Noise Control Filter Research Articles

Method to update the coefficients of the secondary path filter under active noise control

In active noise control systems using a filtered-x algorithm, the secondary path must be identified before the coefficients of the noise control filter are updated. The secondary path, however, has an impulse response that is continually changing in practical systems. This change degrades noise reduction and the stability of the system. Therefore, the coefficients of the secondary path filter must be adjusted to actual impulse response samples at specific intervals. This paper proposes a method to update the coefficients under active noise control without feeding extra noise to the secondary source. Instead, this method uses estimation errors. The coefficients of the noise control filter generally have the different estimation errors whenever the coefficients are updated. To use the estimation error, this method implements the additional filter modeled on the overall path from a detection sensor to an error sensor, through the primary path, the control filter and the secondary path. Two different coefficient sets of the noise control filter derive two equations on this overall path. A solution of these concurrent equations naturally yields the impulse response samples of the secondary path. This paper uses a system identification technique to solve the concurrent equations. This technique is practical in that calculation errors are distributed equally over all coefficients of the secondary path filter. Finally, computer simulations explained in this paper confirm that the concurrent equation method can refresh the coefficients under active noise control.

Ultra-wideband filter for noise control

It is proposed that arrays of periodically positioned metallic bars could serve as a low-tech, low-cost, and effective device for filtering out environmental or industrial noise. The proposed scheme is composed of five sets of periodic arrays in tandem, each characterized by a certain value of the period (of square lattice) a and cylinders of radius r; the filling fraction is f=(r2/a2). The purpose of the tandem structure is to achieve an ultra-wideband filter of noise. The tandem structure is designed such that, for certain values of the period, the stop bands of the respective sets of structure superimpose over one another. It is found that, for f=0.55, the tandem structure should shield from noise in the entire range between 2.0 and 11.0 kHz. This is attributed to the formation of a ‘‘super stop band’’ due to overlapping of the five individual stop bands. An ultra-wideband filter for environmental or industrial noise could thus be fabricated according to the specific requirements. This idea is based on the band structure calculations reported in the preceding abstract.

Passband disturbance reduction in periodic active noise control systems

For narrowband active noise control (ANC), the filtered-X least-mean-square (LMS) algorithm with a sinusoidal reference signal forms an adaptive notch filter. Unfortunately, the effect of the secondary path on the uncorrelated noise is to introduce an undesired passband disturbance. To reduce this disturbance problem, this paper analyzes the passband amplification condition and develops an algorithm that consists of two interconnected adaptive filters using the same internally generated reference signal. The first adaptive filter eliminates the uncorrelated noise components in the residual error, and its output is used to update the second adaptive filter for noise control. Computer simulations were conducted to verify the analysis results and to demonstrate the disturbance reduction for periodic ANC while using a relatively large step size to improve the convergence rate.

A study on an active noise control filter considering the reflection waves

A study on an active noise control filter considering the reflection waves

A predictive filter for active noise control in small spaces

A predictive filter was designed for controlling helium flow noise at up to 115 dB in diving helmets. The transducer configuration consists of an earphone with a single detecting microphone located near the opening of the ear canal. A high‐speed digital signal processor implements the filter in such a way that communications from the surface can be passed into the ear without being cancelled. Frequencies up to 1000–1500 Hz are controlled by the filter and passive means are used beyond that point. The filter in question is not adaptive and was designed from the time domain perspective and so that its coefficients convey physical meaning. Adaptation can be applied to the filter coefficients with the detector microphone doubling as an error microphone.