Feedback ANC System with Long Secondary Path
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Solution Overview
Problem
Feedback ANC systems are not suitable for large rooms like vehicle cabins due to limitations in noise cancellation performance and adaptability, especially with long secondary paths and varying conditions such as different passenger numbers and environmental changes.
Innovation Solution
A feedback ANC system design with a microphone and loudspeaker arranged at distances between the speed of sound divided by 20 times the upper critical frequency and speed of sound divided by two times the upper critical frequency, incorporating a first and second subtractor with useful-signal paths and an ANC filter connected downstream and upstream, respectively, along with a second spectrum shaping filter equal to the inverse secondary path transfer characteristic.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If the distance between microphone and loudspeaker is increased to apply feedback ANC systems in large rooms like vehicle cabins, then the applicable space range is improved, but the noise cancellation performance deteriorates due to long secondary path delays
Solution Approach 1:
The system segments the noise control task into two independent paths: a feedforward path that processes reference signals from noise sources, and a feedback path that processes error signals from the microphone. This segmentation allows each path to be optimized independently, with the feedforward path handling the long secondary path delays and the feedback path providing stability through the subtractor configuration.
Solution Approach 2:
The first subtractor acts as an intermediary that separates the useful signal from the noise signal in the feedback path. By subtracting the useful signal component from the microphone output before feeding it to the ANC filter, the system prevents useful signal cancellation while maintaining noise cancellation effectiveness, thus resolving the contradiction between extended range and performance.
2Area of stationary object
If the distance between microphone and loudspeaker is increased beyond the conventional limit, then the system can be applied in larger spaces, but the system stability deteriorates due to excessive secondary path delay
Solution Approach 1:
The system employs adaptive filters in both the feedforward and feedback paths that can dynamically adjust their transfer functions based on the actual secondary path characteristics. This adaptability allows the system to maintain stability across varying distances and environmental conditions, with the filters optimizing their parameters in real-time to compensate for the long secondary path delay.
Solution Approach 2:
The system changes the parameter configuration by introducing the spectrum shaping filter with transfer characteristic equal to the inverse secondary path transfer characteristic. This parameter transformation effectively compensates for the phase and magnitude distortions introduced by the long secondary path, thereby maintaining system stability while enabling operation in large spaces.
3Speed
If feedback ANC systems are used with long secondary paths, then the frequency range is extended, but the ability to distinguish between wanted and unwanted signals deteriorates
Solution Approach 1:
The first subtractor extracts the useful signal component from the total microphone output signal by processing it through the ANC filter. This extracted useful signal is then subtracted from the original microphone output, leaving primarily the noise component for cancellation. This extraction process maintains signal differentiation accuracy even in extended frequency ranges.
Solution Approach 2:
The system performs preliminary processing of the useful signal through the spectrum shaping filter and ANC filter before the cancellation operation. By pre-processing the useful signal to account for the secondary path characteristics, the system ensures that when the cancellation occurs, the useful signal integrity is preserved while noise is effectively removed, maintaining differentiation accuracy across the extended frequency range.
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
This configuration allows for effective noise cancellation in larger spaces, distinguishing between wanted and unwanted signals, and maintaining stability across varying conditions, achieving up to 10 times higher frequency range attenuation than previously expected, while being capable of differentiating between acoustic warning signals and noise.
Implementation Method 1
The microphone is acoustically coupled to a loudspeaker via a secondary path
Implementation Method 2
The ANC filter filters the signal from the microphone such that the signal that it provides to the loudspeaker and that is radiated by the loudspeaker to the microphone via the secondary path cancels the noise signal
Implementation Method 3
The second useful-signal path comprises a second spectrum shaping filter that has a transfer characteristic that is equal to the inverse secondary path transfer characteristic
Data Source
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AI summary
A feedback ANC system is disclosed that comprises a microphone (1) and a loudspeaker (2) arranged in a distance from each other; the microphone being acoustically coupled to the loudspeaker via a secondary path (3) and the loudspeaker being electrically coupled to the microphone via an ANC filter (6). The distance between the microphone and the loudspeaker is larger than a value that is determined by the speed of sound divided by 20 times an upper critical frequency of the ANC system.