The expression "cone of silence" took on new meaning when British manufacturer Ultra Electronics developed its electronic noise-canceling system for the noisy cabins of turboprop regional airliners. Now Elliott Aviation in Moline, Ill., has improved on the original product to create its Sound Management System (SMS) for the Beech King Air 200, 300 and 350 executive turboprops.
"In a turboprop-powered aircraft, most of the noise is created by propeller blades projecting a shockwave against the side of the fuselage," explained Ron Jennings, Elliott´s director of technical sales support. "Elliott combats this with both active and passive noise-canceling technology, the former working on the same principle as that employed by Bose in its noise-canceling headphones." However, the Ultra system quiets the entire cabin with speakers, microphones and a computer processor that generates a sound wave 180 degrees out of phase from the offensive noise, thus canceling it out.
"The difference between what we do and the original Ultra system is that we didn´t stop at the low-frequency noise," Jennings explained. "The Ultra system works so well in canceling the low-frequency noise that you then hear the high frequencies that you didn´t before because they were drowned out by the low ones." So Elliott addressed the high-frequency noise by installing an improved thermal and acoustic insulation package in the King Air´s cabin and applied foam to the backs of some of the longer interior trim panels to reduce vibration.
"The third thing we did was put isolators between some of the interior panels and the fuselage wall," Jennings said. "So our system is the only one that combats high and low frequencies all the time from takeoff to landing."
The system consists of 12 speakers and 24 microphones distributed from the cockpit to the rear of cabin at a position just before the door. "The reason for all the mikes is that you have to introduce the duplicate wave at precisely the right time in terms of predicting when the aircraft is going to make a certain sound and what that sound is going to be, then introducing that duplicate sound 180 degrees out of phase at precisely the right time," Jennings said.
Ultra Electrics provides a proprietary software package for certification purposes that calculates the optimum location for speakers and microphones. "We claim the system will reduce noise by 50 percent," Jennings said. "It´s comparable to being in the cabin of a typical business jet. It also reduces the fatigue factor for crew and passengers, since sound is pressure."
The SMS adds 79 pounds total for both passive and active components. Typically, a two-week downtime is required for installation, and the cost of the full system ranges from $ 57,000 to $ 62,000, depending on the King Air model.
Elliott has installed 150 systems with 370 systems flying. "When the King Air 350 became popular," Jennings said, "Beech contracted with Elliott to install the systems for a period at the factory in Wichita, hence the 220 additional installations." Beech is now installing its own sound-management system, dubbed ATVA, based on active tuning forks.
In addition to its Moline headquarters, Elliott operates mod shops at Omaha, Minneapolis and Des Moines. Jennings said the cabin refurbishment business has never been better. "We are conducting full completions and refurbs for King Airs, Hawkers, Citations, Falcons and Beechjets. Everyone wants their old airplane to look like the new ones, to replicate the cabinetry, sidewalls, window coverings and so forth. Entertainment systems are big, as are moving maps, DVD players, XM Radio.
You can achieve the factory look by updating the interior shells and seats and installing new cabinetry of contemporary design. We´re doing a Hawker 700 now that will replicate the interior of an 800XP."