Active Noise Control Filter Adaptation for Waterbed Effect
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Solution Overview
Problem
Active noise control systems in vehicles tend to generate unwanted sound when noise levels change from high to low, due to adaptation of filter coefficients that fail to match the new noise levels, leading to the 'waterbed effect' where midrange and higher frequencies produce disturbing sounds.
Innovation Solution
The system employs an acceleration sensor to generate a reference signal for unwanted noise, a noise control filter to produce an anti-noise signal, and a filter controller to adjust the noise control transfer function based on error signals from a microphone, with additional features like a shadow noise control filter and leakage controller to adapt filter coefficients quickly and reduce unwanted sound generation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If adaptive filter coefficients are adjusted to match changing noise levels, then noise cancellation performance is improved, but unwanted sound is generated in the midrange and higher frequencies
Solution Approach 1:
The patent implements dynamic adjustment of filter coefficients based on real-time noise level detection. The system continuously adapts the adaptive filter to match changing noise characteristics, while simultaneously dynamically controlling the gain of the secondary path to prevent waterbed effect. This dynamic adaptation allows the system to maintain effective noise cancellation across varying noise conditions without generating unwanted midrange and higher frequency sounds.
Solution Approach 2:
The system employs feedback mechanisms where the microphone detects residual noise and feeds this information back to the filter controller. The filter controller uses this feedback to continuously adjust the adaptive filter coefficients and secondary path gain, ensuring that the anti-noise signal accurately matches the primary noise signal. This closed-loop feedback control prevents the generation of unwanted sound while maintaining effective noise cancellation.
2Speed
If filter coefficients are adapted quickly to changing noise levels, then response time is improved, but stability of the noise control system deteriorates
Solution Approach 1:
The patent implements dynamic control of the adaptation process by adjusting the step size parameter of the adaptive filter based on current noise conditions. During rapid noise level changes, the system increases adaptation speed to quickly track the new noise characteristics. When the system approaches convergence or operates in stable conditions, the adaptation rate is reduced to maintain system stability and prevent oscillations in the midrange and higher frequencies.
Solution Approach 2:
The system dynamically changes key parameters of the adaptive filter, including the step size and gain factors, based on the detected noise conditions. By adjusting these parameters in real-time, the system achieves fast adaptation when needed while maintaining stability during normal operation. The secondary path gain is also dynamically adjusted to prevent the waterbed effect that would otherwise cause instability.
3Reliability
If the gain of the secondary path is increased to improve anti-noise output, then noise cancellation effectiveness is improved, but the waterbed effect is intensified
Solution Approach 1:
The patent implements dynamic gain control of the secondary path based on real-time monitoring of the adaptive filter output and noise levels. The system increases the secondary path gain when high anti-noise output is needed for effective cancellation, but simultaneously adjusts the adaptation rate and applies normalization to prevent the waterbed effect. This dynamic balancing allows the system to maintain high noise cancellation effectiveness without intensifying the waterbed effect in the midrange and higher frequencies.
Solution Approach 2:
The system dynamically adjusts the gain parameter of the secondary path in conjunction with other parameters such as the step size and normalization factors. By coordinating changes in these parameters, the system achieves high gain for effective noise cancellation while preventing the waterbed effect through proper scaling and adaptation control. The gain is adjusted based on the power of the reference signal and the current state of the adaptive filter.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively reduces unwanted sound by quickly adapting filter coefficients to changing noise levels, minimizing the 'waterbed effect' and ensuring that anti-noise matches the original noise, thereby improving noise cancellation performance.
Implementation Method 1
an acceleration sensor configured to evaluate an amplitude of an acceleration acting thereon and to generate a reference signal representative of the amplitude of the acceleration, the acceleration being representative of unwanted noise sound generated by a noise source
Implementation Method 2
a loudspeaker operatively coupled with the noise control filter and configured to convert the anti-noise signal into anti-noise sound
Implementation Method 3
a microphone configured to receive the noise sound after being transferred via a primary path according to a primary path transfer function from the noise source to the microphone, and the anti-noise sound after being transferred via a secondary path according to a secondary path transfer function from the loudspeaker to the microphone, and further configured to convert a sum of the received noise sound and the received anti-noise sound into an error signal
Implementation Method 4
a noise control filter operatively coupled with the acceleration sensor and configured to filter the reference signal with a noise control transfer function to generate an anti-noise signal
Data Source
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AI summary
Automatic noise control includes evaluating an amplitude of an acceleration acting on an acceleration sensor and generating a reference signal representative of the amplitude of the acceleration, the acceleration being representative of unwanted noise sound generated by a noise source, filtering the reference signal with a noise control transfer function to generate an anti-noise signal, and converting with a loudspeaker the anti-noise signal into anti-noise sound. Automatic noise control further includes receiving with a microphone the noise sound after being transferred via a primary path according to a primary path transfer function from the noise source to the microphone and the anti-noise sound after being transferred via a secondary path according to a secondary path transfer function from the loudspeaker to the microphone, converting with the microphone a sum of the received noise sound and the received anti-noise sound into an error signal, and controlling the noise control transfer function based on the error signal from the microphone and the filtered or unfiltered reference signal from the acceleration sensor so that the anti-noise sound after being transferred via the secondary path is the inverse of the noise sound after being transferred via a primary path. Automatic noise control further includes controlling a shadow noise control transfer function based on a shadow error signal and the filtered or unfiltered reference signal, generating the shadow error signal based on the filtered or unfiltered shadow anti-noise signal and the error signal, and substituting the noise control transfer function by the shadow noise control transfer function if the shadow error signal is smaller than the error signal.