Preface

The subject of acoustic waves might easily be considered a mature one, quite specialized, with narrow and circumscribed fields of interest and of application. The present book is an evidence of the opposite: it witnesses how the concept of acoustic wave, a collective displacement of matter which perturbs an equilibrium configuration, is a pervasive concept, which emerges in very different fields. This type of phenomena can be analyzed from different points of view, it can be exploited in different ways, and is the object of active investigations. The present book, far from pretending to give an exhaustive overview of the subject, offers instead a sampling of various points of view, of applications, and of research objectives which are actively pursued.

It must first be remembered that acoustic waves are supported by all the forms of matter: solids, liquids, gases and plasmas. And if similarities among the different phenomena are deep enough for them to deserve the same name, nevertheless the peculiarities connected to the various media are significant. Although the range of involved length and time scales is huge, going from sub-micrometric layers exploited in microdevices to seismic waves propagating in the Sun's interior, the more profound peculiarities of the various cases concern the very heart of the phenomena, namely the type of forces which, in different types of media, tend to restore the equilibrium configuration.

These phenomena can be approached under different points of view. A first type of approach aims at a better comprehension of phenomena. Many aspects of acoustic waves are nowadays well understood, but the investigation is obviously never ending. A line of research aims at the theoretical exploration, also by relatively sophisticated mathematical analyses, of various aspects of phenomena whose basic laws are well established. Concerning acoustic waves in elastic solids, Huang recalls the characters of such waves in homogeneous isotropic media. Then he exploits recent computational tools to analyze the modifications occurring in media which are periodically inhomogeneous, like composite materials. Alshits, Lyubimov & Radowicz investigate instead the elastic waves in solids which are homogeneous but anisotropic, like single crystals. They show that the addition of a dissipative term to the elasto-dynamic equations has consequences which go far beyond the intuitive introduction of a damping. This term can modify the same topology of the slowness surface, inducing a splitting of acoustic axes. Homentcovschi & Miles review and reformulate in an operational way the 'acoustic analogy' theory which describes how noise is generated in the interaction of gas flow with stationary or mobile bodies; the application of this approach to a range of technologies (jets, propellers, aircrafts) is easily imagined. Khater & Hassan consider various nonlinear evolution equations which are well established in plasma physics and fluid dynamics, and which admit wave solutions, either periodic waves or solitary waves. They seek exact solutions, which helps to understand phenomena more than purely numerical solutions. Karmakar considers various aspects involved in perturbations of plasmas, from ion acoustic excitation to turbulence, and focuses on the effects of the inertia of electrons, which is much smaller than that of ions but is not always completely negligible. He then combines various arguments to give a picture of solar wind plasma, which needs the description of the solar surface boundary.

Preface XI

A third type of approach exploits acoustic waves to perform some kind of manipulation. Foldyna shows how acoustic transducers and waveguides can be exploited to generate and control pulsating water jets, which can be used as machining tools. Zinovev, Veryovkin & Pellin discuss the Laser Induced Acoustic Desorption technique to vaporize solid material to be analyzed by mass spectrometry. This technique is less prone to induce modifications of the analyte than the more widespread MALDI technique, although these authors show by some experiments that the operational mechanism is still not well understood. At the other extreme, that of high intensity laser pulses, Bykovsky & Senatsky demonstrate how stimulated Brillouin scattering can generate shock waves, able to induce permanent modifications of the materials, like phase changes and cracks. Finally, Branski considers the problem of active vibration control of beams, and investigates the optimal distribution of

A fourth type of approach exploits the properties of acoustic waves to design various types of devices, mainly micro devices. The most widely exploited type of device has a simple basic structure: a substrate, at least one layer of piezoelectric material, an interdigitated transducer (IDT) operating as an emitter, and another one operating as receiver. These type of devices were originally introduced as delay lines and filters,

Before discussing this type of device, it must be remembered that other types of devices also exist. Kreft & Blick discuss applications of surface acoustic waves to quantum electronics, made possible by devices like quantum dots and by the interaction of surface acoustic waves with the electron gas. This type of device is nanomechanical, and also exploits IDT, with acoustic waveguides to match their acoustic impedance to that of nanomechanical devices. The chapter by Cuadrado-Laborde and co-workers considers instead the in-fiber photonic devices, and the acousto-optic modulator which obtained exciting traveling or standing acoustic waves by a piezoelectric actuator. This way, a dynamic and controllable modulation of the fiber properties is obtained by the acousto-optic effect. They review a wide variety of configurations, showing how different devices can be obtained, including Q-switched

Returning to the most widespread type of microdevice, its interest is witnessed by the numerous chapters devoted to it. Naumenko reviews the most common design configurations, and presents detailed analyses of their behavior. Hoang presents the most adopted method of analysis of such devices, based on equivalent circuits and the so called Mason model. The method is adopted also in other chapters, and Hoang gives a detailed introduction of the method itself, also presenting the applications to basic configurations. Rocas & Collado analyze, for these devices, the various sources which can introduce a non perfectly linear behavior, leading to 3rd order intermodulation distortion. Most of the devices of this type exploit longitudinal acoustic waves, or surface waves polarized in the plane normal to the surface. Waves

actuators to perform such a control.

lasers and mode locking lasers.

and were then developed also for other purposes.

A second type of approach exploits acoustic waves as probes to gain information about the properties or the behavior of a system. Beghi revises various methods based on acoustic waves which aim at the elastic characterization of materials, namely of thin films. Gao & Pan consider a specific problem of significant technical relevance for the oil and gas industry: the drillability of rocks, and in particular its anisotropy. They shows how the outcome of laboratory acoustic tests correlates with the drilling properties of rocks. Gu & Choi consider instead the acoustic emission from rotating machinery, and show how it can be exploited for the early detection of faults. Burgholzer and coworkers focus on the photoacoustic imaging technique, and in particular on the image reconstruction to achieve the tomographic capability: they analyze methods to compensate for ultrasound attenuation in the media being observed.

Since acoustic waves are relatively a non invasive probe, they can be exploited also on delicate materials and on biological systems. Nassar presents various applications to delicate systems in the agro-industry, like cheese undergoing ripening, for which dedicated low frequency sensors had to be developed. Erzegen and co-workers characterize the performance of the multi-resonant thickness shear mode sensor, exploited with a genetic algorithm for data processing: this type of sensor is devoted to the characterization of biological interfaces. Finally, two chapters present measurements performed in vivo. Michalski, Dziewiszek & Bochnia discuss the performance of phase sensitive techniques to characterize non linear systems, and show how these techniques can be applied to cochlear microphonics to study ear behavior. Mota, Carvalho & Barja present photoacoustic measurements performed on human skin, to characterize the skin itself, and the transdermal drug delivery.

A completely different system is found in the ionosphere, where acoustic-gravity waves are found. Koucka & Mosna show how the ionogram technique can be exploited to investigate the ionosphere, in particular exploiting the waves excited by the shadow of an eclipse.

A third type of approach exploits acoustic waves to perform some kind of manipulation. Foldyna shows how acoustic transducers and waveguides can be exploited to generate and control pulsating water jets, which can be used as machining tools. Zinovev, Veryovkin & Pellin discuss the Laser Induced Acoustic Desorption technique to vaporize solid material to be analyzed by mass spectrometry. This technique is less prone to induce modifications of the analyte than the more widespread MALDI technique, although these authors show by some experiments that the operational mechanism is still not well understood. At the other extreme, that of high intensity laser pulses, Bykovsky & Senatsky demonstrate how stimulated Brillouin scattering can generate shock waves, able to induce permanent modifications of the materials, like phase changes and cracks. Finally, Branski considers the problem of active vibration control of beams, and investigates the optimal distribution of actuators to perform such a control.

X Preface

solar surface boundary.

observed.

the shadow of an eclipse.

splitting of acoustic axes. Homentcovschi & Miles review and reformulate in an operational way the 'acoustic analogy' theory which describes how noise is generated in the interaction of gas flow with stationary or mobile bodies; the application of this approach to a range of technologies (jets, propellers, aircrafts) is easily imagined. Khater & Hassan consider various nonlinear evolution equations which are well established in plasma physics and fluid dynamics, and which admit wave solutions, either periodic waves or solitary waves. They seek exact solutions, which helps to understand phenomena more than purely numerical solutions. Karmakar considers various aspects involved in perturbations of plasmas, from ion acoustic excitation to turbulence, and focuses on the effects of the inertia of electrons, which is much smaller than that of ions but is not always completely negligible. He then combines various arguments to give a picture of solar wind plasma, which needs the description of the

A second type of approach exploits acoustic waves as probes to gain information about the properties or the behavior of a system. Beghi revises various methods based on acoustic waves which aim at the elastic characterization of materials, namely of thin films. Gao & Pan consider a specific problem of significant technical relevance for the oil and gas industry: the drillability of rocks, and in particular its anisotropy. They shows how the outcome of laboratory acoustic tests correlates with the drilling properties of rocks. Gu & Choi consider instead the acoustic emission from rotating machinery, and show how it can be exploited for the early detection of faults. Burgholzer and coworkers focus on the photoacoustic imaging technique, and in particular on the image reconstruction to achieve the tomographic capability: they analyze methods to compensate for ultrasound attenuation in the media being

Since acoustic waves are relatively a non invasive probe, they can be exploited also on delicate materials and on biological systems. Nassar presents various applications to delicate systems in the agro-industry, like cheese undergoing ripening, for which dedicated low frequency sensors had to be developed. Erzegen and co-workers characterize the performance of the multi-resonant thickness shear mode sensor, exploited with a genetic algorithm for data processing: this type of sensor is devoted to the characterization of biological interfaces. Finally, two chapters present measurements performed in vivo. Michalski, Dziewiszek & Bochnia discuss the performance of phase sensitive techniques to characterize non linear systems, and show how these techniques can be applied to cochlear microphonics to study ear behavior. Mota, Carvalho & Barja present photoacoustic measurements performed on

human skin, to characterize the skin itself, and the transdermal drug delivery.

A completely different system is found in the ionosphere, where acoustic-gravity waves are found. Koucka & Mosna show how the ionogram technique can be exploited to investigate the ionosphere, in particular exploiting the waves excited by A fourth type of approach exploits the properties of acoustic waves to design various types of devices, mainly micro devices. The most widely exploited type of device has a simple basic structure: a substrate, at least one layer of piezoelectric material, an interdigitated transducer (IDT) operating as an emitter, and another one operating as receiver. These type of devices were originally introduced as delay lines and filters, and were then developed also for other purposes.

Before discussing this type of device, it must be remembered that other types of devices also exist. Kreft & Blick discuss applications of surface acoustic waves to quantum electronics, made possible by devices like quantum dots and by the interaction of surface acoustic waves with the electron gas. This type of device is nanomechanical, and also exploits IDT, with acoustic waveguides to match their acoustic impedance to that of nanomechanical devices. The chapter by Cuadrado-Laborde and co-workers considers instead the in-fiber photonic devices, and the acousto-optic modulator which obtained exciting traveling or standing acoustic waves by a piezoelectric actuator. This way, a dynamic and controllable modulation of the fiber properties is obtained by the acousto-optic effect. They review a wide variety of configurations, showing how different devices can be obtained, including Q-switched lasers and mode locking lasers.

Returning to the most widespread type of microdevice, its interest is witnessed by the numerous chapters devoted to it. Naumenko reviews the most common design configurations, and presents detailed analyses of their behavior. Hoang presents the most adopted method of analysis of such devices, based on equivalent circuits and the so called Mason model. The method is adopted also in other chapters, and Hoang gives a detailed introduction of the method itself, also presenting the applications to basic configurations. Rocas & Collado analyze, for these devices, the various sources which can introduce a non perfectly linear behavior, leading to 3rd order intermodulation distortion. Most of the devices of this type exploit longitudinal acoustic waves, or surface waves polarized in the plane normal to the surface. Waves

### XVI Preface

transversally polarized in the plane of the device surface are less considered. Yanagitani compares the performances of the two types of operation, showing the possible advantages of transversal waves. Avramov performs a similar comparison, between surface waves of the Rayleigh type and of the transverse type, for devices which are polymer coated to act as gas sensors. Some chapters focus instead on the production and the characterization of various materials which are of interest for the production of this type of devices. Buck considers ultranano crystalline diamond: diamond is the acoustically fastest material, which allows operation at the highest frequencies. Both Kar & Bose and Caliendo consider AlN layers, a piezoelectric material whose properties are interesting under several respects. Caliendo also considers multilayers, including platinum and sapphire layers.

As mentioned above, the various approaches documented in this book represent a sampling of the wide spectrum of methods and techniques involving acoustic waves. This book is offered to the scientific community in the hope of promoting a cross fertilization of ideas and of approaches.

> **Marco G. Beghi** Politecnico di Milano, Energy Department and NEMAS Center, Milano, Italy
