Specific acoustic impedance

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

Impedance and reflection. Another important characteristic of a material is its specific acoustic impedance (Z). The specific acoustic impedance is defined as the ratio of the acoustic excess pressure (P) and the particle velocity (U) ... 

The other important consideration concerns the transmission of ultrasound (and other forms of energy) from one medium to another and the importance of impedance matching . When wave energy is transferred from one medium to another then a part is transmitted and the rest reflected. The ratio of reflected to transmitted energies depends on the characteristic impedances of the two media and the transmission is total if these are matched. In the case of acoustic waves the specific impedance (Z) of a medium is given by the product of the density p and the velocity of sound v. that is... 

The property of the interface which controls the relative amounts reflected and transmitted is termed the specific acoustic input impedance Z which is defined as the ratio of the acoustic pressure to the particle velocity associated with the wave motion at the boundary. Note that this definition is similar to that used previously in defining the specific impedance of the media z. The use of the term impedance for both input impedance at the boundary and specific impedance of the media occasionally leads to some confusion if care is not taken. 

Considering perpendicular sound incidence, the specific acoustic input impedance Z2 at the boundary is simply... 

As the readers may see, quartz crystal resonator (QCR) sensors are out of the content of this chapter because their fundamentals are far from spectrometric aspects. These acoustic devices, especially applied in direct contact to an aqueous liquid, are commonly known as quartz crystal microbalance (QCM) [104] and used to convert a mass ora mass accumulation on the surface of the quartz crystal or, almost equivalent, the thickness or a thickness increase of a foreign layer on the crystal surface, into a frequency shift — a decrease in the ultrasonic frequency — then converted into an electrical signal. This unspecific response can be made selective, even specific, in the case of QCM immunosensors [105]. Despite non-gravimetric contributions have been attributed to the QCR response, such as the effect of single-film viscoelasticity [106], these contributions are also showed by a shift of the fixed US frequency applied to the resonator so, the spectrum of the system under study is never obtained and the methods developed with the help of these devices cannot be considered spectrometric. Recent studies on acoustic properties of living cells on the sub-second timescale have involved both a QCM and an impedance analyser thus susceptance and conductance spectra are obtained by the latter [107]. 

Impedance analysis is also suggested when properties of an attached film, a liquid, or interfaces are of interest. Due to the weak frequency dependence of the acoustic load within a typical measurement range of some 10 kHz at fundamental mode, one measurement point would be sufficient to calculate Zl (Eq. 2). An effective method to decrease statistical errors is to first fit a theoretical curve to the experimental curve or a specific segment, secondly to calculate Zl from the fit, and finally to extract (material) parameters of interest using separate models describing how the acoustic load is generated [37]. 

The effect of the modification of the fibre surface consists in the shift of the sound absorption maximum towards higher frequency range and is accompanied by the decrease of the sound absorption coefficient a. The reduction of the coefficient a is not large. It amounts 2.5 % in the case of composites with pine wood and rapeseed straw Californium. For the samples with beech wood filler the coefficient a remains unaffected after modification. The exception is the composite containing the rapeseed straw Kaszub as a filler that shows the decreasing in the sound absorption of 7%. The reduction of the a coefficient value at the frequency related to the maximum of the sound absorption can be associated with the increase in the density of the composite after modification (Table 1) due to better adhesion between filler particles and polypropylene matrix. The fact implies that the specific acoustic impedance of... 

Table 1 - Specific acoustic impedance of some materials (from Ref. 1)...

In more advanced discussions, there are three kinds of impedances specific acoustic impedance (pressure/particle speed), acoustic impedance (pressure/volume speed) and radiation impedance (force/speed). See Ref. 6 for the details. 

Defect t5q>es frequently relate to the mechanism causing the indication. Many NDT methods have an intrinsic sensitivity for specific types of indications. Examples are ( ) ultrasonic C-scans sensitive to boundaries between materials of different acoustic impedance (eg, voids, delaminations), (2) X-ray radiography sensitive to variations in density (eg, inclusion of foreign objects, voids), and (5) acoustic emission sensitive to microscopic stress release (eg, crack growth). 

The specific acoustic impedance, Z, is the resistance of a medium to the propagation of a sound wave. It can be defined as the ratio of acoustic pressure to the so-called particle velocity at a single frequency (McClements, 1997). At an interface, the proportion of wave energy transmitted or reflected depends on the difference in impedance between the two media. Consequently, this difference determines the coupling between emitting surface and the treated medium. If the impedance difference is large, the proportion of energy reflected will be important and the ultrasound effects will be mainly localized at the interface. However, if the... 

There would appear to be no knowledge of the specific acoustic impedance of foam. It seems probable, however, that the impedance at, for example, the top of... 

Most textiles that are used for acoustic purposes show open porosity due to many interconnected pores or voids inside. The acoustic performance of a porous textile is mainly determined by its (air) flow resistivity, which is an intrinsic property of the textile and is a measure of how easily air can enter and pass through a porous textile material (Cox and D Antonio, 2009). Flow resistivity, also known as static flow resistivity, is related to acoustical properties and plays a critical role in the calculation of many intrinsic acoustic properties of porous textiles, such as the characteristic impedance, the propagation constant, and the sound absorption coefficient. In the S.l. Unit system, flow resistivity is quoted in units of Nsm" and is defined as the unit-thickness specific flow resistance o (Morfey, 2001),... 

Flow resistance is also known as static flow resistance and is the ratio of the pressure drop across a porous element to the volume velocity flowing through it under conditions of steady low speed flow. The flow resistance is almost independent of the volume velocity at low speeds. However, flow resistance is dependent on the acoustic frequency. Dynamic specific flow resistance of a thin (compared to acoustic wavelengths) porous textile layer is the real part of the complex specific flow impedance at a specified frequency, which is defined as the complex ratio of the pressure drop across the layer to the relative face velocity through the layer. When the frequency tends to zero, the dynamic specific flow resistance varies little with frequency, so it is almost equal to the static flow resistance. In the international standard, flow resistance is defined as the real part of the ratio between the pressure drop and the flow velocity through a layer of material of unit thickness (ISO 9053,1991). Flow resistance characterizes a layer of specified thickness, whereas flow resistivity characterizes a bulk material in terms of resistance per unit thickness. 

There are many acoustical methods proposed for measuring flow resistivity (Delany and Bazley, 1971 Smith and Parott, 1983). A method that uses a standard impedance tube directly to measure the static flow resistivity without any additional requirements to tube modification or sensor location change is described in ISO Standard, 10534-2 (1998) and by Tao et al. (2015). In the method, the specific acoustic impedance on the front surface of the test specimen is measured first by using the traditional transfer function method with the test specimen being placed against and with a known interval to the rigid termination, and then the characteristic impedance, the propagation constant, and the static flow resistivity are calculated based on the obtained impedance transfer functions. 

Fig. 5.3 shows a diagram of the impedance tube designed for the ISO 10534.2 implementation, where the test specimen thickness is 21 and the back cavity depth (the distance between the back surface of the specimen and the rigid termination) is L. The specific acoustic impedance at the front surface of the test specimen can be determined by (ISO Standard, 10534-2, 1998) ... 

On the other hand, the specific acoustic impedance at the test specimen s front surface can be formulated with the impedance transfer function (Kinsler et al., 2000) ... 

Assuming that the specific acoustic impedance on the front surface of the test specimen when L is zero and nonzero is measured as Z and Z respectively, the propagation constant can be obtained by substituting Eqs. (6.18) and (6.19) into Eq. (6.17) as Eq. (6.20) ... 

---