Interior Noise of STOL Aircraft and Helicopters

Abstract

A review of interior noise levels in STOL aircraft and helicopters has shown that they can be significantly higher than those in other airborne and surface vehicles. For most STOL aircraft using powered lift devices the interior noise spectrum is broadband, but for helicopters and propeller-driven STOL aircraft the interior noise is dominated by discrete frequency components. Existing noise control methods utilizing add-on treatments are adequate for reducing high-frequency broadband or discrete frequency sound, without imposing unacceptable weight penalties, but new techniques are required to control low-frequency cabin noise. Low-frequency noise reduction is difficult to accomplish in STOL aircraft and helicopters, or indeed in any flight vehicle, because of the severe constraints imposed on weight and space availability. Consequently, research and development work is needed to achieve noise reduction by basic design of the noise source and by modification of the transmission paths in the aircraft mechanical structure. Current work in government agencies and private industrty will contribute to the noise control effort. For example, the NASA Langley Research Center research program on general aviation airplane interior noise will eventually have application to some of the STOL aircraft and helicopter problems, although so far the emphasis has been on source and path identification rather than noise control. This identification procedure is an essential part of the noise control process, but the innovative work required to devise effective low-frequency noise reduction methods has still to be performed. (Authors)