Xiaomi's S40 and H40 run at nearly the same noise level

If you’re worried about noise, both the S40 and H40 operate in the same ballpark — around 55-60 dB on standard mode, which is roughly the hum of a conversation. The S40’s extra suction power means it gets slightly louder when you crank it to max, but the difference isn’t enough to change which model you buy.