The accuracy of some models for the airflow resistivity of nonwoven materials

The airflow resistivity is a key parameter to consider when evaluating the acoustic performance of a fibrous material. The airflow resistivity is directly linked to a fibrous materials acoustic properties which allows for the non-invasive measurements of the fibre diameter and material density from acoustical data. There are several models that relate the airflow resistivity to the acoustic behaviour through the material's density and fibre diameter. It is not always obvious how accurately a model represents the true value of the flow resistivity of a nonwoven material with a fibre size variation. Therefore, the scope of this paper is to compare the performance of several theoretical and empirical models applied to a representative range of nonwoven fibrous media composed of blends of different fibre sizes and types. Being able to understand the performance of these models in application to fibre blends will enable users to characterise these types of fibrous media more precisely. From this work, it was concluded that the Miki model (Miki, 1990) is the most accurate model to invert the airflow resistivity from acoustical surface impedance of a wide range of nonwoven blends

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

Local head loss monitoring using acoustic instrumentation in partially full sewer pipes

After an increase in capital investment in UK sewers to reduce hydraulic capacity problems, the proportion of sewer flooding incidents now linked to blockages has increased. It is clear that if sewer operators are to continue to reduce flooding incidents, then better blockage management is now required. Sewer blockage formation is poorly understood; blockages are intermittent and occur in a number of circumstances. This paper reports on the development of low-cost acoustic instrumentation that can identify the location of a pipe blockage and then estimate the local head loss as a result of the presence of a blockage. A set of experiments were carried out in two full-scale laboratory pipes. The pipes' condition was altered by inserting blockages of different sizes. Acoustic data were recorded and presented in terms of the acoustic energy reflected from the partially blocked pipe. The results of this study show that the total reflected acoustic energy correlates with the measured head loss. A new empirical relation between the reflected acoustic energy and head loss due to a blockage is derived. This knowledge can then be used to estimate the reduction in flow capacity resulting from a blockage based on a single remote measurement

An application of a parametric transducer to measure the acoustical properties of a living green wall

Greening of urban spaces provides a number of environmental benefits. Green living walls (GLW) is a most typical example of greening which is also known for its ability to absorb unwanted noise. However, this ability of GLW to absorb noise is rather hard to quantify, because there is a lack of reliable experimental methods to measure it in-situ. This work reports on a new method to measure the absorption coefficient of LGW which makes use of a highly directional parametric transducer and acoustic intensity method. This method is tested in under controlled laboratory conditions and in a typical street environment. The results of these experiments demonstrate the ability of the method to measure the absorption of a LGW. It also enables us to quantify the effects of the plant type and moisture content in the soil on the ability of the LGW to absorb sound. The proposed method has certain benefits over ISO354-2003 and CEN/TS 1793-5:2003 standard methods

An application of Kozeny–Carman flow resistivity model to predict the acoustical properties of polyester fibre

Modelling of the acoustical properties of polyester fibre materials is usually based on variations of the Bies and Hansen empirical model [1], which allows the calculation of the air flow resistivity of a porous material. The flow resistivity is the key non-acoustical parameter which determines the ability of this kind of materials to absorb sound. The main scope of this work is to illustrate that an alternative theoretical model based on the KozenyCarman equation can be used to predict more accurately the flow resistivity from the fibre diameter and bulk material density data. In this paper the flow resistivity is retrieved from the acoustic absorption coefficient data for polyester fibre samples of different densities and fibre diameters. These data agree closely with the flow resistivity predicted with the proposed KozenyCarman model

Predicting perceived tranquillity in urban parks and open spaces

A method is described that enables the potential tranquillity of an amenity area, such as a park, green, or square to be assessed. The method involves the assessment of traffic noise levels and the measurement of the percentage of natural and contextual features using photographs of the scenes. Examples are taken from three amenity areas in Bradford, West Yorkshire, UK. Using published noise maps, sampling was taken at points in the three parks where visitors were likely to be found and where noise levels were likely to be highest and lowest. At these locations, predictions of the traffic noise levels were made and then the tranquillity rating and the mean value and distribution of ratings were compared. Recommendations for improving the perceived tranquillity are discussed

In-situ acoustic absorption of a living green wall

Data on the ability of a living green wall to absorb sound in-situ is scarce. In this work a directional parametric transducer was used to project sound on the centre of a living green wall to minimise the ground reflection and scattering from it edges. The sound pressure and particle velocity in the incident and reflected sound waves were measured with an intensity probe and used to estimate the acoustic absorption coefficient. These data were also used to estimate the ability of a living wall to scatter the incident sound. It was found that a living wall system that consists of several rectangular cells with plants can support acoustic resonances at frequencies which are controlled by the cell dimension and wall thickness. Some of these resonances are reduced or disappear when the wall is treated with a plant with a relatively high leaf area density. There is evidence that in some cases plants can scatter sound coherently resulting in an apparent decrease in the absorption coefficient. These effects need to be accounted for by a refined numerical model which is yet to be developed

An application of normal mode decomposition to measure the acoustical properties of low growing plants in a broad frequency range

© 2016 Elsevier Ltd This paper presents a new application of the normal mode decomposition to measure the reflection and absorption coefficients of a low growing living plant in a large 300 × 300 mm impedance tube. In this way the higher frequency limit can be extended by a factor of 3 in comparison to that suggested by the standard ISO 10534-2 method for this type of an impedance tube. The adopted method (Prisutova et al., 2014) is based on minimising the difference between the spatial Fourier transform of the measured sound pressure at a range of closely spaced positions along the impedance tube and the predicted transform arising from the normal mode decomposition method. The angular and frequency dependent complex reflection coefficients for the first 5 normal modes are recovered. The acoustical properties of three plants specimen, Pelargonium hortorum, Begonia benariensis and Hedera helix, are measured with the adopted method. These properties are related to the plant morphology through an equivalent fluid model. It is shown that in some cases the predicted and measured data are in close agreement. However, there are cases when the agreement between these data is poor. The possible reasons for this discrepancy are proposed and discussed. This work paves the way for a better understanding of the relations between the plant morphology and its acoustical properties

Rapid detection of sewer defects and blockages using acoustic-based instrumentation

Sewer flooding incidents in the UK are being increasingly associated with the presence of blockages. Blockages are difficult to deal with as although there are locations where they are more likely to occur, they do occur intermittently. In order to manage sewer blockage pro-actively sewer managers need to be able to identify the location of blockages promptly. Traditional closed-circuit television (CCTV) inspection technologies are slow and relatively expensive so are not well suited to the rapid inspection of a network. This is needed if managers are to be able to address sewer blockages pro-actively. This paper reports on the development of an acoustic-based sensor. The sensor was tested in a full scale sewer pipe in the laboratory and it was shown that it is able to find blockages and identify structural aspects of a sewer pipe such as a manhole and lateral connection. Analysis of the received signal will locate a blockage and also provide information on its character. The measurement is very rapid and objective and so inspections can be carried out at much faster rates than using existing CCTV technologies