Thermal-Acoustic Stack Impedance Gradient for Aircraft Noise Reduction
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Solution Overview
Problem
Aircraft with lower mass and higher stiffness composite structures experience a significant increase in noise transmission to the cabin, exceeding noise levels in conventional metallic fuselages, despite using equivalent thermal-acoustic packages.
Innovation Solution
A thermal-acoustic section comprising a stack of juxtaposed porous layers with varying characteristic acoustic impedance and compression levels, positioned along acoustic paths to reflect and absorb noise, featuring a high impedance outermost portion, a central portion with lower impedance for sound trapping, and a progressively increasing impedance inner portion for enhanced noise reduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If lower mass and higher stiffness composite structures are used for the fuselage, then fuel efficiency is improved and emissions are reduced, but noise transmission to the aircraft cabin increases significantly
Solution Approach 1:
The thermal-acoustic package is divided into multiple porous layers with different characteristic acoustic impedances. Each layer segment performs a specific function: the first layer reflects high-frequency noise, the second layer absorbs mid-frequency noise, and the third layer handles low-frequency noise, collectively reducing overall noise transmission through the composite fuselage.
Solution Approach 2:
The invention uses a composite thermal-acoustic package consisting of multiple porous layers with different material properties and acoustic impedances. This composite structure combines the benefits of noise reduction across different frequency ranges while maintaining thermal insulation performance in lightweight composite fuselages.
2Ease of manufacture
If conventional thermal-acoustic packages are used in composite fuselages, then manufacturing simplicity is maintained, but noise transmission increases by about 10 dB or greater compared to metallic fuselages
Solution Approach 1:
The thermal-acoustic package is segmented into multiple porous layers with different characteristic acoustic impedances arranged in a specific sequence. This segmentation allows each layer to target specific frequency ranges, achieving superior noise reduction (exceeding 10 dB improvement) compared to conventional single-layer packages while maintaining compatibility with composite fuselage manufacturing processes.
3Object-affected harmful factors
If multiple porous layers with varying acoustic impedance are arranged in a thermal-acoustic stack, then noise absorption and transmission loss are improved, but device complexity increases
Solution Approach 1:
Each porous layer in the thermal-acoustic stack is assigned a specific characteristic acoustic impedance tailored to its position and function. The first layer has higher impedance for reflecting high-frequency noise, while subsequent layers have progressively lower impedances for absorbing mid and low-frequency noise. This local optimization of material properties achieves comprehensive noise reduction across the frequency spectrum.
Solution Approach 2:
The multiple porous layers act as intermediary elements between the external noise source and the aircraft cabin. Each layer serves as a mediator that progressively attenuates different frequency components of the noise, with the combined effect of all layers providing superior noise reduction compared to single-layer configurations.
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 solution effectively reduces noise transmission by reflecting a substantial portion of noise away and trapping sound energy within the thermal-acoustic stack, achieving improved noise absorption and transmission loss, even in lightweight composite fuselages.
Implementation Method 1
The first outermost portion has a first relatively high characteristic acoustic impedance that is effective to reflect a substantial portion of the noise away from the thermal-acoustic stack
Implementation Method 2
promote absorption of the noise
Implementation Method 3
thermal-acoustic sections that include multiple porous layers arranged together for reducing noise
Implementation Method 4
The thermal-acoustic stack has a gradually decreasing characteristic acoustic impedance from the first outermost portion to the central portion and a gradually increasing characteristic acoustic impedance from the central portion to the second outermost portion
Data Source
Figure 1~2
Figure 3~6
AI summary
Thermal-acoustic section (436) for an aircraft (10) for reducing noise along an acoustic path (22) produced from an acoustic source (16) is provided herein. The thermal-acoustic section (436) comprises a plurality of juxtaposed porous layers (400) forming a thermal-acoustic stack (447) that has a first outermost portion (474), a second outermost portion (476) and a central portion (478). The thermal-acoustic stack has a gradually deecreasing characteristic acoustic impedance from the first outermost portion to the central portion and a gradually increasing characteristic acoustic impedance from the central portion to the second outermost portion, to promote transmission loss of the transmitted portion of the noise through the thermal-acoustic stack.