Calculation of sound reduction by a screen in a turbulent atmosphere using the parabolic equation method

Results from applying a Crank-Nicholson parabolic equation method (CN-PE) are presented in situations with a thin screen on a hard ground in a turbulent atmosphere, and with the acoustic source at ground level. The results are evaluated by comparison with G. A. Daigle's model, which uses the sound scattering cross-section by V. I. Tatarskii together with diffraction theory. The results show a fairly good agreement for situations where the receiver is above ground, thus indicating that both methods are applicable to the problem. When the receiver is at ground level the two methods lead to large differences in insertion loss since only the PE method predicts that turbulence causes an increased sound level in the case without a screen. For the situations considered in this paper a turbulent atmosphere is shown to significantly decrease the sound reduction by a screen. An approximation in the representation of a turbulent atmosphere in the CN-PE method is presented, and is shown to lead to an acceptable error in limited cases

An extended substitute-sources method for a turbulent atmosphere: Calculations for upward refraction

The substitute-sources method (SSM) was previously implemented for a single noise barrier in a turbulent atmosphere by applying a substitute surface between the barrier and the receiver [1, 2]. Here, the method is extended, aiming to more general applicability to traffic noise propagation in urban environments. In the method, multiple substitute surfaces are used along the propagation path. The atmospheric turbulence causes a transfer of the initially coherent field into a residual, random field along the propagation path. The mean sound level at the receiver position is found from uncorrelated addition of the substitute surfaces' contributions. The calculation of each contribution is based on a mutual coherence function (MCF) for a turbulent atmosphere. The strength of the substitute sources and the Green functions to the received pressure are calculated for a non-turbulent atmosphere, here by using a fast field program (FFP). A special MCF for the residual field is derived. Examples are calculated for a turbulent atmosphere with upward refraction or without refraction. The results are compared with those from a parabolic equation method (PE) for the refractive cases and with an analytical solution otherwise. The results show good agreement, which indicates that the SSM could be useful for predictions of outdoor sound propagation

Modelling of a city canyon problem in a turbulent atmosphere using an equivalent sources approach

The sound propagation into a courtyard shielded from direct exposure is predicted using an equivalent sources approach. The problem is simplified into that of a two-dimensional city canyon. A set of equivalent sources are used to couple the free half-space above the canyon to the cavity inside the canyon. Atmospheric turbulence causes an increase in the expected value of the sound pressure level compared to a homogeneous case. The level increase is estimated using a von Kármán turbulence model and the mutual coherences of all equivalent sources' contributions. For low frequencies the increase is negligible, but at 1.6 kHz it reaches 2–5 dB for the geometries and turbulence parameters used here. A comparison with a ray-based model shows reasonably good agreement

Thick barrier noise-reduction in the presence of atmospheric turbulence: Measurements and numerical modelling

Atmospheric turbulence causes scattering of sound, which can reduce the performance of sound barriers. This is an important inclusion in prediction models to obtain a correct picture of the sound reduction at higher frequencies. Here a prediction method is applied that uses the strengths of the wind and temperature turbulence to estimate the scattered power into the shadow zone of a barrier. The predictions are compared to full-scale measurements on a thick barrier, where both acoustic and meteorological data were recorded simultaneously under both calm and windy conditions. Comparison between the measurements and the predictions indicate that the method gives reasonably accurate results for mid to high frequencies and a slight overestimation at very high frequencies

A scale model study of parallel urban canyons

Shielded urban areas are of importance regarding urban citizens’ annoyance and adverse health effects related to road traffic noise. This work extends the existing knowledge of sound propagation to such areas by a scale model study, rather than by model calculations. The scale model study was executed for two parallel urban canyons at a 1 to 40 scale, with a point source located in one canyon. Cases with acoustically hard façades and absorption and diffusion façade treatments were in vestigated. To correct for excess air attenuation of the measurements, a wavelet-based method has been applied. The measurement results in the shielded canyon show that, in contrast to the directly exposed street canyon, the levels and the decay times are quite constant over the length of the canyon. The energy-time curve in the shielded canyon is characterized by a rise time, which can be related to the sound pressure level. The rise times and decays can be explained by separate reflection, diffraction and diffusion processes. A closed courtyard situation enlarges the level difference between acoustically hard façades and applied façade absorption or diffusion treatments at both the directly exposed and shielded side. A comparison between measurements with two different diffusion mechanisms, horizontal and vertical diffusion, reveals that vertical diffusion yields lower levels at the shielded side compared to horizontal diffusion for the investigated situations

An interactive auralization method using real-time sound sources

During recent years, auralization methods have evolved towards using more interactive measures. The use of interactive elements, like navigation in static sound fields, has proven to be very significant in order to better integrate the listener with the simulated soundscape. In this study the possibility of engaging the user by actively contributing to the sound field is explored. Enabling the subject to act as a sound source and allowing communication within the environment, utilizing real-time synthesis of an acoustic environment's response. Auralization allows for a psychoacoustic evaluation of the acoustical space and therefore plays an important part in a wider understanding of different environmental characteristics. With an auralization framework adapting this kind of interaction, experience of the acoustical response is enabled and can thus be used as a tool in the process of subjectively assessing the acoustical space. Real-time convolution software implementing this mode of procedure has been designed. A subjective evaluation has been performed using a listening room equipped with an ambisonics multi-channel reproduction system, and a directional microphone with feedback control. Evaluation results indicate a positive response from the subjects to the added control over the simulated space

Scattering by an array of perforated cylinders with a porous core

In this work multiple scattering by an array of perforated cylindrical shells with a porous core has been investigated. A semi-analytical model to predict scattering from such cylindrical units is presented in the context of the multiple scattering theory (MST), and validated against laboratory experiments. The suggested semi-analytical multiple scattering model uses an impedance expression to include the perforated shell in the scattering coefficients, which is a compact way to describe a composite scatterer in MST. Calculation results of a small array are shown to be in excellent agreement with measured data. Predictions and data show that perforated cylinders with empty cavities exhibit a strong and narrow insertion loss peak at resonance, though simulta- neously suffer from amplification below resonance. By adding porous material in the core of the scatterer adverse effects below the resonance peak were suppressed. In addition, it was found that the reduction peak broadens, though at a cost of a reduced peak amplitude. Finally, it has been shown that adding porous material in a perforated shell will introduce partial absorption of the incoming field, which can be optimized by adjusting the perforation ratio of the shell