Wearable Audio Feedforward Instability Detection

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Solution Overview

Problem

Wearable audio devices such as earbuds and hearing aids can develop parasitic oscillations in a feedforward loop, leading to undesirable instability and squealing due to the feedback of sound pressure from the transducer to the microphone.

Innovation Solution

A system that includes a feedforward instability detector applying two filters to the microphone signal, a peak filter and a notch filter, to detect and compare energy levels within specific frequency bands, indicating instability, and an instability mitigator that adjusts the gain applied to the microphone signal to mitigate parasitic oscillations.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If the feedforward loop is used to detect environmental sounds and enable active noise reduction or transparency mode, then the functionality and user experience are improved, but parasitic oscillations and feedforward instability occur causing squealing

Engineering Contradiction:

Improvefunctional versatilityVSAvoidsystem stability

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The system applies detection filters (peak and notch filters) to the microphone signal in advance to identify parasitic oscillations before they cause audible squealing. The instability detector continuously monitors the filtered signal energy and compares it against thresholds, enabling early detection and prevention of instability issues.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system implements a feedback mechanism where the detected signal energy from the filters is continuously monitored and compared against stability thresholds. When instability is detected through the feedback loop, the system automatically adjusts the gain of the feedforward microphone signal to eliminate parasitic oscillations, ensuring stable operation while maintaining functionality.

Inventive Principle:

Principle #23Feedback

2Measurement precision

If the gain of the microphone signal is increased to improve sound detection sensitivity, then the sensitivity is improved, but parasitic oscillations and instability are exacerbated

Engineering Contradiction:

Improvesound detection sensitivityVSAvoidsystem stability

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The system dynamically adjusts the gain of the feedforward microphone signal based on real-time detection of parasitic oscillations. The gain is not fixed but varies continuously according to the stability conditions detected by the filter-based monitoring system, allowing optimal sensitivity while preventing instability.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system changes the gain parameter of the microphone signal based on the detected energy levels in the frequency bands. When the filtered signal energy exceeds thresholds indicating parasitic oscillations, the gain is reduced to eliminate instability; when stability is maintained, the gain can be increased for improved sensitivity.

Inventive Principle:

Principle #35Parameter changes

3Measurement precision

If frequency-specific filtering is applied to detect parasitic oscillations, then the detection accuracy is improved, but the device complexity increases

Engineering Contradiction:

Improveinstability detection accuracyVSAvoidsignal processing complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The system segments the frequency spectrum into specific bands using detection filters (peak filters and notch filters) centered at frequencies where parasitic oscillations are most likely to occur. This segmentation allows targeted monitoring of critical frequency ranges without processing the entire spectrum, balancing detection accuracy with computational efficiency.

Inventive Principle:

Principle #1Segmentation

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

Effectively detects and mitigates feedforward instability, preventing squealing by reducing gain in affected frequency bands and ensuring stable operation of wearable audio devices.

Implementation Method 1

apply two filters to the microphone signal, wherein a first filter passes more energy in a frequency band than does a second filter

Methodology Applied

Scientific EffectFrequency filtering: Filter (electronic)

Data Source

PatentUS11350204B2Wearable audio device feedforward instability detection

Publication Date: 2022.05.31 BOSE CORP

AI summary

A system for detecting feedforward instability in a wearable audio device. The audio device includes an electro-acoustic transducer that is configured to develop sound for a user, a housing that holds the transducer, a feedforward microphone that is configured to detect sound outside of the housing and output a microphone signal, and an opening in the housing that emits sound pressure from the transducer that can reach the microphone. A feedforward instability detector is configured to apply two filters to the microphone signal. A first filter passes more energy in a frequency band than does a second filter, to develop a filtered signal. The filtered signal is compared to the microphone signal outside of the frequency band, to develop a comparison signal that is indicative of feedforward instability in the frequency band.