Aircraft Interior Noise Reduction and Thermal Control

Business Industry and Outlook

Aircraft have always been loud. For years, a noisy cabin was considered to be the price paid for getting from place to place at top speed. However, certain historical trends, beginning notably around the late 1980s and early 1990s, began to focus more and more attention on the interior environment of transport aircraft.

The first such trend was the increasing levels of complexity and luxury demanded by and offered to more and more affluent, discerning aircraft owners and operators. As technologies became available and the luxury bar was raised, items such as bedrooms, showers, and high-technology cabin entertainment systems became standard offerings. Such "comforts of home" began inadvertently to emphasize the difference between the aircraft interior environment and that of a house or private yacht. Customers in turn began to demand quieter aircraft.

The second trend was the advent of exterior noise standards (the FAA "Stage" system), which is committed to gradually reducing the environmental noise impact that aircraft have on the communities surrounding airports. As a result, the aviation fleet became stratified by noise impact, with newer-technology aircraft significantly quieter than older models. Aircraft owners and operators became more aware of and sensitive to the differences between aircraft, specifically regarding the noise levels inside the cabin, which are significantly affected by engine noise.

The third factor was the increasing public awareness of the impact that long-term exposure to high noise levels has on hearing health and safety. Protective equipment such as headphones or ear plugs, while excellent and low-cost methods of preventing hearing damage, were not considered to be adequate for long-term use due to issues of comfort.

Emerging material technology and advances in applied acoustics were combined to create innovative materials that could be installed in the aircraft fuselage and in turn reduce cabin noise. Early products used materials known in other industries to be effective at noise control, such as lead and heavy vinyl. Many business aircraft were lined with metallic lead or lead-impregnated polymer slabs; this approach, while somewhat effective in reducing cabin noise, was extremely heavy. While initially somewhat inexpensive as well, the negative health and toxic effects of using lead and other heavy metals made them ultimately impractical. There was also a call for materials which were X-ray transparent, and in turn would not need to be removed in order to conduct necessary aircraft structural inspections.

The second generation of acoustic technology used recent advances in polymer and fiber technology to develop replacements for the lead-based systems. Organic foams, either by themselves or mounted to a less toxic metal such as aluminum, were bonded to aircraft skins and interior panels while new fiber felts and batting were pressed into the intercostal cavities. Unfortunately, the effectiveness of these solutions was limited by the temperature sensitivity of the components: organic foams become rigid and acoustically transparent at the extremely low temperatures normally observed in aircraft structures after cold-soaking at altitude. Coupled with new structural techniques such as multi-ply metallic skins and graphite fiber-reinforced plastic panels, aircraft structures became stiffer and more resonant than ever. Also, many esoteric fiber and foam-based materials with novel properties were trickling into the aviation market from areas such as fire protection and space vehicle development. These were marketed as having excellent acoustic properties; however, most of the target market had little understanding of acoustics and lacked the ability to evaluate the materials properly. The result was the installation of high-priced noise reduction elements of questionable and largely unverifiable performance.

It was at about this time that truly effective standards became available for the measurement of acoustic noise levels in aircraft, namely ISO 5129:2001. ISO 5129 was largely rewritten in order to provide an appropriate and comprehensive method to measure the acoustic environment of aircraft in flight. This new standard effectively superseded both ISO 5129:1987 and SAE ARP1323-A.

The third, and current, generation of acoustic noise reduction technology represents a quantum leap in performance and a significant reduction in weight. This approach was pioneered by Olen and Greg Nelson, and has been used with great success for almost a decade. Instead of focusing on the insulation and damping components historically used, Olen and Greg conducted extensive acoustic analyses on a wide variety of aircraft and determined that there are many more sources of noise in the aircraft than were previously known. Each is treated with a noise control solution specifically designed for the unique acoustic properties of the noise source, and all of these solutions are in turn packaged into a single installation kit specially configured for a specific aircraft make, model, and layout. This approach provides dramatically lower noise levels, and is able to do so with significantly less weight than previous approaches. As an added benefit, the cabin sound environment became much more even, with no tonal whistles or whines, and no loud spots. By coupling such an approach with a modular system of basic design standards, Olen and Greg were able to achieve a high level of customization while remaining cost-competitive with many commodity-level engineered kits, and even many raw materials-only approaches.