According to the United States Environmental Protection Agency’s (EPA) 2023 noise Pollution report, the standard sound pressure when the 4-Stroke push mower is operated is 78-82 decibels (dBA), or 12-15% below the 88-92 dba for the standard 2-stroke one, or an equivalent of 60% sound energy reduction. Take an example of the Honda HRX217VKA model. Its muffler and balance shaft system govern the maximum noise of the engine at 84dBA, 9dBA less than the two-stroke version of equivalent power. That is the same as decreasing the perceived loudness of the human ear by 50% (computed in accordance with the ISO 3744 specification). On frequency distribution, four-stroke engines have their noise energy largely in the 200-800Hz range, while two-stroke engines can generate high-level sharp harmonics up to 2000Hz due to the sweeping pulse. Such high-frequency sound is likely to cause hearing damage (WHO 2022 Hearing Protection Guidelines).
The comparison of technical parameters shows that the inherent advantages of the four-stroke design significantly improve the noise profile. The engine Briggs & Stratton EXi170 reduces the amplitude of the combustion pressure oscillation by 34% from ±2.1MPa to ±1.4MPa through optimizing the shape of the combustion chamber, reducing the pulse noise by 34%. The vibration acceleration of 22-inch model of Toro Recycler has been reduced from 3.2m/s² in two-stroke to 1.8m/s². In combination with the rubber vibration isolation bracket, the casing radiated noise has been reduced by 4dBA. Market monitoring data show that among communities employing the 4-stroke push mower, the rate of environmental sound above the limit (>85dBA) when cutting lawns decreased from 37% to 11% (Acoustic Society of America’s sampling statistics of 2022).
Customer feedback data confirm the actual effect of technology innovation. A follow-up survey of 1,500 users by Consumer Reports showed that among the users of the 4-stroke push mower, 82% believed that “the operational comfort was significantly improved,” and the rate of ear discomfort decreased from 43% among two-stroke users to 17%. For identical sound level assessment, the operator of the 4-stroke push mower employed for 1 hour a week has an annual exposure to sound of merely 82dB· hour, or significantly lower than OSHA’s 90dB· hour regulation. Greenworks’ comparative test shows that the speech intelligibility Index (STI) of the four-stroke model at 3 meters is 0.68, which is 0.15 greater than that of the two-stroke model. That is, the intelligibility of dialogue has improved by 40%.
Regulatory drive has prompted the generalization of noise control technology. California’s AB-1346 Act requires that the noise of garden equipment marketed from 2024 and later should be no louder than 85dBA, which directly led the market share of two-stroke models to fall from 35% to 9%. Kohler Company’s 4 stroke push mower, through its double-layer muffler structure, reduces the exhaust noise to 76dBA from 98dBA at 3000rpm with cutting torque of 4.2N·m. Introduction of the EU Eco-Design Directive (EU 2019/1784) has increased four-stroke model noise compliance percentage in the European market from 65% in 2018 to 93% in 2023. The same period of time, the return percentage declined by 42% due to improvement in vibration control. These trends can explain why the “low noise” label appears at a frequency of 78% on the five-star ratings of the 4-Stroke push mower on the Amazon website, 2.3 times higher than the frequency of other dynamic categories.