Sound transducers

Free puncture Puncture or biopsy needle and sound transducer are not connected mechanically. 

Endoscopic ultrasonography promises greater accuracy as it involves placing a specialized sound transducer directly on the liver by means of laparoscopy or surgery. 

RecepU rs Polymers Organelles Microbes Chemical, mass, tighl. heat, sound, pressure. electrical signal Semiconduciitr QCM de ice Phoiolransduccr Sound transducer... 

One further point might be made for clarity. As we have seen, dielectrophoresis is the translational motion evoked by a nonuniform electric field. In the case of some solid materials and in certain semisolid ones (e.g., liquid crystals) there is seen still another mechanical response of a neutral body to an electric field, that of a distortion. This is electrostriction, and refers to the distortional response or strain resulting from an imposed electrical stress. Electrostrictive strains are used in sound transducers, for example. Historically speaking the two effects, translational (dielectrophoresis) and distortional (electrostriction), where both at times referred to as electrostriction with resultant confusion. Modem usage has tended to restrict the term electrostriction to the discussion of distortional strain that has been induced electrically. For the sake of brevity, we shall frequently use the abbreviation DEP response as that referred to properly as dielectrophoresis. One can, of course, couple a moment arm to the dielectrophoretic force (e.g., DEP force) producing a torsion, and possibly a realignment of the body in the field. 

PZT appUcations (see e.g. Herbert 1982 Mattiat 1971 Rogacheva 1994 Setter and Colla 1993 Uchino 1997,2000 Waanders 1991) cover mainly the field of ultrasonics (ultrasound transducers, cleaners, fluid atomizers, welding etc ), sensors (e.g. for acceleration), gas ignitors, ceramic filters for TV and delay fines, sound transducers for buzzers and also various actuators (e.g. PZT bimorphs, ink-jet printer heads) etc. Although PZT compositions are known for many years they are still excellent for many applications. New PZT modifications are imder development. 

Electrode Semiconductor QCM device Phototransducer Sound transducer Thermistor... 

Kressmann R (2001b) New piezoelectric polymer for air-bome and water-borne sound transducers. J Acoust Soc Am 109 1412-1416... 

The rest of this chapter is structured as follows. Section 5.2 gives a brief historical overview together with a description of the synthesis, structure and preparation of ferroelectric polymers. Section 5.3 defines the properties required to evaluate these materials, and includes typical values and brief accounts of the measurement methods. Section 5.4 details the applications of ferroelectric polymers, covering sound transducers, biomedical, pyroelectric and mechanical applications. Conclusions are briefly described in section 5,5. 

Ferroelectric composites can be used as underwater transducers [22] to detect (passive mode) and/or generate (active mode) sound. Transducers made from ceramics have the disadvantage that their density makes it difficult to obtain good impedance matching with water. One of the problems with PVDF (aside from the problems associated with poling) is its low permittivity. This will produce a low element capacitance, which will load the output. The equivalent circuit for a flank array transducer linked to an amplifier via a cable is shown in Fig. 6.15. The open-circuit voltage sensitivity Moc/ is loaded by stray capacitance Cg, and/or cable capacitance Q. The end-of-cable sensitivity Mgc is given by... 

Figure 4 Two side-drilled holes at sound path 50 mm, before and after signal processing. The 10 mm hole is located at transducer position 25 mm and the 8 mm hole at 75 mm.

Some of the problems often encountered during ultrasonic inspection of plane specimens are also found on cylindrical specimens. For example, problems associated with the directional characteristic of the ultrasonic transducer. Furthermore, the discontinuity influences the shape and propagation direction of a reflected pulse, causing wave mode transformation. In addition, the specimen influences the shape and amplitude of the reflected pulse by sound absorption. 

The first example refers to the detection of a 1mm side drilled hole at a depth of 45 mm in a polyethylene plastic material. Due to the high sound absorption in plastics, a low operating frequency is chosen. A probe having a 1 MHz element of 24 mm diameter was selected for this example. The echo pattern of a conventional probe with a PZT transducer is pre-... 

The transducer parameters (position, frequency, diameter) are optimised in order to obtain a sound beam which enables a good echo from the crack. 

The time of flight is measured and because of the fixed distanee of the transducers the actual sound velocity of... 

Fig. 2 shows the CFRP-sandwich specimen and the transducer mounted on the scanner. Fig. 23 presents a C-scan of the specimen as first interesting result. Only the defects visible from the outside are indicated. The distance between transducer and specimen was smaller than the focal length, so that the angle of incidence at the edge of the sound beam converts the longitudinal waves to Rayleigh-waves in the specimen. These waves provide a very sharp image of the surface. This method opens the possibility for a non-contact acoustic microscope. 

In UltraSIM/UlSim the ultrasonic sound propagation from a virtual ultrasonic transducer can be simulated in ray tracing mode in any isotropic and homogeneous 3D geometry, including possible mode conversions phenomenons, etc. The CAD geometry for the simulation is a 3D NURBS surface model of the test object. It can be created in ROBCAD or imported from another 3D CAD system. 

The virtual transducer can be placed in a specific location on the test object surface, it can be moved along a path (e.g. a robot scanning path generated off-line or a path resulting from a real inspection sequence) or it can be moved along the surface, dynamically updating the ultrasonic sound propagation in the material. 

During the attenuation measurements. Transducer 1 was excited with a narrowband tone burst with center frequency 18 MHz, see Figure 1 for a schematic setup. The amplitude of the sound pressure was measured at Tranducer 2 by means of an amplitude peak detector. A reference amplitude, Are/, was measured outside the object as shown at the right hand side of Figure 1. The object was scanned in the j y-plane and for every position, (x, y), the attenuation, a x, y), was calculated as the quotient (in db) between the amplitude at Transducer 2, A[x, y), and Are/, i.e., a(x,y) = lOlogm Pulse echo measurements and preprocessing... 

The sound pressure I at the minute point D produced by the transducer is obtained by the equation(6)... 

The development of active ceramic-polymer composites was undertaken for underwater hydrophones having hydrostatic piezoelectric coefficients larger than those of the commonly used lead zirconate titanate (PZT) ceramics (60—70). It has been demonstrated that certain composite hydrophone materials are two to three orders of magnitude more sensitive than PZT ceramics while satisfying such other requirements as pressure dependency of sensitivity. The idea of composite ferroelectrics has been extended to other appHcations such as ultrasonic transducers for acoustic imaging, thermistors having both negative and positive temperature coefficients of resistance, and active sound absorbers. 

The source and detector of ultrasound in an ultrasound medical imager is called a transducer. The transducer is a piezoelectric crystal which physically changes its dimensions when a potential is appHed across the crystal (38). The appHcation of a force to the piezoelectric crystal which changes its dimensions creates a voltage in the crystal. AppHcation of an oscillating potential to the crystal causes the dimensions of the crystal to oscillate and hence create a sound at the frequency of the oscillation. The appHcation of an oscillating force to the crystal creates an alternating potential in the crystal. 

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