Long time-base observations of surf noise

A year of surf noise observations in the very near shore region of La Jolla Shores beach are presented. Ambient sound levels and surface wave height were recorded for 9 min every hour from July 1997 through June 1998 at a monitoring station located 360 m seaward of the beach in 8-m deep water. Sound segments that were dominated by the noise from breaking surf formed the basis of a correlation analysis between surf noise level and wave height, wave period, wind speed, and mean water depth. The analysis shows that surf noise is primarily determined by wave height, and scales approximately with the wave height squared. The surface wave energy flux onto the beach also scales with wave height squared, leading to the conclusion that the conversion of the mechanical energy of the surface wave field into noise energy is approximately constant. In fact, the ratio of noise energy to surface wave energy flux varies by up to a factor of 3 over the range of energy fluxes considered (100-3000 W per m).     

The effect of temperature on sound wave absorption in a sediment layer

The effect of temperature on sound velocity, absorption, and reflection coefficient in the seabed sediment layer is investigated. Experimental measurements of sound speed, absorption, and the reflection coefficient in a sandy sediment layer have been carried out at several temperatures. An absorption reduction of 75 dB/m and a velocity increase of 65 m/s have been measured at a frequency of 1 MHz when the temperature increases from 5 to 25 degrees C. Because of the absorption temperature dependence the amplitude of the reflected wave from the back surface of the sub-bottom layer after going back and forth across the layer increases with the temperature.     

Factors affecting sound exposure from firing an SA80 high-velocity rifle

The effect of distance on the peak sound pressure level and sound exposure level from an SA80 rifle has been investigated. Sound pressure waveforms were measured in two directions from the gun: downrange, from 50 m to 300 m, and to the left-hand side, from 0.3 m to 32 m. Some additional measurements were made to the right of the gun. Measurements made downrange showed three distinct features of the waveform; the shock wave from the supersonic bullet, the reflection from the ground, and the muzzle blast. The time elapsed between the shock wave and the muzzle blast increased with increasing distance: 94 ms for a distance of 50 m, and 507 ms for a distance of 300 m. The highest peak sound level downrange from a single round was between 151 dB(C) and 148 dB(C) at distances from 50 m to 300 m, and varied little if at all with distance. To the left of the gun, the peak sound pressure level of 161 dB(C) at 0.3 m reduced to 128 dB(C) at 32 m. The peak sound pressure level was estimated to be 137 dB(C) at a distance of approximately 20 m to the left-hand side. Hearing protection must therefore be worn by anyone closer than 20 m to a person firing. The peak sound pressure level was estimated to be 135 dB(C) at a distance of approximately 25 m and therefore hearing protection is recommended at distances of up to 25 m. The sound exposure level of 98 dB(A) at 20 m indicated that an observer at this distance could hear about 1440 rounds without hearing protection before the noise exposure reached the upper exposure action value specified in the Control of Noise at Work Regulations 2005. Peak sound pressure levels were on average 2.4 dB higher at the left ear compared with the right ear.     

Sound propagation in the atmospheric surface layer: Comparison of experiment with FFP predictions

This paper presents a set of acoustical and meteorological data from an outdoor sound propagation experiment. This experiment was done in a farm field near Rock Springs, Pennsylvania, on 7 July 1990. Meteorological and acoustical measurements were recorded simultaneously during six different times in the day. The meteorological measurements permitted determination of the sound speed profiles during each of the measurement sessions, using a method based on surface-layer similarity scaling. The acoustical measurements allowed precise determination of the relative sound pressure levels for a frequency range up to 3150 Hz at six different distances (66, 88, 125, 175, 250 and 350 m). The results show atmospheric conditions have an important effect on sound propagation. At medium and high frequencies, variations of the relative SPL have been measured at distances as short as 62 m. These effects increased with the distances so that variations as great as 30 dB have been measured during that day. Comparisons with the fast field program predictions are also presented, and amply demonstrate the accuracies of this model, especially for the downward refraction cases.     

Air-ground interaction in long range propagation of low frequency sound and vibration—field tests and model verification

An extensive program of intermediate and long range impulsive sound propagation field tests have been conducted. The test program and the performed measurements are presented. Particular focus is given on the air-ground interaction and its effect on low frequency sound and vibration propagation. It is found that the pressure wave interaction with the viscoelastic Rayleigh wave in the ground may have a significant effect on the ground impedance and the sound and vibration propagation. This introduces an important mechanism not covered in commonly used ground impedance models. Numerical simulation models have been developed and verified against the test data. The ground impedance does not only effect the sound pressure propagation. If either acoustically induced ground vibration, or ground to building transmitted vibration, is to be considered, the acousto-seismic impedance has a dramatic effect on the level of ground vibration induced by a given sound pressure. For a site where Rayleigh wave interaction appears at the dominant frequencies of the sound pressure, the ground vibration may be greater than a factor 100 (40 dB) than at a site with ground conditions not making the interaction happen.     

Testing meteorological classifications for the prediction of long-term average sound levels

The paper describes the results of a preparatory study for the planned introduction of a harmonised method to assess long-term sound levels in Europe. The purpose of this study is to quantify the error in the prediction of long-term sound levels when propagation calculations refer to a limited number of between 1 and 121 representative meteorological situations. This error is determined with respect to the average sound level resulting from parabolic-equation simulations for 1452 meteorological situations of consecutive 6-h intervals during one year. The meteorological situations were taken from meteorological tower measurements at Garching near Munich, Germany. The various meteorological conditions enter the model through two-parameter logarithmic-linear vertical profiles of the effective speed of sound which were fitted to the tower measurements. While source power and ground impedance were held constant, the model results elucidate the sound level variations due to the meteorological variability. A spread of 18 dB was found in the calculated instantaneous levels at 200 m range for rigid ground. The spread was 42 dB at 1000 m. Because of the asymmetric frequency distribution of wind directions, a spread of still up to 5.5 dB was found in the calculated annual average levels at 1000 m range for various directions of propagation. As a consequence of the specific meteorological conditions the annual average night-time level exceeds the annual average day-time level by up to 5 dB. Only 25 logarithmic-linear profile classes are necessary to determine the long-term average sound level at 1000 m range with an accuracy better than 2 dB. The use of purely linear profile classes slightly impairs the results, whereas the use of purely logarithmic profile classes cannot be recommended.     

NUMERICAL MODELLING OF THE SOUND FIELDS IN URBAN STREETS WITH DIFFUSELY REFLECTING BOUNDARIES

A radiosity-based theoretical/computer model has been developed to study the fundamental characteristics of the sound fields in urban streets resulting from diffusely reflecting boundaries, and to investigate the effectiveness of architectural changes and urban design options on noise reduction. Comparison between the theoretical prediction and the measurement in a scale model of an urban street shows very good agreement. Computations using the model in hypothetical rectangular streets demonstrate that though the boundaries are diffusely reflective, the sound attenuation along the length is significant, typically at 20-30 dB/100 m. The sound distribution in a cross-section is generally even unless the cross-section is very close to the source. In terms of the effectiveness of architectural changes and urban design options, it has been shown that over 2-4 dB extra attenuation can be obtained either by increasing boundary absorption evenly or by adding absorbent patches on the façades or the ground. Reducing building height has a similar effect. A gap between buildings can provide about 2-3 dB extra sound attenuation, especially in the vicinity of the gap. The effectiveness of air absorption on increasing sound attenuation along the length could be 3-9 dB at high frequencies. If a treatment is effective with a single source, it is also effective with multiple sources. In addition, it has been demonstrated that if the façades in a street are diffusely reflective, the sound field of the street does not change significantly whether the ground is diffusely or geometrically reflective.     

A model for the horizontal directionality of breaking wave noise in the surf zone

A model is presented for the horizontal directionality of the noise generated by individual breaking waves in the surf zone. The model is based on the interaction between sound radiated by ringing bubbles created in a breaking wave crest and the population of acoustically quiescent bubbles left on the seaward side of the wave in the shallow water waveguide. The effect of the quiescent bubbles is to absorb sound from all but the very ends of the breaking crest, resulting in the formation of "acoustic hot-spots." The model calculations are in good agreement with observations of the horizontal directionality of noise from individual breaking waves in the surf zone.     

Noise control for rapid transit cars on elevated structures

Noise control treatments for the propulsion motor noise of rapid transit cars on concrete elevated structures and the noise reduction from barrier walls were investigated by using acoustical scale models and supplemented by field measurements of noise from trains operated by the Port Authority Transportation Corporation (PATCO) in New Jersey. The results show that vehicle skirts and undercar sound absorption can provide substantial cost-effective reductions in propulsion noise at the wayside of transit systems with concrete elevated guideways. The acoustical scale model noise reductions applied to PATCO vehicles on concrete elevated structures show reductions in the A-weighted noise levels of 5 dB for undercar sound absorption, 5 dB for vehicle skirts, and 10 dB for combined undercar absorption and vehicle skirts. Acoustical scale model results for sound barrier walls lined with absorptive treatment showed reductions from 7 dB to 12 dB of noise from vehicles in the far track, depending on the height of the wall, and reductions from 12 dB to 20 dB of noise from vehicles on the near track. Transit vehicles at high speeds where propulsion system noise dominates are 7 dB(A) noisier at 50 ft on concrete elevated structures than on at-grade on tie and ballast. Of this amount, 3 dB is due to loss of ground effect, and 4 dB is due to the absence of undercar absorption provided by ballast.     

Anomalous sea surface reverberation scale model experiments

Low frequency sea surface sound backscattering from approximately 100 Hz to a few kHz observed from the 1960s broadband measurements using explosive charges to the Critical Sea Test measurements conducted in the 1990 s is substantially higher than explained by rough sea surface scattering theory. Alternative theories for explaining this difference range from scattering by bubble plumes/clouds formed by breaking waves to stochastic scattering from fluctuating bubble layers near the sea surface. In each case, theories focus on reverberation in the absence of the large-scale surface wave height fluctuations that are characteristic of a sea that produces bubble clouds and plumes. At shallow grazing angles, shadowing of bubble plumes and clouds caused by surface wave height fluctuations may induce first order changes in the backscattered signal strength. To understand the magnitude of shadowing effects under controlled and repeatable conditions, scale model experiments were performed in a 3 m x 1.5 m x 1.5 m tank at the Technical University of Denmark. The experiments used a 1 MHz transducer as the source and receiver, a computer controlled data acquisition system, a scale model target, and a surface wave generator. The scattered signal strength fluctuations observed at shallow angles are characteristic of the predicted ocean environment. These experiments demonstrate that shadowing has a first order impact on bubble plume and cloud scattering strength and emphasize the usefulness of model scale experiments for studying underwater acoustic events under controlled conditions.     

湍流激励下柔性层贴敷加筋板自噪声的特征机理

为了研究湍流激励下柔性层贴敷加筋板自噪声的特征机理,基于湍流边界层壁面脉动压力功率谱模型、周期加筋板弯曲运动方程和固体波动方程以及力平衡与位移连续边界条件,建立了湍流边界层壁面脉动压力激励下柔性层贴敷单向周期加筋板的振动及内噪声物理模型.研究发现:橡胶外贴时对湍流激励下壳板的减振降噪主要依靠橡胶的厚度模态振动.无筋时,...     

Surf-generated noise signatures: a comparison of plunging and spilling breakers

Range-time-frequency distributions of surf-generated noise were measured within the surf zone during the SandyDuck'97 experiment at Duck, NC. A 24-phone, 138-m, bottom-mounted, linear array located along a line perpendicular to the shore at a depth of 1 to 3 m recorded the surf-generated noise. Concurrent video measurements of the location, size, and time-evolution of the individual breaking waves directly above the array were made from a nearby 43-m tower. Source level spectra are obtained by using a modified fast field program to account for water column and geoacoustic propagation from the distributed source region to an individual hydrophone. The length, location, and orientation of the leading edge of breakers are tracked in time from rectified video images. It is observed that the source levels from spilling breakers are lower (approximately 5-10 dB) than those produced by plunging breakers that occurred during the same time period. Plunging breakers generated time-frequency signatures with a sharp onset while spilling breakers' signatures had a gradual low-frequency precursor. Range-time signatures of plunging breakers indicate a burst of acoustic energy while spilling breakers' signatures depict sound being generated over a longer time period with the source region moving with the breaking surface wave.     

Effect of room absorption on human vocal output in multitalker situations

People increase their vocal output in noisy environments. This is known as the Lombard effect. The aim of the present study was to measure the effect as a function of the absorption coefficient. The noise source was generated by using other talkers in the room. A-weighted sound levels were measured in a 108 m(3) test room. The number of talkers varied from one to four and the absorption coefficients from 0.12 to 0.64. A model was introduced based on the logarithmic sum of the level found in an anechoic room plus the increasing portion of noise levels up to 80 dB. Results show that the model fits the measurements when a maximum slope of 0.5 dB per 1.0 dB increase in background level is used. Hence Lombard slopes vary from 0.2 dBdB at 50 dB background level to 0.5 dBdB at 80 dB. In addition, both measurements and the model predict a decrease of 5.5 dB per doubling of absorbing area in a room when the number of talkers is constant. Sound pressure levels increase for a doubling of talkers from 3 dB for low densities to 6 dB for dense crowds. Finally, there was correspondence between the model estimation and previous measurements reported in the literature.     

Numerical modeling of the sound fields in urban squares

This paper studies the basic characteristics of sound fields in urban squares surrounded by reflecting building fa?ades and the effectiveness of architectural changes and urban design options. A radiosity model and an image source model are developed, and a parametric study is carried out in hypothetical squares. The results show that the reverberation time (RT) is rather even in a square, whereas the early decay time (EDT) is very low in the near field, and then becomes close to RT after a rapid increase. Compared to diffuse boundaries, with geometrical boundaries the RT and EDT are significantly longer and the sound pressure level (SPL) attenuation with distance is generally smaller unless the height/side ratio is high. With a boundary diffusion coefficient of 0.2, the sound field is already close to that resulting from purely diffusely reflecting boundaries. The SPL in far field is typically 6-9 dB lower if the square side is doubled; 8 dB lower if the height of building fa?ades is decreased from 50 m to 6 m (diffuse boundaries); 5 dB (diffuse boundaries) or 2 dB (geometrical boundaries) lower if the length/width ratio is increased from 1 to 4; and 10-12 dB lower if the boundary absorption coefficient is increased from 0.1 to 0.9.     

On the amplification of broad band jet noise by a pure tone excitation

It has been found experimentally that broad band jet noise can be amplified by a pure tone excitation as much as 6 to 7 dB. The jet noise amplification effect takes place at sound pressure levels which are present in real aircraft engines. The experimental investigation was restricted to a cold jet at high subsonic Mach numbers excited by a plane sound wave coming from inside the nozzle. Based on a simplified mathematical model an attenuator has been constructed which is able to reduce the jet noise amplification significantly.     

Outdoor sound propagation under the influence of wind and temperature gradients

The situation investigated is sound propagation from a monopole point source located over an impedance surface. The sound propagation is assumed to be influenced by wind and temperature gradients. A very accurate calculation method for taking into account the effect of wind and temperature gradients on sound propagation outdoors is presented and used for verification of a new approximate calculation model. This comparison shows that the approximate model is accurate. A series of loudspeaker measurements has been carried out over a grass-covered ground for distances up to 80 m. The measurements were carried out for a wind speed fo 2–2·5 m/s measured 10 m above the ground. The measured data agree very well with the calculated results. Furthermore the results from the approximate calculation model agree with results from previous investigations [1,2]. Hence, the main conclusion is that a simple and powerful approximate model for sound propagation under the influence of wind and temperature gradients has been developed. However, the influence of turbulence is not taken into account in this paper, and the wind and temperature gradients are assumed to be constant as functions of height.     

重塑砂质含气沉积物的声响应特性与模型预测分析

海洋沉积物中气泡的存在对沉积物的声学特性有显著影响.为实现在实验室内对不同初始应力条件下含气沉积物的声学特性测量,研制了一套可用于土工三轴仪的双探针声学测量系统,基于CT扫描试验获得重塑含气砂样品中气泡群的尺寸分布,确定其共振频率范围.试验结果表明:细砂沉积物声速随气体含量的增加呈指数型降低,衰减系数随气体含量的增加呈...     

Noise abatement in plant monocultures and plant communities

Three types of vegetation can be discerned with regard to the acoustic climate investigated in 4 different plantations, 2 plant communities, and a grass field. In the first type of vegetation, i.e. beech and ashtree forests, the excess attenuation was at least 10 dB/100 m with the receiver at the same height as the source, 1·2 m, and at least 5 dB/100 m with the receiver at 3·9 m and even more in most $\text{1}\text{3}$-octave bands studied. In the second type of vegetation, i.e. the mixed poplar forest and the Stellario carpinetum, a so-called ‘sound window’ could be detected around 2 kHz and the ground effect was more extended towards the high frequency range compared with the first type of vegetation. In the third type of vegetation consisting of evergreen sprucefir, best excess attenuation was found in the closed forest: at least 10 dB/100 m with the receiver at 1·2 m, and 7 dB/100 m with the receiver at 3·9 m; in the belts the excess attenuation was at least 7 dB/100 m and 4 dB/100 m respectively. Best attenuation was, therefore, found in the closed forest, and not in the belts. It was concluded that the sound attenuating capacity of planted vegetations can be used to abate noise pollution in town and landscape planning if the plantations are at least 12 m wide. To obtain the best effect the rows of trees have to be planted perpendicular to the direction of the sound field.     

NUMERICAL MODELLING OF MEDIAN ROAD TRAFFIC NOISE BARRIERS

Outdoor sound propagation from road traffic is modelled by solving a boundary integral equation formulation of the wave equation using boundary element techniques in two dimensions. In the first model, the source representing a traffic stream can be considered as a coherent line source of sound. The results can then be transformed to derive a pseudo-three dimensional solution to the problem. In the second model the line source is incoherent. For receivers near the ground, the second model predicted significantly higher values of ground attenuation than the first. The first model generally produced better agreement with ground attenuation results obtained using the U.K. traffic noise prediction model. For conditions when a noise barrier was present and the ground was absorbent, the incoherent line source model generally predicted significantly higher values of attenuation than those from the barrier and ground attenuation calculated separately. Over a range of receiver positions and barrier heights a similar, but less marked effect was observed when the coherent line source model was used. On dual carriageway roads, it is possible to incorporate barriers on the central reservation as a noise control measure. These are “median” noise barriers. The incoherent line source model is used to assess the performance of median barriers in reducing noise when installed alone and also with associated roadside barriers. A sound absorbent median noise barrier 1m in height produced consistent values of insertion loss of between 1 and 2dB over the range of receiver positions and ground conditions considered. When the median barrier was used in conjunction with a roadside barrier it produced a consistent improvement in insertion loss of between 1 and 2 dB over the range of conditions considered.     

Comparison of outdoor microphone locations for measuring sound insulation of building facades

Measurement of the acoustical insulation of building facades requires simultaneous measurement of sound pressure levels indoors and outdoors, but there is still some question as to the best location for the outdoor microphone. This paper reports the differences between sound pressure levels at 2 m from a facade and at its surface, for a series of measurements at 33 different houses. The mean results compare well with a modified line source model, and suggest that a 3 dB correction between measurement locations is appropriate on average, except at low frequencies. The variation in the results, from house to house, suggests that establishing a single preferred outdoor microphone location is desirable.