Sintered fiber metal composites are used in aircraft as an acoustic media within environmental ducting, inlet and exhaust systems. The material can be engineered to meet specific acoustic attenuation and noise reduction goals within these applications. Sintered metal fiber composites have proven reliable and effective since the 1950's, but continue to deliver new value through ongoing investment and new use cases.
There are four design options for mitigating noise in the turbine gas flow and in the environmental control system (ECS) of aircraft. In some applications, the use of sintered metal fiber composites may be the most cost- or space-effective approach, deliver unique ancillary benefits or perform better than other technologies that can survive the requisite temperatures.
The engineered metal fabric is available in sheets and can be rolled and formed into parts to meet specific application needs, and is typically welded in place.
The material is used to address internal cabin noise, which is a concern for passenger comfort, and external noise generated by turbines and exhaust, which is a form of regulated noise pollution. External noise may be mitigated through a variety of methods including fiber metal silencers within the gas path, which successfully reduce noise to within allowable levels (Figure 1) while consuming less space within the aircraft than other attenuation methods. Internal noise can be mitigated using sintered fiber composites by mounting silencers inside the ductwork of the ECS.
While the materials have been used for more than 30 years, logging billions of hours of successful in-flight operation, they have in the ensuing years been developed and improved to meet the needs of additional applications, and new benefits of usage are coming to light. Increasing regulatory pressure to reduce noise impacts on flight crews, baggage handlers and other employees, specifically in Europe, mean external noise will be of increasing concern in coming years. Internal cabin noise is a high priority for original equipment manufacturers intent on helping their airline customers improve the passenger experience.
Methods of Acoustic Attenuation
Within the exhaust section of a turbine, there are four primary ways to achieve acoustic attenuation:
- Helmholtz Resonators: A tank-like device connected to a duct by a group of sound ports designed to match the noise duct frequency to be canceled. These are lower-cost, passive devices that do not consume power or create a high-pressure drop.
- Expansion Chambers: A sudden duct enlargement, followed by a contraction back to inlet size that spreads sound across the larger chamber area, then sampling a portion of the sound through a smaller outlet duct opening. Disadvantages include relatively large size, weight and pressure drop. Pressure drop may be a significant problem in the case of high duct velocities as the gas path exits the constrained area of the duct into the less constrained area of the tank, resulting in insertion loss and, potentially, resulting noise at both the inlet and the outlet.
- Active Cancellation Systems: This is an active electronic system including a microphone, amplifier and speakers that sense and analyze noise and create a cancellation effect. Advantages include compact size and performance across a wide frequency spectrum. This technology however has not proven reliable in extreme environments and also requires a power source.
Fiber Metal Silencers: This technology has a long and successful track record on many commercial aircraft in engine fan ducts, jet engine inlet cowls, environment control systems (ECS) and auxiliary power units (APUs). A fiber metal composite is an integral part of this system because it is the only way to attenuate noise in a high-temperature application that performs better than a perforated plate silencer.
While low-bypass turbine design may be a major step towards lower Effective Perceived Noise level in decibels (EPNDB), acoustic liners may reduce EPNDB for high-bypass and low-bypass designs alike. These products perform at up to 932°F when made of austenitic stainless steel and up to 2,000°F when made of FeCrAlY, and may be used in the air intake, exhaust, bypass duct and core nozzle sections of a turbine. The acoustic media can be engineered to specific levels of thickness, strength and acoustic impedance measured in terms of Rayl value. Specifying an acoustic liner requires balancing noise reduction with total area within the turbine occupied by the liner to maximize acoustic attenuation without impeding gas flow.