Loudspeaker Feedback Circuit for Diaphragm Distortion Compensation
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Solution Overview
Problem
Loudspeakers, especially small ones in portable devices, suffer from distortion due to non-proportional diaphragm movement caused by varying magnetic field interactions and springiness, leading to loss of acoustical fidelity.
Innovation Solution
An audio system measures a test current through the loudspeaker to determine its capacitance, using this feedback to adjust the target audio signal and reduce distortion by generating a feedback signal that represents actual diaphragm displacement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a loudspeaker is used to convert electrical signals to sound, then audio output is achieved, but distortion occurs due to non-proportional diaphragm movement
Solution Approach 1:
The patent applies feedback by measuring the actual diaphragm displacement using a capacitance sensor and comparing it to the target displacement from the electrical signal. The difference (distortion) is fed back to an audio driver that adjusts the electrical signal to compensate for the distortion, thereby improving acoustical fidelity. This closed-loop feedback system directly addresses the non-proportional diaphragm movement issue.
2Speed
If the coil and magnet interact to produce diaphragm movement, then sound is generated, but frequency response becomes non-flat due to reactive coil characteristics
Solution Approach 1:
The feedback mechanism measures actual diaphragm displacement across different frequencies and uses this information to adjust the electrical signal. This compensation approach counteracts the non-flat frequency response caused by the reactive coil characteristics, maintaining accurate sound reproduction across the audio spectrum while preserving fast diaphragm response.
3Length of moving object
If the coil position changes inside the magnet, then diaphragm movement is achieved, but distortion increases due to varying magnetic field interaction
Solution Approach 1:
The capacitance sensor provides real-time feedback on the actual coil and diaphragm position. This position information is used to dynamically adjust the electrical signal to compensate for the varying magnetic field interaction, ensuring that the diaphragm movement remains proportional to the target signal even as the coil moves through different positions in the magnetic field.
4Force
If the suspension springiness varies with diaphragm displacement, then mechanical compliance is achieved, but distortion increases due to non-constant spring characteristics
Solution Approach 1:
The feedback system measures the actual diaphragm displacement and uses this information to compensate for the non-constant suspension compliance. By adjusting the electrical signal based on the measured displacement, the system counteracts the distortion caused by varying springiness, maintaining accurate sound reproduction across different displacement ranges.
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 increases audio fidelity by ensuring the loudspeaker's displacement more accurately matches the target audio signal, reducing distortion and improving frequency response.
Implementation Method 1
When an electrical signal is applied to the voice coil, the coil generates a magnetic field that causes the voice coil and its attached diaphragm to move
Implementation Method 2
The test signal is used as feedback to generate a feedback signal that represents an actual displacement of the loudspeaker diaphragm
Data Source
AI summary
An audio system comprises an audio driver configured to receive a target audio signal and a feedback signal and to generate an adjusted audio signal responsive to the target audio signal and the feedback signal. A loudspeaker is configured to convert the adjusted audio signal into acoustical sound. A test signal generator is configured to generate a test signal having a higher frequency than the target audio signal. The test signal causes a test current to flow through the loudspeaker. A current sensing circuit is configured to measure the test current flowing through the loudspeaker and to generate a current sense signal indicative of the test current. A feedback circuit is configured generates the feedback signal responsive to the current sense signal.