In monochromatic description (sections 5.1, 5.2), the results of the studies of the physical
characteristics of unidirectional acoustic sources used in active sound control systems are presented. A discrete
unidirectional source in the form of two phased monopoles (section 5.1) and a planar array of such unidirectional
sources is considered (section 5.2.1). One-dimensional boundary-value problems with two (the two-point problem)
and three (the three-point problem) controlled parallel planar boundaries between homogeneous media with
arbitrary impedances are studied (sections 5.2.2-5.2.6). The boundaries (two or three) are subjected to the action
of external forces. The case of the zero sum of external forces applied to the controlled boundaries corresponds to
a supportless unidirectional source (SUS). It is shown that a unidirectional source can be created within the twopoint
boundary-value problem, whereas a supportless unidirectional source can be created within the three-point
problem (sections 5.2.5, 5.2.6). Such parameters such as transparency, small size, absence of support, and broad
frequency band can be achieved for a unidirectional source in the form of two piezoelectric layers with the same
impedance and velocity of sound as those of the surrounding medium (5.2.7-5.2.9). The aspects of linearity of the
transparent SUS and its application to active sound control problems are described (sections 5.2.10, 5.2.11). A
spatially one-dimensional model of a plane active double layer between two homogeneous elastic half-spaces is
studied analytically in temporal representation (section 5.3). The layer synthesizes a preset smooth trajectory of
the controlled boundary between the media without any mechanical support. The outer layer of the coating is
piezoelectric, and the inner layer is a polymer that is transparent for low-frequency sound and opaque for highfrequency
sound because of dissipation. An algorithm for controlling the piezoelectric elements of the layer on
the basis of signals from surface particle velocity sensors is proposed (section 5.3.6), and a method for measuring
the particle velocity is developed simultaneously (section 5.3.9). Conditions of stability and efficiency of the
synthesis are formulated (section 5.3.7). It is shown that the active layer thickness can be much smaller than the
wavelength corresponding to the minimal time scale of the boundary trajectory to be formed. The accuracy of the
trajectory synthesis depends on the accuracy of measuring, computing, and actuating elements of the system but
does not depend on the vibroacoustic characteristics of the half-spaces separated by the active layer or on the
presence of smooth waves in these half-spaces. For the synthesis to be efficient, the operating frequency band and
the dynamic range of sensors and actuators should be many times greater than the frequency band and the
dynamic range of the trajectory to be formed.
