Browsing by Subject "Ultrasonic waves"

Now showing 1 - 6 of 6

Open Access
Characterization of layered materials by the lamb wave lens
(IEEE, 1990) Atalar, Abdullah; Köymen, Hayrettin; Değertekin, F. Levent
The Lamb wave lens is a type of lens that can replace the conventional lens of an acoustic microscope for some applications. When it insonifies a layered solid structure, it excites a single mode in the structure, provided that the frequency is properly adjusted. Measuring the return signal as a function of frequency results in a characteristic curve showing the excited modes. Since the number of excited modes, the frequencies at which they are excited, and the efficiency of excitation are highly dependent on the elastic and physical parameters of the layered solid, a characterization method emerges. The authors describe this characterization method for the layered structures using the Lamb wave lens. Theoretical and experimental results are presented.
Open Access
Experimental characterization of capacitive micromachined ultrasonic transducers
(IEEE, 2007) Ölçüm, Selim; Atalar, Abdullah; Köymen, Hayrettin; Oğuz, Kağan; Şenlik, Muhammed N.
In this paper, capacitive micromachined ultrasonic transducers are fabricated using a sacrificial surface micromachining process. A testing procedure has been established in order to measure the absolute transmit and receive sensitivity spectra of the fabricated devices. The experiments are performed in oil. Pulse-echo experiments are performed and the results are compared to the pitch-catch measurements using calibrated transducers.
Open Access
Increasing the sensitivity of the scanning acoustic microscope to anisotropy
(IEEE, 1987) Atalar, Abdullah
The response of the scanning acoustic microscope to anisotropic materials is theoretically investigated. For this purpose, the reflection coefficient of plane acoustic waves incident on a liquid-anisotropic-solid interface is calculated. The reflection coefficient depends, in general, on polar and azimuthal angles of incidence. For the acoustic microscope case, a mean reflectance function can be defined which depends only on the polar angle, because there is a circular symmetry. With this mean reflectance function it is possible to explore the effects of changing the lens parameters such as the acoustic field at the back side of the lens. It is found that the response of the scanning acoustic microscope can depend heavily on the orientation of the solid material under investigation, provided that a suitable lens insonification is utilized. The amplitude of the acoustic microscope signal is influenced by the orientation of the material, because there is an interference between the acoustic waves reflected from the material surface at different azimuthal angles. This interference is revealed as a minimum in the mean reflectance function. It is shown by computer simulation that sensitivity to orientation can be increased by use of a ring-shaped transducer in the near field of the acoustic lens. With such lenses, it may be possible to determine the orientation of crystallites in a material.
Open Access
Measurement of sensitivity of different wave modes to subsurface defects
(IEEE, 1988) Atalar, Abdullah; Köymen, Hayrettin; Yemişçiler, O.
Excitation of acoustic wave modes in a layered solid is investigated when the solid is immersed in a liquid. This is done by computing the reflection coefficient of acoustic plane waves at the liquid-layered-solid interface. Then, a method to evaluate the sensitivity of various modes supported in the layered structure to subsurface defects is introduced. The method involves insonification of the layered structure with a conical wave whose axis coincides with the defect. Since all the rays in the conical wavefront hit the planar interface at the same angle, it is possible to excite a single kind of mode in the layer. By adjusting the angle of inclination of the cone, it is possible to excite the modes selectively. Since the conical waves converge to a line focus at the cone axis, the excited mode will focus on the defect. By recording the reflected signal amplitude as the cone angle is varied, a curve is obtained from which it is possible to conclude the sensitivity of various modes to the subsurface flaw. The results of such measurements indicated that the generalized Lamb wave modes are more sensitive to subsurface defects than the Rayleigh waves. An imaging system which makes use of focused Lamb waves was built and the system produced images of very small subsurface defects.
Open Access
Radiation impedance of an array of circular capacitive micromachined ultrasonic transducers
(IEEE, 2010) Senlik, M. N.; Olcum, S.; Köymen, Hayrettin; Atalar, Abdullah
The radiation impedance of a capacitive micromachined ultrasonic transducer (cMUT) with a circular membrane is calculated analytically using its velocity profile for the frequencies up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. A higher radiation resistance improves the bandwidth as well as the efficiency of the cMUT. The radiation resistance is determined to be a strong function of the cell spacing. It is shown that a center-to-center cell spacing of 1.25 wavelengths maximizes the radiation resistance, if the membranes are not too thin. It is also found that excitation of nonsymmetric modes may reduce the radiation resistance in immersion applications.
Open Access
Wafer bonded capacitive micromachined underwater transducers
(IEEE, 2009-09) Olcum, Selim; Oǧuz, Kaan; Şenlik, Muhammed N.; Yamaner F. Y.; Bozkurt, A.; Atalar, Abdullah; Köymen, Hayrettin
In this work we have designed, fabricated and tested CMUTs as underwater transducers. Single CMUT membranes with three different radii and 380 microns of thickness are fabricated for the demonstration of an underwater CMUT element. The active area of the transducer is fabricated on top of a 3″ silicon wafer. The silicon wafer is bonded to a gold electrode coated glass substrate wafer 10 cm in diameter. Thermally grown silicon oxide layer is used as the insulation layer between membrane and substrate electrodes. Electrical contacts and insulation are made by epoxy layers. Single CMUT elements are tested in air and in water. Approximately 40% bandwidth is achieved around 25 KHz with a single underwater CMUT cell. Radiated pressure field due to second harmonic generation when the CMUTs are driven with high sinusoidal voltages is measured. ©2009 IEEE.