projector was designed by use of the developed transformer and a piezoelectric unimorph-type actuator. The sound pressure measured at the mouth of the constructed acoustic projector was compared with the sound pressures evaluated at the same location by numerical calculation to investigate the differences between the numerical simulation model and the actual acoustic projector. Sound pressures generated by several acoustic radiators were calculated numerically and compared with the measured and calculated sound pressures of the developed acoustic projector to evaluate the effects of use of the proposed acoustic transformer. The comparative evaluation shows that the proposed acoustic transformer can provide up to a 10 dB gain over use of a horn in the narrow band low-frequency region from 100 to 200 Hz.     

Measurement of the resistivity of porous materials with an alternating air-flow method

Air-flow resistivity is a main parameter governing the acoustic behavior of porous materials for sound absorption. The international standard ISO 9053 specifies two different methods to measure the air-flow resistivity, namely a steady-state air-flow method and an alternating air-flow method. The latter is realized by the measurement of the sound pressure at 2 Hz in a small rigid volume closed partially by the test sample. This cavity is excited with a known volume-velocity sound source implemented often with a motor-driven piston oscillating with prescribed area and displacement magnitude. Measurements at 2 Hz require special instrumentation and care. The authors suggest an alternating air-flow method based on the ratio of sound pressures measured at frequencies higher than 2 Hz inside two cavities coupled through a conventional loudspeaker. The basic method showed that the imaginary part of the sound pressure ratio is useful for the evaluation of the air-flow resistance. Criteria are discussed about the choice of a frequency range suitable to perform simplified calculations with respect to the basic method. These criteria depend on the sample thickness, its nonacoustic parameters, and the measurement apparatus as well. The proposed measurement method was tested successfully with various types of acoustic materials.     

A numerical study of double-leaf microperforated panel absorbers

Microperforated panel (MPP) absorbers are promising as a basis for the next-generation of sound absorbing materials. Typically, they are backed by an air-cavity in front of a rigid wall such as a ceiling or another interior surface of a room. Indeed, to be effective, MPP absorbers require the Helmholtz-type resonance formed with the backing cavity. Towards the creation of an efficient sound-absorbing structure with MPPs alone, the acoustical properties of a structure composed of two parallel MPPs with an air-cavity between them and no rigid backing is studied numerically. In this double-leaf MPP (DLMPP) structure, the rear leaf (i.e., the MPP remote from the incident sound) plays the role of the backing wall in the conventional setting and causes resonance-type absorption. Moreover, since a DLMPP can work efficiently as an absorber for sound incidence from both sides, it can be used efficiently as a space absorber, e.g., as a suspended absorber or as a sound absorbing panel. The sound absorption characteristics of the double-leaf MPP are analysed theoretically for a normally incident plane wave. The effects of various control parameters are discussed through a numerical parametric study. The absorption mechanisms and a possible design principle are discussed also. It is predicted that: (1) that a resonance absorption, similar to that in conventional type MPP absorbers, appears at medium-to-high frequencies and (2) that considerable “additional” absorption can be obtained at low frequencies. This low-frequency absorption is similar to that of a double-leaf permeable membrane and can be an advantage compared with the conventional type of MPP arrangement.     

基于声场重建的材料吸声系数测量方法

针对现有方法对材料吸声系数进行现场测量时存在低频测量误差大的问题,本文提出了一种利用扬声器线阵列对材料吸声系数进行现场测量的新方法。该方法使用基于能量比值约束的最小二乘法在待测材料表面进行平面波声场重建并结合双传声器传递函数法对材料的吸声系数进行测量。数值仿真表明在100～1600 Hz频率范围内,新方法在未加约束时能够对材料的吸声系数进行准确测量。在半消声室中利用新方法测量了三聚氰胺泡沫的吸声系数,分析了能量比值约束值对测量结果的影响,并和阻抗管以及其它两种现场测量方法的测量结果进行了对比。结果表明该方法能够对吸声材料在160～1600 Hz频段内的吸声系数进行准确测量,并且相较于现存的现场测量方法,新方法具有更低的测量频率下限。     

Using optimized surface modifications to improve low frequency response in a room

A case study of improving sound energy distribution at low frequency in a small orthogonal room is presented in this paper. The effects of the geometric modifications of wall surface on the sound frequency response have been investigated in depth. In order to find the optimal modifications for the wall surface, an optimization procedure, based on finite element analysis, has been developed. The uniqueness of this method is that it takes both modal redistribution and sound diffusion into account during optimization process. As a result, the promising improvements of sound frequency response have been obtained at the frequencies around 100 Hz in all rooms tested, particularly in those where the serious modal concentrations are met. The maximum reduction of sound fluctuation in such a room could reach a mount of 4.6 dB. The work opens up the possibility of improving low frequency sound quality by a means that considers both modal changing and surface scattering at same time.     

Long-range sound propagation in the shallow-water part of the Sea of Okhotsk

Experimental data on the long-range propagation of explosion-generated sound signals in the shallow-water northern part of the Sea of Okhotsk are analyzed. The propagation conditions in this region are characterized by a fully-developed underwater sound channel that captures the rays crossing the channel axis at angles lower than 3°. The experimental data reveal a small increase in the duration of the sound signal in proportion to the range with the proportionality factor lower than 0.00025 s/km. The frequency dependence of attenuation exhibits a pronounced minimum whose position on the frequency axis is close to the critical frequency of the first “water” mode (about 160 Hz). The increase in the attenuation coefficient at lower frequencies is confirmed by the field calculations performed with the wave-field computer code and is explained by the sound energy loss in the bottom sediments. At frequencies higher than 200 Hz, as in the Baltic Sea, the most probable reason for the attenuation to exceed the absorption in sea water is sound scattering by internal waves.     

Performance Evaluation and Effectiveness of the Reverberation Room

This research presents a thorough evaluation of the reverberation room at Acoustics Laboratory in National Institute of Standards (NIS) according to the related international standards. The evaluation aims at examining the room performance and exploring its effectiveness in the frequency range from 125 Hz to 10000 Hz according to the international standard requirements. The room, which was designed and built several years ago, is an irregular rectangular shape free from diffusers. Its volume is about 158.84 m³, which meets the requirement of the ISO 354 standard L_max < 1.9V^1/3. Cut-off frequencies of one and one-third octave are 63 Hz and 100 Hz respectively; however Schroder frequency is 400 Hz. Calculations of cut-off frequency and modal density showed adequate modes that give acceptable uniformity starting comfortably from frequency of 125 Hz. The room has a reverberation time that is suitable for its size over the frequency range of interest. The room sound absorption surface area and its sound absorption coefficient satisfy the criteria given in ISO 3741 and ISO 354. There is an accepted diffuse sound field inside the room due to the standard deviation of measured sound level, which is less than 1.5 dB over all the frequency range. The only exception was 125 Hz which may be due to a lack of diffusivity of the sound field at this frequency. The evaluation proves that the NIS reverberation room is in full agreement with the international standards, which in turns qualifies the room to host measurements inside without concerns.     

Experimental study of sound propagation in a flexible duct

Propagation of sound in a flexible duct is investigated both theoretically and experimentally. Strong coupling of sound and flexural waves on the duct wall is found when the wall-to-air mass ratio is of the order of unity. The axial phase speed of sound approaches the in vacuo speed of flexural waves (subsonic in this case) at low frequencies. However, a speed higher than the isentropic sound speed in free space (340 m/s) is found beyond a critical frequency which is a function of the mass ratio. Experiments using a duct with a finite section of tensioned membrane are compared with the propagating modes pertaining to the infinite membrane model. Satisfactory quantitative agreement is obtained and the measured phase speed ranges from 8.3 to 1348 m/s. In the moderate frequency range, the theory predicts high spatial damping rate for the subsonic waves, which is consistent with the experimental observation that subsonic waves become increasingly undetectable as the frequency increases. Substantial sound reflection is observed at the interface between the rigid and the flexible segments of the duct without cross-section discontinuity, which, together with the high spatial damping, could form a basis for passive control of low-frequency duct noise.     

Ambient habitat noise and vibration at the Georgia Aquarium

Underwater and in-air noise evaluations were completed in performance pool systems at Georgia Aquarium under normal operating conditions and with performance sound tracks playing. Ambient sound pressure levels at in-pool locations, with corresponding vibration measures from life support system (LSS) pumps, were measured in operating configurations, from shut down to full operation. Results indicate noise levels in the low frequency ranges below 100?Hz were the highest produced by the LSS relative to species hearing thresholds. The LSS had an acoustic impact of about 10?dB at frequencies up to 700?Hz, with a 20?dB re 1?μPa impact above 1000?Hz.     

Whole-lung resonance in a bottlenose dolphin (Tursiops truncatus) and white whale (Delphinapterus leucas)

An acoustic backscatter technique was used to estimate in vivo whole-lung resonant frequencies in a bottlenose dolphin (Tursiops truncatus) and white whale (Delphinapterus leucas). Subjects were trained to submerge and position themselves near an underwater sound projector and a receiving hydrophone. Acoustic pressure measurements were made near the thorax while the subject was insonified with pure tones at frequencies from 16 to 100 Hz. Whole-lung resonant frequencies were estimated by comparing pressures measured near the subject's thorax to those measured from the same location without the subject present. Experimentally measured resonant frequencies for the white whale and dolphin lungs were 30 and 36 Hz, respectively. These values were significantly higher than those predicted using a free-spherical air bubble model. Experimentally measured damping ratios and quality factors at resonance were 0.20 and 2.5, respectively, for the white whale, and 0.16 and 3.1, respectively, for the dolphin.     

Acoustic characteristics of composite materials as acoustic window at oblique incidence of sound waves

This paper described a method for estimating the acoustic characteristics of composite materials at oblique incidence of sound waves. Composite materials are used as acoustic windows of SONAR to protect the internal sensors and electronic parts from water. In this study the composite material of glass reinforced plastic and polyurethane was used as the specimen. As the acoustic characteristics the velocities and attenuation coefficients of sound waves through the composite material were measured in the high frequency range. The insertion loss was also measured as a function of incident angle at 200 and 76 kHz, respectively. The attenuation coefficients in the low frequency range were estimated by interpolating the measured attenuation in the high frequency range with power-law form fitting. A four-medium layer model was proposed to estimate the insertion loss of composite materials at oblique incidence of sound waves in the low frequency range. The four-medium layer model well described the experimentally measured insertion loss at the high frequency range. It suggests that the insertion loss of the composite materials can be well estimated as a function of incident angle in the low frequency range.     

Investigation of industrial tea-leaf-fibre waste material for its sound absorption properties

The sound absorption of an industrial waste, developed during the processing of tea leaves has been investigated. Three different layers of tea-leaf-fibre waste materials with and without backing provided by a single layer of woven textile cloth were tested for their sound absorption properties. The experimental data indicate that a 1 cm thick tea-leaf-fibre waste material with backing, provides sound absorption which is almost equivalent to that provided by six layers of woven textile cloth. Twenty millimeters thick layers of rigidly backed tea-leaf-fibres and non-woven fibre materials exhibit almost equivalent sound absorption in the frequency range between 500 and 3200 Hz.     

Radiation characteristics of multiple and single sound hole vihuelas and a classical guitar

Two recently built vihuelas, quasi-replicas of the Spanish Renaissance guitar, one with a small body and one sound hole and one with a large body with five sound holes, together with a classical guitar are investigated. Frequency dependent radiation strengths are measured using a 128 microphone array, back-propagating the frequency dependent sound field upon the body surface. All three instruments have a strong sound hole radiation within the low frequency range. Here the five tone holes vihuela has a much wider frequency region of strong sound hole radiation up to about 500 Hz, whereas the single hole instruments only have strong sound hole radiations up to about 300 Hz due to the enlarged radiation area of the sound holes. The strong broadband radiation of the five sound hole vihuela up to about 500 Hz is also caused by the sound hole phases, showing very consistent in-phase relations up to this frequency range. Also the radiation strength of the sound holes placed nearer to the center of the sound box are much stronger than those near the ribs, pointing to a strong position dependency of sound hole to radiation strength. The Helmholtz resonance frequency of the five sound hole vihuela is influenced by this difference in radiation strength but not by the rosettas, which only have a slight effect on the Helmholtz frequency.