Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures and have led to the development of dynamically reconfigurable, loss-compensating and parity–time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and converting laboratory experiments into useful devices. In this Review, we outline the designs and properties of materials with unusual acoustic parameters (for example, negative refractive index), discuss examples of extreme manipulation of sound and, finally, provide an overview of future directions in the field. Acoustic metamaterials can be used manipulate sound waves with a high degree of control. Their applications include acoustic imaging and cloaking. This Review outlines the designs and properties of these materials, discussing transformation acoustics theory, anisotropic materials and active acoustic metamaterials.

Controlling sound with acoustic metamaterials

Piezoelectric and piezomagnetic materials and their function in transducers

3 – Piezoelectric and Piezomagnetic Materials and Their Function in Transducers

An ultrasonic wave traveling in certain directions in a piezoelectric semiconductor such as cadmium sulfide can be amplified or attenuated by application of a dc electric field. The direct current flowing through the medium in the presence of an ultrasonic wave creates a traveling ac field which interacts with the ultrasonic wave. Amplification occurs when the drift velocity of the electrons exceeds the velocity of sound. For strongly piezoelectric semiconductors, amplification of as much as several percent per wavelength of path is obtainable. Calculations show that for properly prepared material, significant amplification is expected up to the microwave frequencies. At high frequencies, gain is reduced because electron diffusion smooths out the electron bunching necessary for amplification. The dc power required increases rapidly with frequency, and at frequencies above one or two thousand megacycles only pulsed operation seems feasible. Applications as a tool in ultrasonic studies, and for devices such as delay lines and amplifiers, are also discussed.

Amplification of Ultrasonic Waves in Piezoelectric Semiconductors

Properties of piezoelectric crystals and ceramics are reviewed as they affect use of such materials in electroacoustic transducers. Extensive data on lead titanate-zirconate ceramics are presented to help in selection of the appropriate material for a variety of applications in the lower and medium frequency range, up to several megacycles per second. Quartz and several recently discovered piezoelectric crystals will share the higher megacycle range.

Piezoelectric transducer materials

Radiofrequency waves at 15 and 45 Mc are transduced into pressure waves at the same frequencies, and are applied to a CdS crystal. The waves are then converted by a second transducer into a r-f output wave. The signal amplification is found as a function of the illumination intensity on the crystal, and the electric field parallel to the pressure waves. Positive gain may be attained for suitable values of the electric field. (T.F.H.)

D. L. White

Papers

Ultrasonic Amplification in CdS