Scott D. Sommerfeldt

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.

An adaptive filtered‐x algorithm for energy‐based active control

A control algorithm based on the filtered‐x LMS algorithm is developed that has the property of being able to control the energy in the acoustic field. The algorithm is useful for providing greater global control of the field than is obtained by controlling the pressure. This is due to the fact that the energy‐based approach effectively overcomes the observability problems that often limit the performance possible when controlling the pressure field. The control approach is developed for the cases of a predominantly standing wave field, and a predominantly propagating wave field. The approach is applicable for both broadband and narrow‐band excitations. A simple example demonstrating the increased performance possible with the energy‐based control approach is presented for the case of single frequency excitation.

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...

Simulation of a player–clarinet system

A time‐domain simulation model has been developed for investigating the player–clarinet system. The three components that constitute the simulation model consist of the player’s air column, reed, and the clarinet. The player’s air column is represented in terms of an analogous circuit model to obtain the mouth pressure. The reed is represented as a damped, driven, nonuniform bar. The clarinet is represented in terms of a scaled version of its input impedance impulse response. A convolution of the impulse response with the volume velocity determines the mouthpiece pressure. Use of the model is valid for both small‐ and large‐amplitude reed oscillations. Many of the nonlinearities associated with the clarinet are incorporated in the model in a rather natural way. Several vocal tract configurations are investigated to determine the influence of the vocal tract on the player’s air column impedance and the concomitant effect on the clarinet tone.

Global attenuation of broadband noise fields using energy density control

The performance of the energy density control algorithm for controlling a broadband noise is evaluated in a one-dimensional enclosure. To avoid the noncausality problem of the control filter, which often happens in a frequency domain optimization, analyses presented in this paper are undertaken in the time domain. This approach provides the form of the causally constrained optimal controller. Numerical results are presented to predict the performance of the active noise control system, and indicate that improved global attenuation of the broadband noise can be achieved by minimizing the energy density, rather than the squared pressure. It is shown that minimizing the energy density at a single location yields global attenuation results that are comparable to minimizing the potential energy. Furthermore, unlike controlling the squared pressure, the energy density control does not demonstrate any dependence on the error sensor location for this one-dimensional field. A practical implementation of the energy...

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.

Adaptive control of structural intensity associated with bending waves in a beam

This paper examines a method to control adaptively the structural vibration intensity in a beam. An algorithm is developed to estimate the total, instantaneous structural intensity, using finite‐difference techniques. In addition, algorithms based on the filtered‐x least‐mean‐squares algorithm are developed to adaptively control the intensity. To investigate the effectiveness of adaptive control of structural intensity, a number of control actuator/error sensor configurations are used. Adaptive control is implemented at resonance and off‐resonance frequencies, and the performance is evaluated by means of a separate accelerometer located in the structural far field. Experimental results demonstrate several trends. First, controlling the acceleration is considerably more effective when the error sensor is located in the far field rather than in the near field. Furthermore, controlling acceleration is more effective than controlling intensity, when the error sensors are in the far field. Conversely, when the...

A fast adaptive noise control algorithm based on the lattice structure

Abstract In this paper, a fast adaptive noise control algorithm is presented and applied to the active control of broadband noise in a one-dimensional enclosure. This algorithm employs the lattice predictor which decouples the filtered reference signals. As a result, the algorithm delivers good performance even with a wide variance in the eigenvalue spread ratio of the input correlation matrix. In addition, the algorithm can be implemented with a computational complexity that increases linearly with the filter order. To avoid the primary signal estimation process, the order-update for the estimation error is modified to obtain local estimation errors directly from the signal sensed by the error microphone. Computer simulations and laboratory experiments are conducted to investigate the convergence of this noise control algorithm in a one-dimensional enclosure. The results illustrate the fast convergence capability of the lattice-based noise control algorithm.

A hybrid modal analysis for enclosed sound fields.

A hybrid modal expansion that combines the free field Greens function and a modal expansion will be presented in this paper based on a review and an extension of the existing modal analysis theories for the sound field in enclosures. The enclosed sound field will be separated into the direct field and reverberant field, which have been treated together in the traditional modal analysis. Studies on a point source in rectangular enclosures show that the hybrid modal expansion converges notably faster than the traditional modal expansions, especially in the region near the source, and introduces much smaller errors with a limited number of modes. The hybrid modal expansion can be easily applied to complex sound sources if the free field responses of the sources are known. Damped boundaries are also considered in this paper, and a set of modified modal functions is introduced, which is shown to be suitable for many damped boundary conditions.

Eigenvalue equalization filtered-x algorithm for the multichannel active noise control of stationary and nonstationary signals

The FXLMS algorithm, which is extensively used in active noise control, exhibits frequency dependent convergence behavior. This leads to degraded performance for time-varying and multiple frequency signals. A new algorithm called the eigenvalue equalization filtered-x least mean squares (EE-FXLMS) has been developed to overcome this limitation without increasing the computational burden of the controller. The algorithm is easily implemented for either single or multichannel control. The magnitude coefficients of the secondary path transfer function estimate are altered while preserving the phase. For a reference signal that has the same magnitude at all frequencies, the secondary path estimate is given a flat response over frequency. For a reference signal that contains tonal components of unequal magnitudes, the magnitude coefficients of the secondary path are adjusted to be the inverse magnitude of the reference tones. Both modifications reduce the variation in the eigenvalues of the filtered-x autocorrelation matrix and lead to increased performance. Experimental results show that the EE-FXLMS algorithm provides 3.5-4.4 dB additional attenuation at the error sensor compared to normal FXLMS control. The EE-FXLMS algorithms convergence rate at individual frequencies is faster and more uniform than the normal FXLMS algorithm with several second improvement being seen in some cases.