Speaker Q-Factor Tuning With Class-D Current Feedback
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Solution Overview
Problem
Conventional acoustic apparatuses face challenges in improving efficiency over the whole frequency range, including the lowest resonant frequency, due to the Q factor being below a predetermined lower limit, leading to insufficient sound pressure in the low frequency range.
Innovation Solution
Incorporating a class-D amplifier with a current feedback circuit to increase the Q factor of the speaker system by controlling the output impedance, using a speaker unit with high force factor and low voice coil resistance, and employing current feedback to maintain the Q factor within the ordinary range without increasing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the Q factor is decreased to improve efficiency at mid-high frequencies, then efficiency is improved, but sound pressure decreases around the lowest resonant frequency
Solution Approach 1:
The patent applies dynamics by making the output impedance of the amplifier adjustable and frequency-dependent. The amplifier's output impedance is dynamically changed based on the frequency range: at mid-high frequencies, the impedance is set to decrease the Q factor and improve efficiency, while at low frequencies around the resonant frequency, the impedance is adjusted to maintain adequate sound pressure. This dynamic adjustment resolves the contradiction between efficiency and sound pressure across different frequency ranges.
Solution Approach 2:
The patent employs parameter changes by modifying the output impedance parameter of the amplifier across different frequency ranges. Specifically, the output impedance is changed from a fixed value to a variable parameter that depends on frequency. This allows the system to optimize the Q factor at different frequencies: reducing Q factor at mid-high frequencies for improved efficiency while maintaining appropriate Q factor at low frequencies for adequate sound pressure.
2Object-affected harmful factors
If the Q factor is increased to maintain sound pressure at low frequencies, then sound pressure is maintained, but efficiency decreases over the whole frequency range
Solution Approach 1:
The patent applies local quality by implementing different Q factor characteristics for different frequency ranges. Instead of using a single uniform Q factor across all frequencies, the system provides localized optimization: at low frequencies around the resonant frequency, the Q factor is maintained higher to ensure adequate sound pressure, while at mid-high frequencies, the Q factor is reduced to improve efficiency. This localized differentiation resolves the contradiction between maintaining sound pressure and improving efficiency.
3Loss of energy
If a speaker unit with high force factor is used to improve efficiency, then efficiency is improved, but the Q factor falls below the predetermined lower limit
Solution Approach 1:
The patent introduces the amplifier's output impedance as an intermediary element that mediates between the speaker unit's inherent Q factor characteristics and the desired system performance. Even though the speaker unit has a low Q factor due to high force factor, the amplifier's adjustable output impedance acts as an intermediary to effectively increase the overall system Q factor at low frequencies, ensuring adequate sound pressure while maintaining the efficiency benefits of the high force factor speaker unit at mid-high frequencies.
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
Enhances efficiency over the entire frequency range, including the lowest resonant frequency, by maintaining sound pressure levels and reducing electric power consumption.
Implementation Method 1
The current feedback circuit is configured to increase the second output impedance of the class-D amplifier by feeding a current flowing to the voice coil back to an input of the class-D amplifier
Implementation Method 2
a speaker system including a voice coil driven by the class-D amplifier
Data Source
AI summary
An acoustic apparatus includes a class-D amplifier including a current feedback circuit, and a speaker system including a voice coil driven by the class-D amplifier. The speaker system is configured such that, in a case where the speaker system is driven by an ordinary amplifier having a first output resistance lower than a second output impedance of the class-D amplifier, a Q factor of the speaker system falls below a predetermined lower limit of an ordinary Q factor range of an ordinary speaker system. The current feedback circuit is configured to increase the second output impedance of the class-D amplifier by feeding a current flowing to the voice coil back to an input of the class-D amplifier so as to increase a Q factor as the acoustic apparatus higher than the predetermined lower limit of the ordinary Q factor range and within the ordinary Q factor range.