Rotary power lawn mower noise

Abstract

The purpose of this study was to examine the noise of a typical rotary power lawn mower. Hearing loss and annoyance criteria were examined to determine acceptable mower noise limits. It was found that if reduced from its present level of 91 dBA at the operator to 85 dBA, mower noise would pose almost no damage risk for a typical exposure. When the spectral content of the mower noise had been determined it was found that structural radiation was responsible for most of the noise in the range 500–10 000 Hz and that both the blade and exhaust were major sources below 500 Hz. The spectral content of the noise when mowing was found to be similar to that when not mowing. The three major noise sources—exhaust, blade, and structural radiation—were studied seperately. Sources of blade noise were identified by applying the theories of propellor and fan noise generation. Of the blade parameters studied, which included fan lift, sharp trailing edge, and multiple blades, none reduced the A-weighted level of the blade noise. A 1-dBA reduction was observed when the blade was statically balanced. A vibration isolator was designed and built to reduce the vibrations of the blade enclosure and resulted in an additional 112-dBA reduction. Existing theories of exhaust system design were researched and used to design resonator-type systems. Within the assumptions of classical acoustics, no acceptable solutions were found. Applying the theory of acoustic radiation from a hole, a system was designed, built, and tested which resulted in an excess attenuation over the stock muffler of 212 dBA. With the isolator, a blade with less lift, and the improved muffler, the final mower noise level was 86 dBA at the operator. Cost of the solutions is estimated at less than $ 10.00 and none of them is expected to impair the mower's operation.