Using a Noise Reduction Plan to limit uncertainty

Insulation Bags

Our last blog post talked about the level of confidence we have that any given aircraft insulation project will achieve the desired outcome. This time we will take a closer look at the process we go through when evaluating and treating an aircraft.

Cabin noise reduction is a rather complex subject, since the acoustic environment in a constrained system such as an aircraft has multiple, interdependent noise sources. Any specific source, if untreated, will not only have the primary effect of increased noise directly attributable to that source, but will also have ancillary effects on any acoustically coupled system. Hence, we have found that a “holistic” approach, wherein every aspect of the aircraft is analyzed and treated as required to attain a specific noise level and/or weight, is the most effective manner in which to address aircraft interior noise.

“Piecemeal” application of each component, followed by acoustic measurements, will usually not exhibit the expected gradual reduction in noise levels. This is typically the case because the reduction in noise provided by one component may only uncover noise generated by another source (not currently treated and masked by background noise). Correspondingly, laboratory tests in simplified, isolated conditions (such as transmission loss or absorbance testing), while providing meaningful data about the performance of a specific material or design, do not adequately predict the performance of such a design when applied to the aircraft. Again, this is primarily due to the fact that many noise sources in an aircraft are interrelated and therefore non-discrete.

Regardless of the detailed configuration of the aircraft, it has been our experience that the most satisfactory cabin noise reduction solution requires the dressing of the interior panels (damping and isolation), environmental control system (diffuser selection, flow balancing and mufflers), and the thermal/acoustic insulation blanket system (providing reduction of aerodynamic and structure-borne noise). For a given mission profile (VIP, medevac, commercial, etc.), these systems can be configured modularly. In the case of the interior and ECS, company standard practices can be developed or augmented to apply the principles of acoustic noise reduction in a consistent manner. Thus, these principles can be applied to a general arrangement drawing, or simplified floor plan, to determine the approximate weight impact, cost, and performance levels attainable. The same can be said of the thermal/acoustic insulation system – this element can also be tailored more specifically to accommodate different noise level and weight requirements in specific portions of the aircraft. It is essential that all three (interior, ECS, and insulation) be installed together, or any single untreated component will mask the results obtained by the other two.

For example, in one case an acoustically efficient insulation system was installed on an aircraft without treating the environmental control system or isolating the interior (two major required noise reduction components). The measured sound levels were approximately 5 dB-SIL higher than a similar aircraft whose ECS and interior were properly treated.

In another case, an aircraft’s ECS was muffled and the interior isolated, but an effective insulation system was not installed (the existing insulation was left in place). This resulted in measured sound levels approximately 7-10 dB-SIL higher than an aircraft equipped with an effective insulation system. This difference was verified on the noisy aircraft by installing a new insulation system, and reducing the noise levels by up to 10 dB-SIL.

Although it is possible, on the basis of a floor plan and general aircraft model data, to predict the weight and performance of the system to within ±5% weight and ±2 dB-SIL3, a detailed approach must be taken to ensure that the noise reduction treatments are completely and properly implemented. Past history has shown that, without the last step, noise sources as much as 20 dB above the noise floor can go untreated, resulting in an increase in the room’s average noise level of as much as 5-10 dB. As this amount can be the difference between the target noise level and the original untreated aircraft noise level, it is clear why treatment of such noise sources is absolutely critical to the success of the project.

The noise reduction plan approach is intended to locate and address noise sources that cannot be addressed from analysis of the floor plan or its components. It addresses complex noise sources, such as the transmission of noise emanating from a single source, yet propagating through multiple interior components (such as air inlet noise/vibration and turbulence over the wing fairing). The concerted effort to locate and address every anomalous noise source continually throughout the completion is the key to achieving consistent results. Approximately 75% of these sources are found before the aircraft flies for the first time, and the remaining sources (typically the more complex ones) during in-flight analysis. These days, the testing is done using acoustic imaging software, which gives us a very detailed visual representation of the aircraft’s noise signature. More details about this process can be found here.

Our experience locating and addressing such noise sources in completed aircraft minimizes the time spent testing and evaluating the aircraft – a critical necessity at this latest state of the completion, and essential for a successful aircraft insulation project.