Acoustical Materials and Acoustical Treatments for Aircraft

Abstract

Soundproofing treatments for aircraft must be designed to provide high degrees of both attenuation of sound transmitted through the fuselage and absorption of sound within the cabin. At low frequencies, treatments of weights which are practical for aircraft provide little or no attenuation of transmitted sound, hence the sound absorption at low frequencies should be made as high as possible by spacing the treatment one to three inches from the dural. At intermediate and high frequencies, the sound absorption is approximately the same for most treatments which have a blanket of acoustical material at least one-half inch thick on the side toward the interior of the cabin, and a highly porous trim cloth. If the trim cloth is not porous, the absorptive properties of the structure will be poor at low frequencies, which must be compensated for by sufficient absorption in the seat upholstering, carpets, etc. The flow resistance of the acoustical material used in the treatment should not be greater than about 500 g cm−2 sec.−1 per inch of thickness. Attenuation of sound by impervious panels, such as windows, dural, etc., at the intermediate and high frequencies is almost entirely dependent upon the surface (SS) density of the panels. The attenuation by an acoustically-treated fuselage structure is at least as great as would be predicted from its surface density. The presence of air spaces, absorptive materials, and impervious septa in the treatment will tend to give greater attenuation than predicted by “weight law.” The relative “efficiency” of a treatment may therefore be characterized by a Merit Factor, which is the ratio of “db better than weight-law” attenuation (at 5000 c.p.s.) to the surface density of the treatment. For both attenuation and absorption, the most effective of the simple treatments designed at this laboratory consists of two blankets of acoustical material with an impervious septum between, the treatment being mounted with an air space of one to three inches between it and the dural, and covered with a highly porous trim cloth. The choice of an acoustical material depends upon many mechanical and economic factors in addition to the acoustical requirements. Materials should be chosen on the basis of complete data on their acoustical transmission (attenuation) and absorption characteristics measured by the standard laboratory techniques referred to in this paper. A rapid estimate of the relative effectiveness of various materials in a given treatment may be obtained by measurement of the flow-resistance of samples of equal surface density, at several thicknesses (obtained by compressing the samples). From a plot of flow-resistance R vs. thickness T, the value of RTx for each material may be found. The material with the highest value of RTx will generally provide the highest attenuation in a given treatment. The validity of the laboratory measurements of acoustic attenuation and absorption discussed in this paper has been established by many measurements in airplanes of various types, in flight, with a wide variety of acoustical treatments.