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Basic Principles of Ultrasound

Juan A. Gallego-Juárez1,2

1 Instituto de Tecnologías Físicas y de la Información (ITEFI), Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain

2 PUSONICS S. L, Arganda del Rey (Madrid), Spain

1. 1.1 Introduction
2. 1.2 Generation and Detection of Ultrasonic Waves: Basic Transducer Types
3. 1.3 Basic Principles of Ultrasonic Wave Propagation
4. 1.4 Basic Principles of Ultrasound Applications
   1. 1.4.1 Low-intensity Applications
      1. 1.4.1.1 Non-destructive Testing of Materials
      2. 1.4.1.2 Ultrasonic Imaging
      3. 1.4.1.3 Process Control
   2. 1.4.2 High-intensity Effects and Applications: Power Ultrasound
      1. 1.4.2.1 Cleaning
      2. 1.4.2.2 Atomization
      3. 1.4.2.3 Mixing, Homogenization, and Emulsification
      4. 1.4.2.4 Defoaming
      5. 1.4.2.5 Drying and Dewatering
      6. 1.4.2.6 Supercritical Fluid Extraction Assisted by Ultrasound
      7. 1.4.2.7 Bioremediation
      8. 1.4.2.8 Particle Agglomeration
      9. 1.4.2.9 Sonochemical Processes
5. 1.5 Conclusions

• Acknowledgments
• References

1.1 Introduction

As is well known, acoustics is the science of elastic waves, a broad interdisciplinary field which comprises such diverse areas as life and earth sciences, engineering, and arts. It may be divided into three main branches according to the frequency spectrum and the hearing characteristics to which the human auditory system responds: infrasound, sound, and ultrasound.

Infrasound is the branch dealing with frequencies below the human hearing range (0-20 Hz), sound refers to the human audible range (20Hz-20kHz), and ultrasound covers the very wide range of elastic waves from 20 kHz up to the frequencies associated to wavelengths comparable to intermolecular distances (about 1012 Hz). The basic principles and equations of acoustics are used to explain the general behavior of waves in the three branches. Nevertheless, the special characteristic of the ultrasound and infrasound waves of being inaudible establishes a fundamental difference in their applications with respect to the audio frequency field. The applications and uses of ultrasound are totally different from those of infrasound due to the very large differences in their wavelength ranges, as wavelength is inversely proportional to frequency. Infrasound waves are very long waves (wavelengths in the range of meters) generated by some natural phenomena, such as earthquakes or volcanic eruptions, or by human processes, such as sonic booms or explosions. Ultrasound waves are very short waves (wavelengths in the range of centimeters to nanometers) generally generated by specifically designed technological sources and are applied in many industrial, medical, and environmental processes. However, the human use of ultrasound was long preceded by use by animals, for example bats and dolphins, who use ultrasound for navigation and communication.

Beside the range of frequency, the range of wave intensity broadly influences the phenomena related to the production, propagation, and application of acoustic waves. As a consequence, a sub-classification within each of the three branches of acoustics should be adopted related to the use of low- or high-intensity waves. In this way ultrasound may be divided into two areas, dealing respectively with low- and high-intensity waves. The boundary between low- and high-intensity waves is very difficult to pinpoint, but it can be approximately established for intensity values which, depending on the medium, vary between 0.1 W/cm2 and 1 W/cm2.

As mentioned earlier, the general feature of ultrasound is its short wavelength, which determines its applications. In fact the short wavelength implies a high degree of discrimination and a high concentration of energy, therefore ultrasonic waves can be used as a means of exploration, detection, and information, and as a means of action. They can also be used as a means of communication, particularly for propagation in water, where electromagnetic waves have many limitations. In exploration, detection, and information, ultrasonic signals are able to determine the characteristics and internal structure of the propagation media without modifying them. For action applications, ultrasonic waves of high intensity are able to produce permanent changes in the medium on which they act. As a means of communication an ultrasonic signal can be modulated and transmit information.

The applications in which ultrasound waves are used as a means of exploration, detection, and information constitute the area of low-intensity ultrasound or signal ultrasound. The applications in which the ultrasonic energy is used to produce permanent changes in the propagation medium constitute the area of high-intensity ultrasound or power ultrasound. One specific use of ultrasound for communication is underwater acoustics, where sonic as well as ultrasonic waves are used to detect submerged objects, and for echo ranging, depth sounding, etc.

Typical applications of low-intensity ultrasound include non-destructive testing (NDT), process control, and medical diagnosis. High-intensity applications include a great variety of effects such as cleaning, drying, mixing, homogenization, emulsification, degassing, defoaming, atomization, particle agglomeration, sonochemical reactions, welding, drilling etc. High-intensity ultrasound also plays an important role in medical therapy.

The history of ultrasound is a modern part of the history of acoustics. In fact, although some studies on high acoustic frequencies were carried out in the 19th century, the real history of ultrasound began in 1915 with Paul Langevin, a prominent French physicist at the School of Physics and Chemistry in Paris. During the First World War, France and Britain launched programs for submarine detection and for this purpose Langevin designed and constructed underwater ultrasonic transducers consisting of a quartz plate sandwiched between two metal pieces (Langevin, 1920a,b, 1924). Following Langevin's work, in the 1920s Wood and Loomis conducted interesting experiments with high-intensity ultrasonic waves (200-500 kHz), for example the formation of emulsions, flocculation of particles, etc. (Wood and Loomis, 1927). During the 1930s new effects related to the application of ultrasonic energy were discovered and more than 150 studies were published. In the period 1940-1970 the development of new transducer materials as well as rapid advances in electronics made the production of commercial ultrasonic systems possible. Since 1970, the field of ultrasonics has grown rapidly and presently ultrasound is considered an emerging and expanding field covering a wide range of applications in the industrial, medical, and environmental sectors. Behind any application of ultrasound there is a fundamental scientific basis and the corresponding technology for generation, propagation, and detection of the ultrasonic waves, therefore the development of each specific application requires knowledge of the related basic principles and technologies.

1.2 Generation and Detection of Ultrasonic Waves: Basic Transducer Types

Any device capable of generating and/or detecting ultrasonic waves is called an ultrasonic transducer. As is well known, a transducer converts energy from one form to another. The most common conversion is electrical to ultrasonic energy in the case of transmitters, and ultrasonic to electrical energy in the case of receivers. The main types of electrical transducers are piezoelectric, magnetostrictive, capacitive or electrostatic, and electromagnetic. There are other kinds of transducers that are actuated mechanically, such as whistles and sirens, but in practice they have only historical value.

Piezoelectric transducers are based on the piezoelectric effect and are by far the most commonly used transducers in ultrasonics, therefore we will cover them in more detail later.

Magnetostrictive transducers utilize the magnetostriction effect that is produced in ferromagnetic materials that change dimensions under the application of a magnetic field. Conversely, if the material is deformed as a result of an external perturbation a variation in its magnetic properties is observed. The classical materials that have this effect are iron, nickel, cobalt and their different alloys, and also ceramic materials consisting of cubic ferrites (Mattiat, 1971). Since the 1970s new magnetostrictive materials based on rare earth compounds have been developed with large magnetostrain and high energy density (Clark, 1988).

Capacitive or electrostatic transducers are flat condensers in which one electrode is a very thin membrane very close to the other rigid electrode. The application of an alternate voltage superimposed on a bias voltage means that the membrane moves at the same frequency as the alternate voltage. The use of these transducers dates back to the 1950s (Khul, 1954), but recently the application of micromachining techniques for manufacturing transducers has strongly promoted their development for high-frequency imaging applications (Oralkan et al., 2002).

Electromagnetic transducers make use of the...