The Source of Propeller Noise
A 1937 NASA technical report on propeller noise, a scientific paper with figures plotting directional sound patterns. The record gives no place or intended audience beyond technical readers. The pictures are polar diagrams drawn as black line grids on cream paper, with radial and arc gridlines, dashed and solid curves, frequency labels in hertz, and figure captions, which a designer might take as a source of technical graphic styling and measurement-grid layouts.
