Cabin noise is a huge issue for business jets, especially those that are on the higher end of the cost spectrum and that fly ultra-long distances. Business aircraft manufacturers consequently expend an extraordinary amount of time starting early in the design process to maximize noise reduction and increase passenger comfort.
In a laboratory deep inside the St. Cloud, France headquarters of Dassault Aviation, expert engineers analyze and mitigate sources of noise in new Falcon designs. This effort ramped up when the Falcon 7X was in development and more recently with the 8X and upcoming 6X.
What the lab produces is a more accurate evaluation of the perceived noise inside an aircraft cabin, which Dassault terms “comfort note.” The higher the comfort note number, the better the noise level (perceived comfort) inside the aircraft.
“This was developed as a way to measure perceived differences,” said acoustics specialist Yann Revalor. “We wanted it to be an objective, quantifiable measurement.” To go further, the lab uses the measurements to boost the comfort note by tuning and balancing elements of soundproofing and other factors in the cabin that affect noise but without increasing the weight of the aircraft. “The challenge is to stay in the envelope of weight,” he said.
Typical noise measurements are dBA, which attempts to measure the audible frequency range, and dBSIL, which tries to quantify the noise level related to the range of frequency involved in speech.
According to Dassault, “The acoustic comfort perceived by a human being combines noise levels and other factors, called ‘sound quality,’ that are not taken into account by the dBSIL and dBA indicators. To master acoustic comfort, sound quality—human perception—should be quantified.”
As an example, a relatively quiet airliner, the Airbus A380 (upper deck, left side, row 14, Mach 0.75, FL360) generates levels of noise measured by Dassault at 69.7 dBA and 52.4 dBSIL.
By comparison, Dassault’s measurement of a G650 that it chartered produced 67 dBA and 49 dBSIL. The Falcon 8X (same seat location as the G650) measured 65 dBA and 47 dBSIL.
However, the comfort note for those three aircraft is 5.6 for the A380, 6.3 for the G650, and 10 for the 8X. The 7X’s comfort note is about 8.
Factors that affect noise perception inside an aircraft vary and include the shape of the interior, how engines are mounted, and the materials covering interior furnishings. Glass bulkheads, while attractive, present a huge challenge in noise reduction. Granite floors are also reflective, but “we can deal with [them],” said Revalor.
To illustrate the differences in comfort note, Revalor and his engineering team created a space where customers and others can evaluate the comfort note of a particular aircraft. A seat in the lab is surrounded by speakers, and during a recent visit to the lab, I tried out the comfort note demonstration.
I’ve flown in the A380, including in the upper deck business class section, and the sound coming from the speakers did seem quite similar to the sound of the real airliner. It’s a bit hard to describe, but I perceived the A380 noise as deep and throaty.
Revalor switched on the G650 sound and this seemed less throaty with a hint of “pipe-ness” that made the sound appear to be coming from inside a tube.
Finally, the 8X felt even quieter, without the "pipe-ness" or throatiness of the G650 and A380. Overall, the variations in comfort note as measured by Dassault seemed to match the comfort note numbers derived in the lab.