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The fundamentals of energy transfer in an acoustic field are addressed and it is shown that describing the flux of energy in an acoustic field with the active intensity alone is inaccurate. A single active intensity vector describes only the time-average energy flux at a point in space, but not where the energy came from nor where it is going. Consequently, the instantaneous intensity must be used to properly describe energy flux as a time-dependent process. The phenomenon of the acoustic vortex is examined and, from the perspective of active intensity, it is seen to represent a resultant wave rotating around a zero pressure line or point at which the pressure phase is discontinuous. It is shown that this resultant wave travels with a phase speed cp, which is generally different than the plane-wave phase speed c. The instantaneous intensity, however, shows that energy is flowing through the vortex and not with the resultant waves. Although the complex intensity vector is normally separated into the active and reactive components (i.e., rotational and solenoidal parts), the active intensity can be further represented by two parts, one with zero and another with nonzero curl, which permits the representation of the coupling between sources or parts of a source. In general, this article emphasizes the physical meaning and interpretation of energy related quantities.

The investigation of an active control system based on acoustic energy density has led to the analysis and development of an inexpensive three-axes energy density sensor. The energy density sensor comprises six electret microphones mounted on the surface of a 0.025-m (1 in.) radius sphere. The bias errors for the potential, kinetic, and total energy density as well as the magnitude of intensity of a spherical sensor are compared to a sensor comprising six microphones suspended in space. Analytical, computer-modeled, and experimental data are presented for both sensor configurations in the case of traveling and standing wave fields, for an arbitrary incidence angle. It is shown that the energy density measurement is the most nearly accurate measurement of the four for the conditions presented. Experimentally, it is found that the spherical energy density sensor is within +/- 1.75 dB compared to reference measurements in the 110-400 Hz frequency range in a reverberant enclosure. The diffraction effects from the hard sphere enable the sensor to be made more compact by a factor of 3 compared to the sensor with suspended microphones.

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Many applications of active sound transmission control ͑ASTC͒ require lightweight partitions, high transmission loss over a broad frequency range, simple control strategies, and consistent performance for various source and receiving space conditions. In recent years, researchers have begun to investigate active segmented partitions ͑ASPs͒ because of their potential to meet such requirements. This paper provides a theoretical and numerical analysis of four ASP module configurations that are candidates for these applications. Analogous circuit methods are used to provide normal-incidence transmission loss and reflection coefficient estimates for their passive and active states. The active control objective for each configuration is to induce global vibration control of various transmitting surfaces through direct vibration control of a principal transmitting surface. Two characteristic single-composite-leaf ͑SCL͒ configurations are unable to use the strategy effectively. However, design adjustments are investigated to improve their performances. Two double-composite-leaf ͑DCL͒ configurations use the strategy much more effectively to produce efficient global control of transmitting surface vibrations and achieve high transmission loss over a broad frequency range. This is achieved through a minimum volume velocity condition on the source side of each module. One DCL configuration enhances module isolation in full ASP arrays while satisfying other design and performance criteria.