Quieting aircraft cabin noise

By experimenting with the materials in cabin interior elastomeric isolators used to dampen the sound of flight, engineers from ITT Enidine, an ITT Corp. brand, have significantly reduced cabin noise. Vibration within the frequency range of 20Hz to 16,000Hz is called sound; it’s the range that a normal, adult ear can detect.

To develop elastomeric isolators that cope with noise attenuation at critical frequencies of hearing, the engineers work with a customer’s noise and vibration control engineers and the assembly team to understand their needs. Among the requests to dampen noise is the desire to expedite the installation process.

Major sources of noise in aircraft come from the engines and air flow, which transmit through the plane’s fuselage and structure, and are often perceived as a rumble or roar. Making a plane quieter typically requires a combination of elastomeric isolators, thermal/acoustic blankets, and sound-damping materials. At times the elastomeric isolators are located on the airframe side of the thermal acoustic blanket and must be designed to perform in an extremely cold environment associated with higher cruise altitudes.

A specialty of ITT Enidine is engineering elastomeric isolators. Its engineers include these elastomers in the attachment of interior trim panels, stowage bins, and monuments to the frame. Originally, the elastomeric isolators were thought of by customers as simple rubber connectors. But now the highly engineered components – designed to ensure the required degreed of noise attenuation – reduce aircraft weight, save interior installation time, and protect the passengers during emergencies.
 

Reducing cabin noise

The isolator is an elastomeric device that attaches the cabin interior trim panels to the frame of the aircraft.

Cabin interior isolators have reduced sound by up to 5 to 6 A-weighted decibels (dBA) in some of the world’s newest aircraft. A-weighted decibels express the relative loudness of sounds perceived by the human ear. The A-weighted system reduces decibel values at low frequencies because the human ear is less sensitive to audio frequencies below 1,000Hz.

Simply placing an elastomeric isolator between the frame of the aircraft and the interior can lead to amplification of noise. Instead, engineers first listen to the customer to understand the entire system and requirements, then they engineer the dynamic performance of elastomers and the interaction of this material with the cabin interior.

ITT Enidine’s patented, customized solutions are the outcome of 10 years of studying the performance of elastomeric isolators and the resulting reduction in cabin noise levels. The company’s vision is to help an aircraft maker reach cabin noise levels of 50dBA, equivalent to the noise level in a typical hotel room, by creating an optimal cabin noise control system.
 

The effects of noise

A reduction in cabin noise takes a system approach and an understanding of the dynamic characteristics of the elastomeric isolators, including the high-frequency performance. Elastomeric material responds very differently depending on the speed or frequency at which it is exercised. To demonstrate the different effects, imagine slowly pulling some Silly Putty so it stretches. Pull the putty too quickly, though, and it snaps in half. This is a simple example of the non-linear behavior of elastomeric materials.

Families of elastomeric compounds also behave very differently, and an elastomer optimized for noise reduction can help reduce cabin noise by 10dBA to 25dBA versus a non-optimized elastomer. As aircraft makers strive to reduce cabin noise, noise attenuation strategies – and the understanding of the interior’s true attenuation performance – will become increasingly critical.

There are three primary challenges with the existing design of elastomeric isolators:

• First, elastomeric isolators are currently analyzed and designed for low- to mid-frequency performance, when a significant portion of noise occurs at higher frequencies.
• Second, the noise attenuation of the isolators is not balanced with the attenuation of the acoustic blankets (or layer between the fuselage and cabin interior), increasing the overall weight of the interior.
• Third, isolators are not always designed to allow quick and efficient assembly of the cabin interior.

ITT Enidine has made improvements in all these areas. Two factors determine the materials to use when constructing isolators. Performance is the first element, which entails load, stiffness, damping, and fatigue. The second set of factors, or environmental requirements, include temperature, smoke, toxicity, and flammability.

A full understanding of the trade-offs and benefits of various families of elastomeric materials is essential to the proper design of an isolator. The company formulates and tests is own proprietary elastomeric compounds specifically for the requirements of aircraft cabin elastomeric isolators.
 

Studying noise with a purpose

ITT Enidine has conducted fundamental research and published papers that explore the methods used to predict the noise attenuation performance of elastomeric isolators, including the contribution at higher frequencies. These studies show the attenuation of the elastomeric isolator must include the consequence of the terminating effect of the interior trim panel. The dynamic properties of elastomeric material is also highly dependent on ambient temperature, and the elastomeric isolators must be designed for (and tested at) these temperatures to accurately predict the noise attenuation. This is essential for understanding the true performance of the elastomeric isolator and balancing the noise attenuation performance and weight as well as the entire noise control system.

The isolators designed by ITT Enidine also include features that allow for an efficient installation of the aircraft cabin interior via quick-attach push-pins found on the isolators. The isolators attach to the fuselage, and the cabin interior snaps into place on the isolator. Unlike other elastomer makers’ designs, ITT Enidine has been adding manufacturing techniques that allow molding its isolator directly into the cabin interior’s metal or plastic bracket, saving weight, space and part count by up to 75%. These features help reduce the overall cost of the interior and its installation time.

At the 2013 Business Jet Interiors World Expo in Cannes, France, ITT Enidine was able to show that in some cases its elastomeric devices have reduced noise by up to 6dBA. These same isolators are standard in the new composite fuselage of the Boeing 787 Dreamliner. ITT is providing all the interior trim isolators for the 787, for which the requirements are stringent. Customers such as Boeing are extremely satisfied with lower noise levels as well as reduced installation time and the overall number of elastomers needed to connect the cabin interior with the fuselage.

Noise-damping materials are typically applied to the inside of the fuselage skin in areas not braced by stringers and ribs and therefore prone to vibration. Damping materials are also applied to the cabin flooring, and they have been used as a coating on the inside of sidewalls and overhead composite materials that are prone to reflect and even amplify noise.
 

What’s ahead

Engineers across many companies are looking at innovative ways to tackle the problem of excessive interior cabin noise. However, much of the historic knowledge has been on aircraft with aluminum construction. But the new class of business and commercial jets with a composite fuselage is growing. According to the newspaper Le Mauricien, Boeing’s use of composites as a percentage of its aircraft jumped from 12% on the 777 to 50% on the 787, while Airbus moved from 10% on the A340 to 25% on the A380 and finally to 53% on the A350XWB. The use of composite materials make the fuselage stiffer and will have lower damping than aluminum, thus transmitting acoustic energy much more efficiently. A systematic approach to noise control products, analysis, and methods must be incorporated to meet this challenge.

Looking to challenges beyond creating elastomeric isolators for aircraft interiors, the company has tested a new type of damping material called Enidamp. The new particle-based damping provides good low-frequency performance, especially for aircraft flooring. Passengers spend a majority of any flight with their feet resting on the floor, and the vibration from the cabin floor is a fatigue issue every bit as important as the noise emanating from behind the interior trim panel.

ITT Corp.
www.itt.com

About the author: Jeff Weisbeck is vice president of Industrial Sales for ITT Control Technologies. He holds patents for the development of elastomeric isolators, noise attenuation systems, and shock and vibration isolation technology. He holds a master’s degree in mechanical engineering from Rensselaer Polytechnic Institute and a bachelor’s degree in mechanical engineering from Clarkson University. He can be reached at jeff.weisbeck@itt.com.