Keeping the noise down - materials for soundproofing aircraft
Flying in an aeroplane is a noisy experience. Adith Sujan and Mike Brock, of global specialty material products manufacturer Rogers Corporation, describe how to keep the sound level down.
For aeroplane passengers the noise level inside the cabin of a typical commercial jet is seldom considered. But for those who can afford to purchase and retrofit their own private aircraft, it is often a design requirement to have a quiet cabin. Commercial or business jets modified to meet certain specifications are called VIP aircraft.
There are two modes of noise found on aircraft, structure and airborne. The former is generated by interior component vibration and the latter is the sound of the engines or aerodynamic flow over the fuselage. Good acoustic design reduces both modes and requires specific materials.
A systems design approach develops materials with a balance between flammability, acoustic performance, durability and weight. The sound barrier blocks a sound wave through its mass at the surface area, measured in terms of areal density. This implies that a heavier barrier material will reflect sound better. However, in order to minimise fuel costs, weight restrictions are enforced for all engineered materials.
Due to the enclosed environment in an aircraft cabin, the flammability of the material used is a critical property. Substances that isolate vibration of interior components or block outside noise must meet strict Civil Aviation Authority requirements. Many materials meet the 12-second and 60-second vertical burn requirements (FAR25.853), but there are fewer choices for the radiant panel test (FAR25.856), which both ensure fire does not spread through an aircraft.
The typical noise level in the economy section of a commercial aircraft is approximately 75dBA Speech Interference Level (SIL). A variety of materials have been developed to address this. The systems design approach, using isolation and barrier materials, improves noise levels in excess of 65dBA SIL.
Acoustic insulation for cabin noise is located between the aircraft skin and interior side panels. A mass layer, such as a flame retardant silicone rubber, is also used in a composite insulation blanket, which blocks sound. It must meet regulations for aircraft insulation (FAR25.856). The composite often includes both a noise barrier and an absorbing substance such as fibreglass batting or foam.
Silicone materials and composites are often chosen for aircraft applications due to their high performance. Silicone-based polymers maintain physical properties over a wide temperature range while displaying low water absorption and little ageing, making them a reliable choice.
The formulation of a lightweight silicone polymer with cross-functional ingredients that possess the flammability and durability characteristics is required.
Silicone rubbers for high performance applications use customised fillers to provide the performance function. There needs to be a balance between fire performance and mechanical properties. Adding high levels of flame retardant fillers drives up viscosity, making the composite difficult to process. The volume of filler causes the finished product to become brittle, which may not be acceptable for the application.
Both the loading level and the type of fillers used are vital. Many filler properties affect the finished product, including particle size distribution, filler shape, specific gravity and filler treatment. The challenge is finding the right combination of fillers to meet processing and product requirements.
Rogers Corporation, headquartered in Connecticut, USA, recently launched a new product family for vibration and acoustic mitigation, the BISCO L3 Series. The range includes silicone foam materials for vibration isolation on high speed trains (floating floors) and an acoustic sound barrier engineered silicone rubber designed for use aboard aircraft. An acoustic noise barrier (mass layer) of filled silicone rubber for the aircraft industry is due to launch later this year.