Sintered Fiber Metal Composites for Aircraft Acoustic Attenuation

Sintered Fiber Metal Composites for Aircraft Acoustic Attenuation

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 passenger experience.

Figure 1:  Silencer performance on an alternate power unit (APU) application allowed this aircraft to comply with noise certification guidelines. Supplied by Technetics Group.

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, and 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 932o F when made of austenitic stainless steel and up to 2,000oF 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.

Acoustical Design Factors

Any time a sound wave is reflected from a hard surface of a silencer, a porous material placed across the resulting wave pattern can convert the sound to heat energy. Per Figure 2, a number of factors go into designing an appropriate acoustic media for an application, including but not limited to sonic qualities like nominal and maximum acoustic impedance and physical characteristics including dimensional and tensile strength requirements. Specifying the material itself is a crucial step because if the acoustic media is too hard, it will act like a solid surface and reflect the sound waves. If the media does not provide sufficient resistance, the sound wave will simply pass through the material unabated. Because there are so many variables involved, it often requires several design iterations to take all of these factors into account and arrive at an ideal acoustic media.

Figure 2: This list of criteria will drive an informed specification for acoustic media. Supplied by Technetics Group.

The acoustical impedance of the fiber metal itself should also be matched to the impedance of the air in the duct, which in turn varies with temperature and pressure (Figure 3). 

Figure 3: In tuning the duct liner, the fiber metal is particularly effective for those frequencies that have standing wavelengths that match the duct cross-sectional dimensions. The simplest design results when the two facing liners match a full wave cross mode. Supplied by Technetics Group.

Case In Point: 787 Dreamliner

Fiber metal silencers in particular are becoming more important in ductwork serving the aircraft cabin. On the Boeing 787 Dreamliner for instance, sintered fiber composites are used strategically to attenuate noise from turbulent flow in the ECS ducting, improving the passenger experience. While sintered fiber composites are the only acoustic media that can meet the heat resistance requirements within some turbine exhaust and auxiliary power unit applications, it is favored in the ECS system because it can be engineered to attenuate a broader range of frequencies and performs more consistently over time than an alternative technology like metal mesh.

Accelerated life cycle testing performed by Technetics’ customers show that FELTMETAL™ isn’t plagued by phase shifts over time and temperature nor shifts in Rayl value due to airborne contaminants the way that competing sintered materials do. This means it will deliver the same acoustic attenuation properties over the duration of the maintenance cycle, while the Delta P/pressure drop and attenuation properties of competing sintered metal products experience reduced effectiveness and need to be replaced well in advance of their scheduled maintenance.

In Boeing’s new no-bleed design, engine efficiency is increased by reducing the amount of pressurized engine air used to power auxiliary systems. Instead of redirecting pressurized air from the engine, for instance, interior cabin pressure is driven by compressors introducing fresh air that originates from twin exterior air ducts on the belly of the aircraft. This preserves power, eliminates waste as heat energy must be removed from the air prior to its introduction into the cabin. This no-bleed design also reduces weight because ducting used to pass the pressurized air through the aircraft employs check valves, pre­coolers, and ducting of heavy materials like titanium. Among other places in the Dreamliner ECS, Feltmetal™ is used to mitigate noise in the air handling system servicing these exterior ducts.

Physical Properties

Sintered metal fiber composites are engineered materials with a density range between 10 percent and 50 percent and ultimate tensile strength between 500 and 3000 pounds per square inch (psi) or 3.5 – 20.7 megapascals (MPa). Products like FELTMETAL™ acoustic media can be engineered further for each program and application. Sintered metal composites are created by first shearing metal fibers into short strands of specific lengths. Materials include austenitic stainless steels (Types 316L and 347) for service temperatures up to 932o F. For higher temperatures up to 2012o F, engineers specify FeCrAIY—a super alloy of chromium, aluminum, silicon, manganese, yttrium, zirconium, carbon and iron. The fibers are distributed evenly across a substrate and then felted using various methods including a roller felter. The felt is sintered, or baked, in a vacuum furnace to form diffusion bonds between the fibers.

Sintered fiber metal composites may be a mature technology, but they may be the best tools for solving certain acoustic attenuation problems. The material can be engineered to a specific application, and offer advantages not found with competing technologies.

 

 

This article originally appeared in the October 2017 issue of Aerospace & Defense Technology Tech Briefs. You can view the article here.