Broadband cabin noise, which can greatly fatigue passengers in even the most finely appointed business aircraft, could be a thing of the past if a new system from Britain’s Ultra Electronics and Qinetiq makes it to market. Early next year the partners are expecting to flight test new-generation hybrid active/passive mounts positioned between the fuselage and cabin trim panels.
In ground tests on a cabin section provided by Bombardier, the Airat (active isolator research for aircraft trim) system has been found to reduce noise by between 15 and 30 dBA. The overall objective is a 10- dBA cut, and the partners are confident that this can be achieved, resulting in an actual audible noise reduction of as much as 70 percent.
Active and passive elements in the new mounts are intended to neutralize broadband noise, which originates with the airflow over the fuselage and is then transmitted through the airframe into the cabin panels, causing them to vibrate. The mounts replace existing rubber mounts and blankets, which can dampen noise only in the 1,000- to 1,500-Hz range. Generally, active systems are most effective at low frequencies, while passive systems cover the high frequencies.
The trim mounts developed by Cambridge-based Ultra incorporate a spring to absorb the vibrations (the passive element) and an electrical coil, which cancels them out. Farnborough-based aerospace and defense technology company Qinetiq has provided structural dynamics expertise for the program.
According to Ultra v-p of marketing and support Rob McDonald, Airat effectively blocks the dominant route for vibrations emanating from airflow over the fuselage. “Controlling this broadband low-frequency noise is the Holy Grail in reducing overall cabin noise,” he told AIN.
The Airat partners hope to have a production unit ready for flight demonstration in 2005, and retrofitting could be under way later that year. However, key issues such as the system’s final weight, cost and power requirements first must be resolved. In their current form, each unit weighs slightly more than existing passive rubber mounts and has a five-watt power requirement. The partners are looking into the possibility of installing the system zonally to quash cabin noise where it is most severe.
McDonald argued that some business aircraft are prime applications for Airat on the grounds that light to midsize models suffer “unacceptably high levels of noise compared with airliners–in the high 80s [dBA] and low 90s.” This is because their smaller fuselages mean that cabin panels are closer to the outside of the aircraft.
Ultra pioneered active noise control for higher-frequency tonal noise problems. It has developed its UltraQuiet cabin active-noise and vibration-control system for the Bombardier Challenger (for which it is available as an option), as well as the Saab 2000, King Air 350 and Bombardier Q300 and Q400. These systems use tiny cabin speakers to introduce noise of equal frequency–but opposite phase–to cancel trim vibration.