Attenuation of vehicle noise over short grass

Field experiments to investigate the attenuation of noise from single vehicles over short grass indicate that for a microphone height of 1·2 m and a source/receiver distance <30 m, the rate of attenuation is 4·8 dB(A) per doubling of distance. Some evidence is presented to suggest that for source/receiver distances >30 m the rate of attenuation increases with distance.     

Pilot study on railway noise attenuation by belts of trees

Sound levels from passing trains were recorded. Both maximum A-weighted sound pressure levels L_{A, max} and equivalent levels L_Aeq over 60 second time periods containing each pass-by were measured. Recordings of 15 pass-bys were made at each of two different sites. At each site attenuation over level, grass-covered ground and through shelter belts was measured 1.5 m above ground. Attenuation differences at each site were due to both minor variations in terrain configuration (track above/below adjacent terrain) and to attenuation in vegetation. The latter cannot be separated. L_Aeq attenuations were smaller than L_A,max attenuations, as should be expected. Noise reduction by shelter belts—i.e., the approximate difference between attenuation over grass-covered ground and thouugh vegetation, respectively—was nearly the same, expressed in L_{A, max} and L_Aeq values. Shelter belts selected for this investigation were carefully maintained. Their overall structure, therefore, was very uniform. Behind a dense, 15 year old shelter belt, 50 m wide, consisting of beeches and various conifers planted between older birches and elms, noise levels were 8 to 9 dB lower than in level grass-covered country. Behind a dense, 10 to 20 year old shelter belt, 25 m wide, consisting of oaks, hornbeams, poplars, silver firs and various sorts of bushes, noise levels were 6 or 7 dB lower than in level grass-covered country. The attenuations measured seemed to be of such an order of magnitude that similar belts of trees and bushes could be a means of practical noise reduction. Further investigations, therefore, seem to be worthwhile.     

The barrier attenuation of garden fences

A study has been undertaken of the barrier attenuation produced by 40 different finite garden fences shielding suburban homes from traffic noise. Two alternative techniques of measuring barrier attenuation were used and an empirical relationship between the results is obtained. The attenuation measured in terms of $\text{L}{}_{\mathrm{<mtext>1</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$ or $\text{L}{}_{\mathrm{eq}}$ generally agrees within ± 1 dB(A) for a given barrier. Similarly constructed fences can produce different attenuations, depending on their surroundings. The effects of unshielded ends, open gates and vegetation are considered. It is postulated that buildings reflect back sound, the effect depending on building size and distance from the fence. A reasonable trend is obtained for brick barriers; however, the wooden fence results may be obscured by sound leakage through the fence.     

An experimental investigation of the noise produced by air jet impingement on flat plates

The sound power level produced by an air jet impinging upon a flat solid boundary is investigated. Measurements are recorded for nozzle diameters and exit velocities in the ranges $\text{0·75 in < D < 1·5}$ in and $\text{270 ft/s <}\text{U}\text{?}\text{< 700 ft/s}$. The effect of varying the nozzle to plate spacing is investigated for plates normal to and inclined to the jet axis.At low velocities and large nozzle diameters the sound power level was found to increase continuously as the nozzle to plate spacing decreased, the maximum increase being 7 dB above that of the free jet. Under certain conditions, usually small nozzle diameter and high velocities, a maximum in sound power level was observed in the nozzle to plate spacing range, increases of 27 dB above that of the free jet levels being recorded and a distinct tonal character being detected.The reasons for this phenomenon cannot be fully explained, but it is likely that this can be avoided by inclining the plate to the jet axis.     

A study of the spectral characteristics of traffic noise attenuation by vegetation belts in Delhi

Traffic noise attenuation at different 1/3-octave frequencies is measured at three vegetation sites and a control site in Delhi, the capital city of India. The study indicates that attenuation generally increases with frequency. At low frequencies, maxima (between 10 and 16 dB) in relative attenuation are observed in the frequency interval between 315 and 400 Hz. Comparatively greater relative attenuation (>20 dB) is observed in the high frequency range between 10 and 12.5 kHz. A significantly higher relative attenuation of more than 24 dB is observed characteristically at 3.15 kHz at all the vegetation sites. The results indicate that vegetation belts could be used as effective barriers for traffic noise control along the roadsides.     

Road traffic noise attenuation by belts of trees

Measurements were made at a number of sites of road traffic noise propagating through belts of trees and bushes and above grass-covered ground, respectively. The belt widths were between 3 and 25 m. The distance from the road to the front of the belts also varied from site to site. The microphones were placed 1·5 m above the ground. A comparison between attenuations obtained, expressed as differences in equivalent constant A-weighted sound pressure levels, L_Aeq, showed no significantly higher attenuation values for propagation through belts of trees than for propagation above grass-covered ground. Only in the frequency range above 2 kHz were attenuations significantly higher through the belts of trees and bushes. The belts of trees selected consisted mainly of deciduous trees and bushes between 5 and 10 years of age. Such types and widths are representative of what could often be used in normal urban situations in an attempt to provide practical noise reduction. According to the results of this investigation, however, these do not significantly reduce L_Aeq 1·5 m above the ground. Planting of belts of trees and bushes between roads and dwellings might influence the environmental quality of residential areas due to nonacoustic factors or reduce nuisance due to spectral changes not affecting L_Aeq. This has not been investigated.     

Averaged electrical parameters of forest vegetation and the height of forest cover

A device and a method for measuring the electrical parameters (resistivity ρ and permittivity ?) of forest vegetation are described. The measuring method and the data processing technique are detailed for a forest area the resistivity and permittivity of which measured in the ranges of long and very long waves are ρ = 37 ± 12 kΩ m and ? = 1.6 · 0.3, respectively. It is found that coordinate measurements in forest vegetation with a single-frequency GPS receiver always involve a one-sided systematic drift, since the permittivity is other than unity. This circumstance makes it possible to develop a way of determining the average height of forest cover. For example, the average height of trees is roughly equal to 21 m in the given forest area.     

Theoretical and phantom based investigation of the impact of sound speed and backscatter variations on attenuation slope estimation

Quantitative ultrasound features such as the attenuation slope, sound speed and scatterer size, have been utilized to evaluate pathological variations in soft tissues such as the liver and breast. However, the impact of variations in the sound speed and backscatter due to underlying fat content or fibrotic changes, on the attenuation slope has not been addressed. Both numerical and acoustically uniform tissue-mimicking experimental phantoms are used to demonstrate the impact of sound speed variations on attenuation slope using clinical real-time ultrasound scanners equipped with linear array transducers. Radiofrequency data at center frequencies of 4 and 5 MHz are acquired for the experimental and numerical phantoms respectively. Numerical phantom sound speeds between 1480 and 1600 m/s in increments of 20 m/s for attenuation coefficients of 0.3, 0.4, 0.5, 0.6, and 0.7 dB/cm/MHz are simulated. Variations in the attenuation slope when the backscatter intensity of the sample is equal, 3 dB higher, and 3 dB lower than the reference is also evaluated. The sound speed for the experimental tissue-mimicking phantoms were 1500, 1540, 1560 and 1580 m/s respectively, with an attenuation coefficient of 0.5 dB/cm/MHz. Radiofrequency data is processed using three different attenuation estimation algorithms, i.e. the reference phantom, centroid downshift, and a hybrid method. In both numerical and experimental phantoms our results indicate a bias in attenuation slope estimates when the reference phantom sound speed is higher (overestimation) or lower (underestimation) than that of the sample. This bias is introduced via a small spectral shift in the normalized power spectra of the reference and sample with different sound speeds. The hybrid method provides the best estimation performance, especially for sample attenuation coefficient values lower than that of the reference phantom. The performance of all the methods deteriorates when the attenuation coefficient of the reference phantom is lower than that of the sample. In addition, the hybrid method is the least sensitive to sample backscatter intensity variations.     

Excess attenuation for sound propagation over an urban canyon

Because quiet areas in dense urban environments are important to well-being, the prediction of sound propagation to shielded urban areas is an ongoing research focus. Sound levels in shielded areas, such as canyons between rows of buildings, are strongly influenced by distant sources. Therefore, propagation factors such as metrology, screening, and intermediate canyons—as occur between a source canyon and a receiver canyon—must be addressed in an engineering propagation model. Though current models address many important propagation factors, engineering treatment of a closed urban canyon, subject to multiple internal reflections, remains difficult.A numerical investigation of sound propagation across the open tops of intermediate urban canyons has been performed, using the parabolic equation and equivalent sources methods. Results have been collected for various canyon geometries, and the influences of multiple canyons, canyon/rooftop absorption, variable rooftop height, wind gradient, and correlated versus uncorrelated source models have been investigated. Resulting wideband excess attenuation values ranged from −1 dB to −4 dB per canyon, and were fairly constant with frequency in many useful cases. By characterizing the excess attenuation of canyons intermediate to the source and receiver, the influence of these intermediate canyons could be addressed simply, without the overhead of a detailed numerical calculation.     

Attenuation of noise by windbreaks

Sound attenuation by narrow forest belts, under quasi-line source conditions has been investigated. Experiments were conducted on windbreaks of Casuarina and Eucalyptus belts, along three sites at Nobria.Windbreaks of Casuarina were found to act as sound barriers, which reduce the highway noise resulting from trucks, cars and other traffic. Reduced or even negative attenuation is, however, recorded in some locations behind mixed windbreaks of Eucalyptus and Casuarina as a result of downward scattering of acoustic propagation.     

Use of a digital voltmeter to measure the output of a sound level meter

The use of a digital voltmeter and associated computer program to measure the varying output of a sound level meter is discussed. The computer program converts the voltmeter readings into sound levels and calculates quantities such as $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$, $\text{L}{}_{\mathrm{eq}}$, etc. Using the dc output of a sound level meter, the dynamic range is about 25 dB. This increases to at least 45 dB on the ac output. In general, values found for $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$, $\text{L}{}_{\mathrm{eq}}$, etc. are within 1 dB of results obtained by other methods.     

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.     

Sound attenuation in forests

Many measurements of sound attenuation rates in forests have been made but there is little in common in the measuring procedures used or the results obtained. Consequently there is a considerable divergence of opinion on the effectiveness of vegetation as a noise control measure. In this paper the factors controlling the transmission of sound through vegetation are examined and the attenuation rates achieved in pine plantations are presented.     

A study of the subjective equivalence of noise and whole-body vibration

An experiment has been conducted to determine the subjective equivalence of 1000 Hz pure tone noise and 10 Hz sinusoidal whole-body vertical vibration. Each of 20 male subjects was exposed to all 64 possible combinations of 8 levels of noise (65 dB to 100 dB SPL) and 8 levels of vibration (0·20 m/s² r.m.s. to 1·2 m/s² r.m.s.). The noise was presented via circumaural headphones and the vibration exposure was by means of a flat hard seat. The method of constant stimuli was used. Both stimuli were presented simultaneously for a period of ten seconds and subjects were asked to indicate whether, if they were to be presented with the combination again, they would prefer that the noise or the vibration should be reduced.It was concluded that the subjects were relatively self-consistent and that the major source of variability was due to intersubject differences. The conditions for equivalence for 50% of the subjects ranged from about 0·2 m/s² r.m.s. at 69 dB to 1·2 m/s² r.m.s. at 94 dB. The results are presented in a form that enables an estimate to be made of the percentage of subjects who prefer reduced noise or vibration at any of the given combinations of the two stimuli. Further studies to extend the range and establish the general applicability of these results are suggested. It is considered that such results could be employed as a guide to reducing either the noise or the vibration in some environments.     

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...     

Turbulence measurements in a resonance tube

Experimental investigations of acoustically induced turbulence in a resonance tube have been performed. Frequency (f) and sound pressure level (I_p) effects have been studied. Measurements were made at various spatial locations on loops and nodes. Sampled data were processed to estimate the characteristics of turbulence. It is found that the acoustically induced turbulence appears when I_p exceeds 160 dB under the experimental conditions of f = 680–2740 Hz and I_p = 160–166 dB. The turbulent spectrum (F) and the wave number (κ) are found to satisfy a power law F ∝ K^s with $\text{s ? ?1·6}\text{to}\text{? 2·1}$. The r.m.s. turbulent velocity ($\text{u}\text{?}$) is experimentally found to have an $\text{I}{}_{\mathrm{p}}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence, yet is relatively insensitive to the variation of f. Throughout the whole measuring range of f and I_p, the rate of energy dissipation per unit mass (ε) is estimated to be in the order of 10⁶–10⁷cm²/s³.     

Distribution display attachment for a sound level meter

The use of a sound level meter to estimate $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>50</mtext>}}$ and $\text{L}{}_{\mathrm{<mtext>90</mtext>}}$ for a small number of noise samples has appealed to many users as being simple, effective and economical. The more conventional ways of using a statistical distribution analyser or a community noise analyser are justifiable only with a large number of samples and over extended periods. This paper describes a recent development—a portable sound level meter attachment—which is aimed at eliminating the need for accurate meter reading. In the attachment, intermittent noise levels grouped in 3 dB classes are displayed by an array of LED's. Data recording is reduced to registering the occurrences of flash from each LED. When used on traffic noise this method gives values of $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>50</mtext>}}$, $\text{L}{}_{\mathrm{<mtext>90</mtext>}}$ and $\text{L}{}_{\mathrm{eq}}$ within 0·7 dB(A) of values obtained by a Bruel and Kjaer type 4426 Statistical Analyser. It is found that this method is most accurate for noise with a high level of fluctuation.     

Non-linear attenuation of sound in circular lined ducts

The attenuation of high intensity sound in circular ducts lined with fibrous material has been investigated. With no mean flow, the sound pressure levels are varied to illustrate the linear and non-linear absorption characteristics of the liner. Effects of liner thickness, perforation ratio of the duct wall and the $\text{d}\text{t}$ ratio are analysed.Optimum combinations of the perforation geometry and liner thickness are found to be of stable attenuation characteristics over a wide frequency range and at high sound levels.     

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.     

Statistical analysis from a sound level meter

Increasingly, and particularly for motorway noise, sound level meters are being used to estimate $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$, the noise level just exceeded for 10 per cent of the time, by noting the meter readings at regular intervals and manually analysing the array of data so obtained. In this paper we describe a simple attachment to a sound level meter, which is intended to ease the operator's task in making such measurements and to minimise operator bias. Comparisons are made between the results of sound level meter measurements of three types of noise and the results obtained using the more conventional technique which employs a level recorder and statistical analyser.It is concluded that, at least for motorway noise, the sound level meter technique gives values of $\text{L}{}_{\mathrm{<mtext>10</mtext>}}$ and of $\text{L}{}_{\mathrm{<mtext>90</mtext>}}$ within 1 dB(A) of the values obtained by the normal method. Furthermore, it is probable that the technique will be useful for the measurement of other types of noise in statistical terms, but further investigations of accuracy are required.