June 1, 1998
By Peter A. Bedell
Looking at the design of conventional turboprop twins, it's easy to see why they can be downright noisy inside. Consider the fact that two large propellers swing in close proximity to the nose, which happens to be shaped like a megaphone with the large end aimed squarely at the cockpit and cabin. Add to that the fact that the engines have a pipeline, the wing-spar carrythrough structure, to bring their droning right into the cabin. Logically, there is a lot of racket to contend with, and the folks in the back who pay the bills, as well as the pilots, can benefit in surprising ways from a reduction of that din.
Today, there are ways to quiet the cabins of popular new and used general aviation turboprops with a technology familiar to those who use noise-canceling headsets. Unlike a headset, which cancels noise in a volume no larger than a few cubic inches, these systems are required to reduce noise in an entire cabin — something not easily done. Here's how it works.
By taking a series of microphones and speakers and strategically placing them about the cabin, a noise-canceling system analyzes the incoming noise at specific stations in the cabin. That noise is sampled by a central control unit, which, in turn, transmits a signal 180 degrees out of phase to the original, effectively canceling much of the incoming racket. It seems like a lot of gee-whiz gadgetry, but the end result is something that makes a difference in cabin noise and comfort.
Traditional methods of quieting an airplane's cabin consist of lining the interior with lots of insulation and/or lead-based soundproofing blankets. The downside of this practice, especially the lead blankets, is the weight. When it comes to weight versus noise reduction, the active system has the lead blankets beat. Lord Corporation's NVX system, as installed in a Beech King Air 200, weighs in at 55 pounds. Elliott Aviation's Sound Management System (SMS), which is a combination of an active system and passive sound insulation, weighs 78 pounds as installed in all new Raytheon Beech King Air 350s. The active part of Elliott's SMS is the UltraQuiet system developed by Cambridge, England-based Ultra Electronics. Currently, one or both of the systems are available for installation in King Air 90, 200, 300, 350; Cessna Conquest I and II; and the Twin Commander series.
Fatigue is the most common side effect of a noisy cabin. The condition is certainly familiar to those who fly piston-powered airplanes. How often have you walked away with a headache after a long flight during which you sat behind a large-displacement four-cylinder engine swinging a fat two-blade prop? Couple that racket with the thin air at altitude and a pinching headset and it's no surprise that a wicked headache could start soon after takeoff. About eight hours of constant 90-decibel noise is considered an irritant or minor threat to hearing, according to the U.S. Department of Labor's noise regulations. For comparison, the cockpit of a Beech Bonanza registers about 90 dB, using A-scale weighting (more on that later).
Generally, turboprop aircraft are easier on the ears both because of slower-turning propellers and less reciprocating mass inside the engine. However, regardless of the superior smoothness of the turboprop, anytime you add more power and more speed to any airframe, the noise level goes up — either way, you're going to pay.
Handing out earplugs or planting a headset on a $150 fresh-out-of-the-salon coiffure is not going to go over well with executives in the back. If they're paying the bills for the airplane, they'd like to be comfortable. In addition, if they want to accomplish any business — or rest — during the trip, a quiet cabin makes life much easier. Any pilot would love to convince the accounting department that the company needs the quietness of a jet, but the bean counters may not be ready for such an investment, making the $35,000 cabin-noise-canceling system look more attractive.
We took a ride in a King Air 200 that is operated by Stevens Aviation, located at the Greenville-Spartanburg Airport (GSP) in Greer, South Carolina. This particular airplane had the Lord NVX system installed under a supplemental type certificate. Pilots will recognize the Lord name because the majority of the Dynafocal engine mounts dampening the vibrations of Lycoming and Continental engines are made by the company.
Lord's NVX system does not make use of any supplemental passive noise reduction materials as does Elliott's SMS. Regardless, the installation of either system would be unnoticed by those who didn't know that the system is there. The Lord NVX-equipped King Air 200 has two button-sized microphones placed at each seat in the cockpit and cabin, for a total of 16. Generally, one is mounted in the headliner and another just below shoulder height in the cabin sidewall. The system is designed to cancel noise at the shoulder level and above. This means that if you lean forward in your seat and lower your head, the noise level is noticeably higher. A speaker is hidden under each seat to transmit the canceling noise. If your airplane has drawers under the seats, they may be partially or completely consumed by the new speakers.
The Lord system operates automatically by turning itself on and off via the landing gear squat switch. In flight, the system can be turned off by a switch mounted behind the copilot's seat. Using a handheld sound level meter, we measured 94 dB on the C scale at head level during climb. At lap level the noise rose to 103 dBC. The C-scale weighting curve is nearly uniform over the frequency range from 32 to 10,000 Hertz, which gives a better indication of overall sound level than does the A-scale. The aforementioned Bonanza cockpit scored 105 dBC for comparison purposes.
After leveling off at 17,500 feet and setting typical cruise power, we measured the difference in noise at various stations in the cockpit and cabin with the system on and off. We saw overall sound reductions anywhere from 3 to 8 dBC, depending on the seat. Although single-digit reductions don't look very dramatic on paper, you must keep in mind that sound measurement is on a logarithmic scale — an increase of 10 dB is three times louder than the original figure. A 6 dBC reduction in a 90- to 100-dBC environment is equivalent to ridding 25 percent of the sound pressure, said Michael McPeters, manager of technical products development at Elliott Aviation.
With the NVX system activated, the lowest reading we got in the main club seating area was 92 dBC. The highest reading with the system on — 96 dBC — was taken at the copilot's seat, where the din rose to 103 dBC when the system was deactivated. Our crude method of measuring overall sound is not completely fair, as the NVX and UltraQuiet systems target propeller noise only. Wind noise and other sounds are generally left unabated. In addition, the noise can vary quite a bit from seat to seat, but reductions of four to five dBC were average in the NVX-equipped King Air 200.
To the typical passenger, the difference heard by turning the system on and off is not earth-shattering, but it is definitely noticeable. Cabin noise canceling systems do the best job attacking low-frequency prop noise. According to Lord, the prop noise reduction is 70 percent in the King Air 200. Our testbed was equipped with four-blade propellers, so it can be assumed that the system's performance would be more noticeable on an airplane with larger three-blade propellers, which emit a somewhat lower frequency. Similarly, the system would perform better at higher altitudes where the thin air would lower overall noise levels (since the air noise is dramatically reduced), leaving the system to target the majority of the racket coming from the propellers. Both systems have sensors in the nose that detect the passing of each propeller blade and then generate an out-of-phase signal designed to reduce the intensity of the wow-and-flutter humming that twins are so inclined to produce.
An STC installation would take at least 100 hours and at least a week of down time. Typically, owners elect to coordinate the installation with a major inspection or phase check in which all or most of the interior will already be removed. The system's control box can be located under the floor or somewhere else out of the way but accessible for maintenance.
Both the Lord and Elliott cabin noise canceling systems are of the set-it-and-forget-it type that requires no action from the pilot. If the system were to have some sort of failure, the self-diagnostics will illuminate an annunciator on the cockpit control panel. A technician can then hook up a laptop computer to the control unit and allow the system to spill out its problem. Lord's system carries a three-year warranty on the electronics and a one-year warranty on the speakers. Elliott's warranty is two years on aftermarket installations, while Raytheon's warranty covers the SMS system for five years on new King Air 350s.
For the retrofit market, price appears to be the deterring factor for most turboprop owners. Lord admits that it hasn't sold as many NVX systems as it originally anticipated. So far, 25 systems have been sold. Elliott says that it has sold 55 aftermarket systems and that more than 10 have been installed in new King Air 350s. Lord temporarily put its Twin Commander STC on the shelf while the company concentrates its efforts on the Douglas DC-9/MD-80 NVX systems. Logically, the more expensive the airplane, the easier it is to absorb the cost of such a setup.
As mentioned before, Elliott is enjoying a recently announced deal with Raytheon Aircraft in which every new Beech King Air 350 is delivered with the SMS system installed as standard equipment. Considered the noisiest of the King Air fleet, the $4.9 million (fully equipped) 350 easily absorbs the cost of the system. On the other hand, an owner or operator of an older Twin Commander may spend a sizable percentage of the airplane's worth by installing such a system. Remanufactured Renaissance Commanders from the Twin Commander Aircraft Corporation will have the UltraQuiet system installed on an OEM basis.
There's promise for these systems in the form of blossoming jet and airline markets. Elliott's SMS is being installed at the Bombardier factory as optional equipment in new Canadair Challenger 604s, and there's talk of retrofitting Cessna Citations. Besides the DC-9/MD-80 work, Lord has active vibration isolation systems installed as standard equipment on Cessna Citation X airplanes. Lord and Elliott representatives hinted at the possibility of an active noise-canceling system for light GA aircraft, withprices considerably lower than they are now. But don't throw your headsets away yet; it'll probably be a few years.
For more information about the Elliott system, contact Elliott Aviation, Quad City Airport, Post Office Box 100, Moline, Illinois 61266-0100; telephone 309/799-3183; or visit the Web site ( www.elliottaviation.com). For information about the Lord system, contact Lord Corporation, Mechanical Products Division, 110 Corning Road, Cary, North Carolina 27511; telephone 919/468-5981 ext. 6402; or visit the Web site ( www.lordcorp.com/nvx/NVX_Home.html).
Links to all Web sites referenced in this issue can be found on AOPA Online ( www.aopa.org/pilot/links.shtml). E-mail the author at firstname.lastname@example.org.
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