Jiri Tichy

Instantaneous and time‐averaged energy transfer in acoustic fields

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...

Narrowband and broadband active control in an enclosure using the acoustic energy density

An active control system based on the acoustic energy density is investigated. The system is targeted for use in three-dimensional enclosures, such as aircraft cabins and rooms. The acoustic energy density control method senses both the potential and kinetic energy densities, while the most popular control systems of the past have relied on the potential energy density alone. Energy density fields are more uniform than squared pressure fields, and therefore, energy density measurements are less sensitive to sensor location. Experimental results are compared to computer-generated results for control systems based on energy density and squared pressure for a rectangular enclosure measuring 1.5 x 2.4 x 1.9 m. Broadband and narrowband frequency pressure fields in the room are controlled experimentally. Pressure-field and mode-amplitude data are presented for the narrowband experiments, while spectra and pressure-field data are presented for the broadband experiment. It is found that the energy density control system has superior performance to the squared pressure control system since the energy density measurement is more capable of observing the modes of a pressure field. Up to 14.4 and 3.8 dB of cancellation are achieved for the energy density control method for the narrowband and broadband experiments presented, respectively.

Adaptive control of a two-stage vibration isolation mount

An adaptive control system has been developed that may be used for active noise and vibration control problems involving one‐dimensional propagation. Based on the least‐mean‐squares (LMS) algorithm, the adaptive controller performs both system identification and control in real time, without the need for a priori measurements of the system. Since the controller is adaptive in nature, it is possible to track changes in the system while maintaining optimal control. In the present application, the adaptive control system was applied to the problem of minimizing the force transmitted through a two‐stage vibration isolation mount. The control system was implemented in real time using a Motorola DSP56000ADS signal‐processing board and applied on a physical vibration isolation mount. For periodic excitations, the adaptive controller was capable of providing 30‐ to 40‐dB attenuation of the transmitted vibration. For broadband excitation, some limitations exist, but the controller was still capable of providing ab...

Error analysis of a practical energy density sensor

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.

Effect of rotating diffusers and sampling techniques on sound‐pressure averaging in reverberation rooms

The paper summarizes different means and techniques for measuring sound‐power and ‐pressure averaging in reverberation rooms. Stationary reflectors, properly located in the nearfield of the source and properly oriented with respect to the floor, can contribute considerably to the scattering of the energy radiated by the sound source. The final effect consists of reduction of the dependence of the radiated sound power on the source position. Rotating diffuser vanes are important for determining the sound power radiated at single frequencies. There are three effects: (i) The motion of the vane causes fluctuations of the sound pressure at each point of the chamber and, thus, reduces the dependence of the sound pressure on the microphone location. (ii) The motion of the vane causes a Doppler effect; an originally single‐frequency tone becomes a frequency‐modulated tone. Thus, a single line spectrum becomes a multiline spectrum, which reduces the spatial variations of the sound pressure and also increases the ...

Sound radiation from an elastically supported circular plate

The sound radiation from an elastically supported circular plate in an infinite baffle, which is excited by a concentric voice coil, was discussed taking account of the reaction of the acoustical fluid. Eigenvalue, elgenfunction, and volume velocity of each mode of the plate with different edge stiffness were calculated. It was found that the effect of elasticity was smaller for higher order of modes. There was a remarkable difference between the frequency response curves for light and heavy loading. For light loading(air), the peaks and dips were the results of resonances between the mass of the voice coil and the reactance of the mechanical impedance of the plate. On the other hand, for heavy loading(water), peaks and dips occurred when the reactance of mechanical impedance of the plate and radiation impedance canceled each other. The effect of several factors such as the loss factor of the plate, the mass and the radius of the voice coil on the frequency response curves were also discussed.

Sound radiation from convex and concave domes in an infinite baffle

The sound radiation from convex and concave rigid diaphragms represented by portions of a spherical surface was investigated for the purpose of modeling a direct radiator loudspeaker. Pressure responses, radiation impedances, directivity patterns, and some other characteristics were obtained and compared with those of a circular flat diaphragm of the same radius. While the on axis pressure response of the rigid flat piston remains constant, the response of the convex dome decreases for about ka?1.0 as the height of the dome increases. The concave dome has a wide smooth peak around ka=1.5 and several peaks and dips for larger values of ka. The radiation resistance of the convex dome increases by the rate of 12 dB/oct or less up to ka=2.0 and remains constant in the higher frequency regions. On the other hand, the radiation resistance of the concave dome has its largest peak at the frequency corresponding to the peak of sound pressure response. The directivity patterns of either the convex or concave dome d...

Acoustic intensity analysis: Distinguishing energy propagation and wave‐front propagation

Three arguments are presented to demonstrate the distinction between energy propagation and wave‐front propagation in an acoustic field where the reactive intensity is nonzero. This distinction is especially useful for understanding the acoustic near field. Physical interpretations of the active and reactive intensity vectors and their contributions to the energy propagation described by the instantaneous intensity are summarized. Also, two differential equations are developed that show the interdependence of the active and reactive intensity vectors. Other important results show the contribution of the reactive intensity to the wave fronts propagating at speeds other than the speed of sound, and also show that energy associated with the reactive intensity does propagate to the far field.

Adaptive control of a two-stage vibration mount

A two-stage vibration mount has been investigated as a means of reducing the force transmission to the foundation of the mount. Adaptive closed-loop control was applied to the system, thereby giving both active reduction at the lower frequencies and passive reduction at the higher frequencies. The control force generator was mounted on the intermediate mass, thus preventing the static force of the force generator from being transmitted to the foundation. The adaptive closed-loop system, based on the least-mean-squares algorithm, was investigated to determine its stability properties and optimal performance characteristics.<<ETX>>

Near‐field identification of vibration sources, resonant cavities, and diffraction using acoustic intensity measurements

Near‐field acoustic intensity measurements are a powerful analysis and diagnostic tool when all available measured quantities are utilized. The advantage of considering the reactive intensity and complex specific acoustic impedance along with the active intensity is stressed. An added feature to the data presentation is the pressure phase that is reconstructed from the time‐independent measured data. The active intensity plots are thus enhanced by superimposing the lines of constant phase so that the wave‐front propagation is visualized. In the first measurement configuration, an active source absorbing power is differentiated from a resonant cavity absorbing power and diffraction is localized. In the second configuration, the mode shape of a vibrating plate is identified.