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Noise reduction is the needs of the time even for (demanded even from) the outboard motors which are typically used under heavy load and in higher engine speed for an extended period of time. For the sound emitted by an outboard motor, the acoustic impression to human ears largely depends on the high frequency intake noise. The high frequency intake noise is generated by the air flow that passes through the intake pipes at high speed. The test performed with a flow stand bench demonstrated that the sound pressure increment relative to the increasing flow velocity is definitely larger in high frequency band compared with that in low frequency band. Pressure wave form inside the intake pipe was measured relative to the crank angle. The measurement revealed that the high frequency sound is generated in the later part of the intake stroke when the flow velocity within the intake pipe becomes the highest.

The interaction phenomena of the internal flow and the acoustic resonance noise in expansion cavity type mufflers was investigated. The generation conditions of this phenomena and the predominance of acoustic resonance noise were examined. According to the distribution of fluctuating velocity generated in the cavity, the location where the vortexes were generated in the cavity and the amplifying process of the vortex rings were clarified.