One thing to note about decibels is that they aren't linear, they're logarithmic. So if you hear something that's 80dB, it's not twice as loud as something that's 40dB, it's much much louder. As a general rule, a ten-decibel increase means double the perceived noise. So if someone's bike idles at 75dB and someone else's bike idles at 95dB, the 95dB bike sounds four times as loud. This means that the higher numbers you post, the more each number means. The difference between 75 and 80dB will not seem like much compared to the jump from 100 to 105. It's a 50% increase both times, but that second 50% is a lot bigger. This is also why the 3-5dB difference your helmet makes, will seem like a very big deal indeed if you've got a bike that's rocking 110dB.