Hambric Stephen A, Shepherd Micah R, Schiller Noah H, Snider Royce, May Carl
ARL/Penn State University, PO Box 30, State College, PA 16804.
NASA Langley Research Center, Hampton, VA 23681.
J Am Helicopter Soc. 2017 Jan;62(1):1-10. doi: 10.4050/JAHS.62.012009.
A rotorcraft roof composite sandwich panel has been redesigned to optimize sound power transmission loss (TL) and minimize structure-borne sound for frequencies between 1 and 4 kHz where gear meshing noise from the transmission has the most impact on speech intelligibility. The roof section, framed by a grid of ribs, was originally constructed of a single honeycomb core/composite facesheet sandwich panel. The original panel has acoustic coincidence frequencies near 600 Hz, leading to poor TL across the frequency range of 1 to 4 kHz. To quiet the panel, the cross section was split into two thinner sandwich subpanels separated by an air gap. The air gap was sized to target the fundamental mass-spring-mass resonance of the panel system to less than 500 Hz, well below the frequency range of interest. The panels were designed to withstand structural loading from normal rotorcraft operation, as well as 'man-on-the-roof' static loads experienced during maintenance operations. Thin layers of viscoelastomer were included in the facesheet ply layups, increasing panel damping loss factors from about 0.01 to 0.05. Transmission loss measurements show the optimized panel provides 6-11 dB of acoustic transmission loss improvement, and 6-15 dB of structure-borne sound reduction at critical rotorcraft transmission tonal frequencies. Analytic panel TL theory simulates the measured performance within 3 dB over most frequencies. Detailed finite element (FE)/boundary element (BE) modeling simulates TL slightly more accurately, within 2 dB for frequencies up to 4 kHz, and also simulates structure-borne sound well, generally within 3 dB.
一种旋翼机顶部复合夹芯板已被重新设计，以优化声功率传输损失（TL），并在1至4千赫的频率范围内将结构传声降至最低，在该频率范围内，传动装置的齿轮啮合噪声对语音清晰度影响最大。顶部部分由肋条网格构成框架，最初由单个蜂窝芯/复合面板夹芯板制成。原始面板的声学吻合频率接近600赫兹，导致在1至4千赫的频率范围内TL较差。为了使面板安静下来，其横截面被分成两个较薄的夹芯子面板，中间由气隙隔开。气隙的尺寸设计为将面板系统的基本质量-弹簧-质量共振频率目标设定为低于500赫兹，远低于感兴趣的频率范围。这些面板设计用于承受正常旋翼机运行时的结构载荷，以及维护操作期间经历的“人在顶部”静载荷。面板层压板中包含薄的粘弹性体层，将面板的阻尼损耗因子从约0.01提高到0.05。传输损失测量表明，优化后的面板在关键旋翼机传输音调频率下，声传输损失提高了6至11分贝，结构传声降低了6至15分贝。解析面板TL理论在大多数频率下模拟测量性能的误差在3分贝以内。详细的有限元（FE）/边界元（BE）建模模拟TL的精度略高，在高达4千赫的频率下误差在2分贝以内，并且对结构传声的模拟效果也很好，一般误差在3分贝以内。
