Off-Ear Headphone Acoustic Cavity Resonance Control
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Solution Overview
Problem
Intermodulation distortion (IMD) in acoustic cavities of off-ear headphones limits the maximum loudness due to varying motor force constants and standing waves, which cause undesired frequency components, particularly at higher sound pressure levels.
Innovation Solution
Incorporating a second sound-emitting outlet with an acoustically resistive element, such as a tightly woven mesh screen, in the acoustic cavity to reduce the amplitude of the fundamental resonance and shift the standing wave frequency, thereby minimizing IMD. The optimal acoustic impedance for this outlet is between one and five times that of air, with around 1000 pascal-seconds per meter often being effective.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If off-ear headphones are driven at higher amplitude to provide desired sound levels, then sound pressure level is improved, but intermodulation distortion increases
Solution Approach 1:
The acoustic cavity is segmented into multiple resonance modes by introducing a second outlet, which divides the standing wave patterns into distinct frequency components. This segmentation prevents the motor force constant from varying due to单一 resonance, thereby reducing intermodulation distortion while maintaining high sound pressure levels
Solution Approach 2:
The patent changes the acoustic parameters of the cavity by adding a second outlet with specific impedance characteristics (between one and five times that of air). This parameter change shifts the resonance frequencies and standing wave patterns, allowing high amplitude operation without the harmful motor force constant variations that cause IMD
2Object-generated harmful factors
If a second sound-emitting outlet is added to reduce resonance amplitude, then intermodulation distortion is reduced, but device complexity increases
Solution Approach 1:
The second outlet serves multiple functions simultaneously: it acts as a sound emission path, a resonance control element, and an impedance matching component. By making this single structural element multi-functional, the patent reduces IMD without proportionally increasing device complexity
Solution Approach 2:
The patent employs an acoustically resistive element (such as a tightly woven mesh screen) at the second outlet, which provides the necessary acoustic impedance (one to five times that of air). This porous/resistive material approach achieves the desired acoustic parameter control with a relatively simple structural addition
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 reduces IMD, allowing for higher sound pressure levels with lower distortion, directing most sound towards the ear while minimizing spillage and maintaining audio quality across a wide frequency range.
Implementation Method 1
If the second sound-emitting outlet is designed to incorporate an acoustically resistive element, such as a tightly woven mesh screen, the amplitude of the resonance can be significantly reduced
Implementation Method 2
An acoustic cavity with a single sound-emitting outlet has a fundamental resonance, wherein a standing wave within the cavity has a high amplitude at a location opposite the outlet
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
An audio device with an acoustic radiator that emits acoustic radiation from a first side, a housing that defines an acoustic cavity that receives the acoustic radiation emitted from the first side of the acoustic radiator, and first and second sound-emitting outlets in the housing and acoustically coupled to the acoustic cavity such that the outlets emit sound from the acoustic cavity. The second sound-emitting outlet has a greater equivalent acoustic impedance than the first sound-emitting outlet.