Sound absorption by clamped poroelastic plates

Measurements and predictions have been made of the absorption coefficient and the surface acoustic impedance of poroelastic plates clamped in a large impedance tube and separated from the rigid termination by an air gap. The measured and predicted absorption coefficient and surface impedance spectra exhibit low frequency peaks. The peak frequencies observed in the absorption coefficient are close to those predicted and measured in the deflection spectra of the clamped poroelastic plates. The influences of the rigidity of the clamping conditions and the width of the air gap have been investigated. Both influences are found to be important. Increasing the rigidity of clamping reduces the low frequency absorption peaks compared with those measured for simply supported plates or plates in an intermediate clamping condition. Results for a closed cell foam plate and for two open cell foam plates made from recycled materials are presented. For identical clamping conditions and width of air gap, the results for the different materials differ as a consequence mainly of their different elasticity, thickness, and cell structure

The effect of continuous pore stratification on the acoustic absorption in open cell foams

This work reports new data on the acoustical properties of open cell foam with pore stratification. The pore size distribution as a function of the sample depth is determined in the laboratory using methods of optical image analysis. It is shown that the pore size distribution in this class of materials changes gradually with the depth. It is also shown that the observed pore size distribution gradient is responsible for the air flow resistivity stratification, which is measured acoustically and non-acoustically. The acoustical absorption coefficient of the developed porous sample is measured using a standard laboratory method. A suitable theoretical model for the acoustical properties of porous media with pore size distribution is adopted. The measured data for open porosity, tortuosity, and standard deviation data are used together with this model to predict the observed acoustic absorption behavior of the developed material sample. The transfer matrix approach is used in the modeling process to account for the pore size stratification. This work suggests that it is possible to design and manufacture porous media with continuous pore size stratification, which can provide an improvement to conventional porous media with uniform pore size distribution in terms of the attained acoustic absorption coefficient

A model of the free surface dynamics of shallow turbulent flows

Understanding the dynamic free surface of geophysical flows has the potential to enable direct inference of the flow properties based on measurements of the free surface. An important step is to understand the inherent response of free surfaces in depth-limited flows. Here a model is presented to demonstrate that free surface oscillatory spatial correlation patterns result from individual surface features oscillating vertically as they advect over space and time. Comparison with laboratory observations shows that these oscillating surface features can be unambiguously explained by simple harmonic motion, whereby the oscillation frequency is controlled by the root-mean-square water surface fluctuation, and to a lesser extent the surface tension. This demonstrates that the observed “complex” wave pattern can be simply described as an ensemble of spatially and temporally distributed oscillons. Similarities between the oscillon frequency and estimated frequency of near-bed bursting events suggest that oscillon behavior is linked with the creation of coherent flow structures

A Non-invasive wave monitor

This work presents a new acoustical method for remote measurement of the surface characteristics of the dynamic air–water interface in turbulent free-surface flows. The technique uses the reflection of a monochromatic ultrasonic wave by the dynamically rough air–water interface to measure the water surface position. It is found that with careful selection of the acoustical components and their configuration, thephase of the reflected signal responds to the local fluctuations in the fluid interface at the point of specular acoustic reflection. In order for the method to be applicable, three criteria must be satisfied: (1) the dominant wavelength of the surface under investigation must be greater than the first Fresnel zone corresponding to the wavelength and component geometry of the acoustical system; (2) the mean magnitude of the instantaneous local surface gradient must not exceed 0.025; and (3) the root-mean-square wave height must be greater than 1% of the acoustic wavelength. Under these conditions the mean error of the system is within 5% (and usually within 1%) of the acoustic wavelength, and is generally within 10% of the wave amplitude for turbulence generated waves, and 3% of the amplitude for gravity waves. This error may be reduced by optimising the acoustic wavelength for the surface of interest. For turbulent depth limited flows, the surface waves fall well within the criteria, and the absolute errors are independent of wave height, so for larger wave heights, the relative error can be considerably lower. The technique provides a robust system for monitoring the dynamics of free surface fl ows, which is non-invasive, low cost, and low power. The method has been tested on laboratory flows but should be applicable to remote sensing of free surface properties on a local scale in field environments where invasive techniques are difficult to implement such as might be found in coastal, river and wastewater environments

A method to determine the acoustic reflection and absorption coefficients of porous media by using modal dispersion in a waveguide

The measurement of acoustic material characteristics using a standard impedance tube method is generally limited to the plane wave regime below the tube cut-on frequency. This implies that the size of the tube and, consequently, the size of the material specimen must remain smaller than a half of the wavelength. This paper presents a method that enables the extension of the frequency range beyond the plane wave regime by at least a factor of 3, so that the size of the material specimen can be much larger than the wavelength. The proposed method is based on measuring of the sound pressure at different axial locations and applying the spatial Fourier transform. A normal mode decomposition approach is used together with an optimization algorithm to minimize the discrepancy between the measured and predicted sound pressure spectra. This allows the frequency and angle dependent reflection and absorption coefficients of the material specimen to be calculated in an extended frequency range. The method has been tested successfully on samples of melamine foam and wood fiber. The measured data are in close agreement with the predictions by the equivalent fluid model for the acoustical properties of porous media

Padé approximants for the acoustical properties of rigid frame porous media with pore size distributions

Expressions for the viscosity correction function, and hence bulk complex impedance, density, compressibility, and propagation constant, are obtained for a rigid frame porous medium whose pores are prismatic with fixed cross-sectional shape, but of variable pore size distribution. The lowand high-frequency behavior of the viscosity correction function is derived for the particular case of a log-normal pore size distribution, in terms of coefficients which can, in general, be computed numerically, and are given here explicitly for the particular cases of pores of equilateral triangular, circular, and slitlike cross-section. Simple approximate formulae, based on two-point Pade´ approximants for the viscosity correction function are obtained, which avoid a requirement for numerical integration or evaluation of special functions, and their accuracy is illustrated and investigated for the three pore shapes already mentione

Simulation of the behavior of biologically-inspired swarm robots for the autonomous inspection of buried pipes

The use of robots for the inspection of buried pipelines has gained popularity over the past decade. In this paper we move the vision forward by examining what behavior and attributes would be required for these robots to become autonomous and pervasive within buried water pipe infrastructure. We present the results from novel simulations to evidence the inspection capability of autonomous robots, investigating operation, cooperation and communication attributes. The simulation uses a biologically-inspired behavior that provides complete and consistent coverage of real life example clean water distribution management areas. We show that autonomous robots could operate without a centralized controller and benefit from having some degree of in-pipe communication. We evidence the ability to adapt to changes in communication, speed, and flow conditions. The mathematical model that we derive through the simulation is scalable with the change of network length, topology, robots’ speed and number. This work paves the way and sets the specifications for practical development of autonomous pervasive robots for the inspection of complex pipe networks

Frequency-wavenumber spectrum of the free surface of shallow turbulent flows over a rough boundary

Data on the frequency-wavenumber spectra and dispersion relation of the dynamic water surface in an open channel flow are very scarce. In this work, new data on the frequency-wavenumber spectra were obtained in a rectangular laboratory flume with a rough bottom boundary, over a range of subcritical Froude numbers. These data were used to study the dispersion relation of the surface waves in such shallow turbulent water flows. The results show a complex pattern of surface waves, with a range of scales and velocities. When the mean surface velocity is faster than the minimum phase velocity of gravity-capillary waves, the wave pattern is dominated by stationary waves that interact with the static rough bed. There is a coherent three-dimensional pattern of radially propagating waves with the wavelength approximately equal to the wavelength of the stationary waves. Alongside these waves, there are freely propagating gravity-capillary waves that propagate mainly parallel to the mean flow, both upstream and downstream. In the flow conditions where the mean surface velocity is slower than the minimum phase velocity of gravity-capillary waves, patterns of non-dispersive waves are observed. It is suggested that these waves are forced by turbulence. The results demonstrate that the free surface carries information about the underlying turbulent flow. The knowledge obtained in this study paves the way for the development of novel airborne methods of non-invasive flow monitoring

Doppler spectra of airborne sound backscattered by the free surface of a shallow turbulent water flow

Measurements of the Doppler spectra of airborne ultrasound backscattered by the rough dynamic surface of a shallow turbulent flow are presented in this paper. The interpretation of the observed acoustic signal behavior is provided by means of a Monte Carlo simulation based on the Kirchhoff approximation and on a linear random-phase model of the water surface elevation. Results suggest that the main scattering mechanism is from capillary waves with small amplitude. Waves that travel at the same velocity of the flow, as well as dispersive waves that travel at a range of velocities, are detected, studied and used in the acoustic Doppler analysis. The dispersive surface waves are not observed when the flow velocity is slow compared to their characteristic velocity. Relatively wide peaks in the experimental spectra also suggest the existence of nonlinear modulations of the short capillary waves, or their propagation in a wide range of directions. The variability of the Doppler spectra with the conditions of the flow can affect the accuracy of the flow velocity estimations based on backscattering Doppler. A set of different methods to estimate this velocity accurately and remotely at different ranges of flow conditions is suggested

Efficient calculation of two-dimensional periodic and waveguide acoustic Green’s functions

New representations and efficient calculation methods are derived for the problem of propagation from an infinite regularly spaced array of coherent line sources above a homogeneous impedance plane, and for the Green's function for sound propagation in the canyon formed by two infinitely high, parallel rigid or sound soft walls and an impedance ground surface. The infinite sum of source contributions is replaced by a finite sum and the remainder is expressed as a Laplace-type integral. A pole subtraction technique is used to remove poles in the integrand which lie near the path of integration, obtaining a smooth integrand, more suitable for numerical integration, and a specific numerical integration method is proposed. Numerical experiments show highly accurate results across the frequency spectrum for a range of ground surface types. It is expected that the methods proposed will prove useful in boundary element modeling of noise propagation in canyon streets and in ducts, and for problems of scattering by periodic surfaces