Sound quality evaluation of noise emitted from brush cutters

Engine-driven brush cutters generally generate noise with a high sound pressure level. Although such noise should be attenuated as a first countermeasure, improved design of the sound quality is another valid approach. The perceived quality of noise emitted by brush cutters operated at various engine speeds was measured using a semantic differential method. Stimuli induced, for example, muddy, rough, noisy and unpleasant impressions. There was a difference in the impression of undulation between two-stroke-engine sounds and four-stroke-engine sounds. Furthermore, correlation analysis between rating results and metrics indicated that the perceived quality was affected by loudness. We therefore then employed stimuli having the same loudness. Results of semantic differential experiments revealed that two-stroke-engine sounds were rated as being significantly more undulating and unpleasant than four-stroke-engine sounds at various engine speeds. Furthermore, the impression of unpleasantness was significantly correlated with that of undulation for a wide range of engine speeds. It was supposed that the impression of undulation is related to the fluctuating temporal envelope created by unstable salient peaks in the waveform of stimuli, which are likely caused by the incomplete internal combustion of fuel.