Acoustic transducers

Shear Horizontal (SH) waves generated by Electromagnetic Acoustic Transducer (EMAT) have been used for sizing fatigue cracks and machined notches in steels by Time-of-Flight Diffraction (TOED) method. The used EMATs have been Phased Array-Probes and have been operated by State-of-the-art PC based phased array systems. Test and system parameters have been optimised to maximise defect detection and signal processing methods have been applied to improve accuracy in the transit time measurements. 

Fig.6. Appearance of installation for the testing of sleeves of diesel engines. 1- ultrasonic flow detector 2- electronic blocks 3- electromechanical drives 4- acoustic transducers in a sliding bar 5- controllable detail.

The use of electromagnetic acoustic transducers (EMAT) obviates the coupling problems already referred to, and has been applied successfully to the inspection of boiler tubes. Ultrasonic time of flight diffraction (TOFD), developed by the Harwell Laboratory" , is utilised to fingerprint flaws (cracks) in process plant. Subsequent examination at, say, six month intervals can indicate any growth or extension of the crack. It is claimed that changes in crack height of 0-5 mm or less be estimated. 

A semi-industrial pilot plant has been developed in which air-borne ultrasound has been applied to the reduction of particle emissions in coal combustion fumes [62]. The installation basically consists of an acoustic agglomeration chamber with a rectangular cross-section, driven by four high-power and highly directional acoustic transducers operating at 10 and/or 20 kHz, and an electrostatic precipitator (ESP). In the experiments, a fluidised bed coal combustor was used as fume generator with fume flow rates up to about 2000 m /h, gas temperatures of about 150 °C. and mass concentrations in the range 1-5 gm. The acoustic filter reduced fine particle emissions by about 40 %. 

The pyroelectricity or depolarization current is closely related to the piezoelectricity of Groups (B) and (C), because the pyroelectric current is caused by the thermal activation of charges in the film. The application of polypeptide film to an acoustic transducer was proposed by Fukada, Tamura, and Yamamoto (1968). 

Mercury binding leads to an increase of mass of the gold layer which can be detected by electro-acoustic transducers based on quartz microbalance (QMB the abbreviation QCM = quartz crystal microbalance is also widely used), surface acoustic waves (SAW)—devices [20] or microcantilevers [21,22], Adsorption of mercury vapour increases resonance frequency of shear vibrations of piezoelectric quartz crystals (Fig. 12.2). This process can be described by Sauerbrey equation [23]. For typical AT-cut quartz, this equation is... 

AOTFs (101, 102) work on a different principal. This type of filter functions by the interaction of light with a traveling acoustic wave in an anisotropic medium. Both crystals and polymers have been used for the anisotropic medium. An acoustic transducer is bonded to one end of the material and an... 

Figure 3-35 AOTF Crystal showing the RF source, the acoustic transducer, the Acoustic absorbed, the Zero order ray and the two 1st order rays. (Reproduced with permission from Ref. 103.)...

SUUC OSCIUATOR CRYSTALS FOR FREOUERCV CONnOL hELECTROMAONETK ACOUSTIC TRANSDUCER (EMATI... 

Acoustic transducers operating at 10 and (or) 20 kHz are capable of defoaming liquids provided the acoustic source is placed above the liquid surface upon which the foam is being generated. 

Applicability of piezoelectric acoustic emission sensors to end-point determination has been studied since the beginning of this century.F l The technique is very promising, especially because it is non-invasive, sensitive, and relatively inexpensive. Granulation process signatures obtained with acoustic transducer can be used to monitor changes in particle size, flow, and... 

Figure 4 An acousto-optic tunable filter [a, tellurium dioxide (Te02) crystal b, incident or input beam c, acoustic transducer d, rf input e, monochromatic light (ordinary beam) f, nonscattered light beam g, monochromatic light (extraordinary beam) h, acoustic wave absorber].

Acoustic transducers have also been used in immunosensors for water analysis (Guilbault, 1992). The resonant frequency of an oscillating piezoelectric crystal can be affected by a change in mass at the crystal surface. Piezoelectric immunosensors are able to measure a small change in mass. Recent publications have been based on immunosensors using a quartz crystal microbalance (QCM) for the detection of trace amounts of chemical compounds, such as dioxins (Kurosawa, 2003). 

Any type of acoustic transducer, such as quartz crystal microbalance (QCM) or surface acoustic wave device (SAW), is fundamentally based on the piezoelectric effect. This was first described in 1880 by Jacques and Pierre Curie as a property of crystalline materials that do not have an inversion centre. When such a material is subjected to physical stress, a measurable voltage occurs on the crystal surfaces. Naturally, the opposite effect can also be observed, i.e. applying an electrical charge on a piezoelectric material leads to mechanical distortion, the so-called inverse piezo effect. These phenomena can be used to transfrom an electrical signal to a mechanical one and back, which actually happens in QCM and SAW. Different materials are ap-pHed for device fabrication, such as quartz, Hthium tantalate, lithium titanate... 

But why are acoustic transducers referred to as mass-sensitive Evidently, the plate thickness determines the resonance frequency of the quartz, but it... 

Hz this means absolute detection limits of 1 ng in the worst case, sometimes even lower, depending on the standards applied. For chemical sensing the detection of mass is one of the best label-free possibihties, because any analyte has a mass. Compared with other methods, acoustic transducing is... 

Acoustical properties of soybeans can be used to help distinguish between healthy and diseased soybeans. Misra et al. (1990) measured acoustic properties of soybeans by transmitting sound waves through soybeans using acoustic transmission and by an impact force method. In the impact force method, a seed is dropped on an acoustic transducer creating an impulse wave. The acoustic transmission method was slow but was able to predict the mass of individual soybeans. The impact force method showed that diseased soybeans had a narrower bandwidth than healthy soybeans. Soybeans with wrinkled surfaces and diseased and damaged soybeans were detected from healthy soybeans based on wide variations at low frequencies. 

Cellulose Aerobacter, Acetobacter, Achromobacter, Agrobacterium, Alcaligenes, Azotobacter, Pseudomonas, Rhizobium and Sarcina Food (natade coco), Diaphragms of acoustic transducers and wound dressing... 

Acoustic waves are often used in microscopy techniques for failure analysis and reliability testing of modem devices. Although they have a quite large wavelength up to several centimeters, they can be well used for nanoscopic investigations by introduction of near-field conditions [1], e.g. with microprobes. These microprobes can be used either as an acoustic source [2,3] or as a detector [4,5] together with a comparably large acoustic transducer. 

Baravelli E, Senesi M, Ruzzene M, De Marchi L, Speciale N. Double-channel, frequency-steered acoustic transducer with 2-D imaging capabilities. IEEE Trans Ultrason Ferroelectr Freq Control 2011 58(8). http //dx.doi.org/10.1109/tuffc.2011. 2000. 1430 and 1706. 

Piezoelectricity links the fields of electricity and acoustics. Piezoelectric materials are key components in acoustic transducers such as microphones, loudspeakers, transmitters, burglar alarms and submarine detectors. The Curie brothers [7] in 1880 first observed the phenomenon in quartz crystals. Langevin [8] in 1916 first reported the application of piezoelectrics to acoustics. He used piezoelectric quartz crystals in an ultrasonic sending and detection system - a forerunner to present day sonar systems. Subsequently, other materials with piezoelectric properties were discovered. These included the crystal Rochelle salt [9], the ceramics lead barium titanate/zirconate (pzt) and barium titanate [10] and the polymer poly(vinylidene fluoride) [11]. Other polymers such as nylon 11 [12], poly(vinyl chloride) [13] and poly (vinyl fluoride) [14] exhibit piezoelectric behavior, but to a much smaller extent. Strain constants characterize the piezoelectric response. These relate a vector quantity, the electrical field, to a tensor quantity, the mechanical stress (or strain). In this convention, the film orientation direction is denoted by 1, the width by 2 and the thickness by 3. Thus, the piezoelectric strain constant dl3 refers to a polymer film held in the orientation direction with the electrical field applied parallel to the thickness or 3 direction. The requirements for observing piezoelectricity in materials are a non-symmetric unit cell and a net dipole movement in the structure. There are 32-point groups, but only 30 of these have non-symmetric unit cells and are therefore capable of exhibiting piezoelectricity. Further, only 10 out of these twenty point groups exhibit both piezoelectricity and pyroelectricity. The piezoelectric strain constant, d, is related to the piezoelectric stress coefficient, g, by... 

Acoustic transducers (e.g., in ultrasonic meters, sound-delay lines, sound absorbers)... 

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