Ultrasonic Wave and Electromechanical Impedance Spectroscopy— Not What You Thought!!!
In this article at the beginning I will explain and describe what is an ultrasonic wave. After that, I will explain how to generate this kind of waves and what are their ranges. At the end I will count different types of ltrasonic wave and explain what is an electromechanical impedance spectroscopy (EMIS).
1. Introduction
Ultrasonic wave is an inaudible sound wave with high frequency for human. A sound wave is defined as “a vibration that is transmitted through a medium, such as air, water, and metals“. Basically, any sound wave which is not intended to be heard called ultrasonic wave. We talking about sound wave above 20 kHz (i.e., 20,000Hz).
2. How is ultrasonic wave generated?
In order to duplicate ultrasonic frequencies, humans have harnessed the electrical properties of materials. When a specially cut piezoelectric quartz crystal is compressed, the crystal becomes electrically charged and an electric current is generated: the greater the pressure, the greater the electric current.
If the crystal is suddenly stretched rather than being compressed, the direction of the current will reverse itself. Alternately compressing and stretching the crystal has the effect of producing an alternating current.
It follows that by applying an alternating current that matches the natural frequency of the crystal, the crystal can be made to expand and contract with the alternating current. When such a current is applied to the crystal, ultrasonic waves are produced.
Depending on which way the crystal is cut, the waves can be focused along the direction of ultrasound propagation or at right angles to the direction of propagation. Waves that travel along the direction of propagation are called longitudinal waves; as noted above, these waves travel in the direction in which molecules in the surrounding medium move back and forth.
Waves that travel at right angles to the propagation direction are called transverse waves; the molecules in the surrounding medium move up and down with respect to the direction that the waves propagate.
Ultrasound waves can also propagate as surface waves; in this case, molecules in the surrounding medium experience up-and-down motion as well as expanding and contracting motion.
3. What is the range for ultrasonic waves?
The term “ultrasonic” applied to sound refers to anything above the frequencies of audible sound, and nominally includes anything over 20,000Hz . Frequencies used for medical diagnostic ultrasound scans extend to 10 MHz (10,000,000Hz) and beyond.
Frequency less than 20Hz is infrasonic, frequency greater than 20,000Hz is ultrasonic, 20Hz to 20,000Hz is audible range for the human ear.
4. What are the different types of ultrasonic waves?
Ultrasonic waves are acoustic waves whose frequency is more than 20kHz .They travel with the speed of sound. Hence their wave length is smaller than 333200cms-1/ 20000Hz = 1.66 cm/ These waves possess a number of properties of sound waves and exhibit some new phenomena also.
Ultrasonic waves can propagate through a medium as stress or strain waves depending upon the elastic properties of medium. Based on particle displacement of the media, ultrasonic waves are are classified into four types or modes:
- Longitudinal or Compressional or Pressure ultrasonic Waves- In the longitudinal waves particles of medium vibrate back and forth parallel to the direction of propagation of wave. These waves propagate through the medium as a series of alternate compression and rarefaction. These waves are most widely used in the ultrasonic inspection of materials. This mode is exhibited when medium of propagation has no boundaries i.e. it has infinite span. Due to propagation of these waves both pressure and density of medium fluctuate periodically.
- Transverse or Shear ultrasonic Waves- In the transeverse waves particles of the medium vibrate perpendicular to the direction of waves propagation. In this case the medium undergoes shear deformations periodically. These waves can propagate through this rods.
- Surface or Rayleigh Waves- The surface waves travel along the flat or curved surface of thick solids without influencing the bulk of medium below the surface. The depth to which these waves propagate below the surface with considerable intensity is approximately equal to wavelength of the wave.Practically all of its energy is attenuated within this depth. These waves are used to detect cracks or flaws on or near the surface of test objects. During the propagation of surface waves, the particles of medium describe elliptical orbits.
- Lamb or Flexural or Plate Waves- The lamb waves are produced in thin metal, whose thickness is comparable to the wavelength of ultrasonic wave.
5. What is Electromechanical Impedance Spectroscopy (EMIS)?
Electromechanical impedance spectroscopy (EMIS) is a popular structural health monitoring (SHM) technique, which had found applications in many fields of engineering: mechanical, aerospace, civil and others.
Piezoelectric wafer active sensors (PWAS) are lightweight and inexpensive transducers that enable a large class of SHM applications such as the high-frequency modal sensing, i.e., EMIS method.
PWAS can be used as active sensing devices that provide bidirectional energy transduction from the electronics into the structure, and also from the structure back into the electronics. PWAS can be used as collocated electromechanical impedance sensor-actuators that permit effective modal identification in a wide frequency band.
In an embedded sensing system, the PWAS can be embedded into the structures by mounting them directly onto the structure and then leaving them in place to perform their structural health-monitoring task (such a thing would be unthinkable with conventional ultrasonic transducers, which are bulky, obtrusive, and expensive).
About the Author
Roi Polanitzer, CFV, QFV, FEM, F.IL.A.V.F.A., FRM, CRM, PDS, is a well-known authority in Israel the field of business valuation and has written hundreds of papers that articulate many of the concepts used in modern business valuation around the world. Mr. Polanitzer is the Owner and Chief Appraiser of Intrinsic Value — Independent Business Appraisers, a business valuation firm headquartered in Rishon LeZion, Israel. He is also the Owner and Chief Data Scientist of Prediction Consultants, a consulting firm that specializes in advanced analysis and model development.
Over more than 17 years, he has performed valuation engagements for mergers and acquisitions, purchase price allocation (PPA) valuations, goodwill impairment test valuations, embedded option and real option valuations, employee stock option (ESOP) valuations, common stock valuations (409A), splitting equity components and complicated equity/liability instrument valuations (PWERM / CCM / OPM), contingent liability, guarantees and loan valuations, independent expert opinions for litigation purposes, damage quantifications, balancing resources between spouses due to divorce proceedings and many other kinds of business valuations. Mr. Polanitzer has testified in courts and tribunals across the country and from time to time participates in mediation proceedings between spouses.
Mr. Polanitzer holds an undergraduate degree in economics and a graduate degree in business administration, majoring in finance, both from the Ben-Gurion University of the Negev. He is a Full Actuary (Fellow), a Corporate Finance Valuator (CFV), a Quantitative Finance Valuator (QFV) and a Financial and Economic Modeler (FEM) from the Israel Association of Valuators and Financial Actuaries (IAVFA). Mr. Polanitzer is the Founder of the IAVFA and currently serves as its chairman.
Mr. Polanitzer’s professional recognitions include being designated a Financial Risk Manager (FRM) by the Global Association of Risk Professionals (GARP), a Certified Risk Manager (CRM) by the Israel Association of Risk Managers (IARM), as well as being designated a Python Data Analyst (PDA), a Machine Learning Specialist (MLS), an Accredited in Deep Learning (ADL) and a Professional Data Scientist (PDS) by the Professional Data Scientists’ Israel Association (PDSIA). Mr. Polanitzer is the Founder of the PDSIA and currently serves as its CEO.
He is the editor of IAVFA’s weekly newsletter since its inception (primarily for the professional appraisal community in Israel).
Mr. Polanitzer develops and teaches business valuation professional trainings and courses for the Israel Association of Valuators and Financial Actuaries, and frequently speaks on business valuation at professional meetings and conferences in Israel. He also developed IAVFA’s certification programs in the field of valuation and he is responsible for writing the IAVFA’s statement of financial valuation standards.