I'm well aware of how sound and the decibel scale work. 3 dB represents the minimum noticeable increase in sound level outside of a controlled research lab. 60-70 dB is pretty typical for a city, which isn't great for annoyance thresholds, but in terms of sleep deprivation, that usually has more to do with structural insulation of residences. I don't have the measurements for other cities, but I imagine a place like NYC averages at least 70 dB if not higher.