Many real-world applications of active noise control are characterized by transfer functions that vary significantly and unpredictably. The controller's transfer-function models must adapt to these variations. Presented here is a class of adaptive filters that accomplish quasiperiodic system identification updates for feedforward control by using blocks of input-output histories. The algorithms form a one-dimensional family linking normalized least-mean squares (LMS) adaptive filters and block recursive least-squares, termed "block projection" algorithms, and generalize the noninvasive system identification studied by Sommerfeldt and Tichy. The system identification proceeds noninvasively, producing nonparametric impulse responses. Simulations show that the algorithm's convergence is faster than that of normalized LMS, even after the additional overhead of computing the update is taken into account. Both the multichannel generalization and application of these algorithms to system identification are novel. Simulations of the algorithms' performance using measured data are presented here, while experimental results of an implemented algorithm are contained in the companion paper.

This paper presents experimental validation of a class of algorithms designed to enable active noise control (ANC) to function in environments when transfer functions change significantly over time. The experimental results presented are for broadband, local quieting in a diffuse field using a multichannel ANC system. The reverberant enclosure is an ordinary room, measuring approximately 1.4 x 2.4 x 2.4 m3 and containing a seated occupant, with six microphones defining the quiet zone near the occupant's ears. The control system uses a single reference signal and two error channels to drive four secondary sources. Using an ideal reference sensor, reduction in sound pressure level is obtained at the quiet-zone microphones averaged over the frequency range 50 to 1000 Hz with an occupant seated in the room. Two main results are presented: first for an adaptive cancelling algorithm that uses static system models, and second for the same algorithm joined with a noninvasive real-time system identification algorithm. In the first case better than 23 dB of performance is obtained if the occupant remains still through calibration and testing. In the second case, approximately 18 dB is obtained at the error microphones regardless of the motion of the occupant.

The fmal demonstrations of the ARPA SPICES (Synthesis and Processing of Intelligent Cost Effective Structures) program test the control of two active vibration mounts manufactured from composites with embedded actuators and sensors. Both mount demonstrations address wide band control problems for real disturbances, one at low frequency and the other at high frequency. The control systems for both are two-level hierarchies, with an inner active damping augmentation loop and an outer vibration control loop.We first review the control design requirements for the demonstrations and summarize our control design approach. Then we focus on presenting the experimental results of the final demonstrations. For the low frequency demonstration, two alternative control approaches were demonstrated, one involving finite impulse response modeling and the other state space modeling. For the high frequency demonstration only the finite impulse response modeling approach was used because of computational limitations due to the complex system dynamics.The goal of the ARPA Synthesis and Processing of Intelligent Cost Effective Structures (SPICES) program3 is to develop and demonstrate new techniques for constructing and applying smart materials. The separate advances in actuators, sensors, composite fabrication, modeling, and control systems have been brought together into two final demonstration active vibration mounts constructed of smart materials.In the demonstrations, the requirement was to attenuate vibration along the vertical axis by 10 dB (with a goal of 30 dB) by use of SPICES vibration isolation mounts, with no out-of-band enhancement. The commercial compressor application ("the plate") focused on the band 1 -4 kHz for attenuation, and the military motor application ("the combined mount") focused on the band 5 -100 Hz. This requirement specifies attenuation with respect to the vibration of the compressor or motor attached to a rigid mount (insertion loss). Nonetheless, our experiments have focused on obtaining attenuation with respect to vibration transmitted through the uncontrolled SPICES mounts (control performance). This was largely a matter of convenience, and post-experiment analysis indicates that insertion loss is superior to the control performance achieved.The plate demonstration requirement specified that the attenuation was to be measured using a running compressor as the disturbance source. The combined mount demonstration requirement specified both that performance would be evaluated with the motor running at two speeds, 64 and 71 Hz, and that the required attenuation was to be obtained over the 5 -100 Hz band.These design objectives of the control systems were modified during the demonstrations due to difficulties with or limitations of the equipment. In sum, the design bandwidth was reduced for the plate demonstration, and for the motor demonstration two control systems were designed: one that could attenuate the vibration over 5 to 100 Hz due to the motor running at any frequency from 63 to 71 Hz, and ano...

Many real-world applications of active noise control are characterized by transfer functions that vary significantly and unpredictably. The controller’s transfer function models must adapt to these variations. Presented here is a class of adaptive filters that accomplish quasiperiodic system identification updates for feedforward control by using blocks of input–output histories. The algorithms form a one-dimensional family linking normalized LMS adaptive filters and optimal Wiener filters, and are termed ‘‘block projection’’ algorithms. The system identification proceeds noninvasively, producing nonparametric (FIR) impulse responses. The multichannel generalization and application of these algorithms to system identification, as presented here, is novel. Considerations are described that arise from the algorithms’ implementation in the context of system identification; in particular, the proper weighting of input and output data pairs is discussed. The resulting multichannel control algorithms have been implemented successfully for quieting of a compact distributed source in an anechoic environment, and for local quieting of a diffuse field in a reverberant room. In both cases, error microphones could be moved about, providing a ‘‘mobile quiet zone,’’ and performance was obtained for bandwidths exceeding a decade.

Active noise control (ANC) is often discussed in the context of reducing noise in vehicles. The acoustic reverberation of typical vehicle cabins makes the application of ANC to quieting interior noise a stiff challenge. While some ANC approaches take advantage of the modal structure of reverberant enclosures to provide reduction throughout the enclosure, these approaches become infeasible for many frequency bands of practical interest because of increasing modal density with increasing frequency. For high-bandwidth, diffuse fields, noise reduction is achieved locally by specifying a quiet zone within the enclosure. Experimental results for broadband, local quieting in a diffuse field using a multichannel ANC system are presented. The reverberant enclosure is an ordinary room, measuring approximately 1.4 m×2.4 m×2.4 m and containing a seated operator with six microphones defining the quiet zone near the operator’s ears. The control system uses a single reference signal and two error channels to drive four control speakers. An average of 20-dB reduction in sound pressure level is found at the quiet-zone microphones for the frequency range 50 to 1000 Hz. These results demonstrate the viability of real-time, multichannel ANC for locally attenuating random noise in diffuse acoustic fields.