Noise from Two-Blade Propellers

The two-blade propeller, one of the most powerful sources of sound known, has been studied with the view of obtaining fundamental information concerning the noise emission. In order to eliminate engine noise, the propeller was mounted on an electric motor. A microphone was used to pick up the sound whose characteristics were studied electrically. The distribution of noise throughout the frequency range, as well as the spatial distribution about the propeller, was studied. The results are given in the form of polar diagrams.

The mechanical power radiated in the form of sound was measured for three different pitch settings of the propeller. It was found that the percentage of the acoustical power going into the fundamental note (the "roar") became very large as the power supplied to the propeller was increased.

The effect of such sounds upon the ear was investigated both theoretically and experimentally. Computations of loudness level about the propeller at fire distances were made. Attempts to check these computations experimentally showed discrepancies; explanations are given for the direction and magnitude of the deviations from the calculated loudness levels.

An appendix of common acoustical terms is included.