A measuring scale was developed years ago to get a handle on calibrating the wide range of discernible sound levels. This scale is called the decibel range, and goes from 0 - 140 in intensity, with one decibel being the normal threshold of hearing for most people and 140 being the point where noise causes acute pain to the ear. Roughly stated, a noise source doubles in volume with every 6 - 8 decibel increase, and is halved when reduced by corresponding decibel amount.
Common noise sources and their decibel values are listed below:
40 dB : quiet residential neighborhood
70 dB : normal speech at 12" distance 90 dB - heavy city traffic
95 - 100 dB : diesel engine room, not soundproofed
120 dB : jet take-off
Because the decibel scale compacts the entire range of hearing into a 0 - 140 scale, what would seem to be modest noise reductions of 4 dB can mean up to a 25% reduction in perceived noise and well worth the trouble to achieve. Many sound barrier type materials boast decibel reductions of 15 - 40 dB, but these are measured under perfect laboratory conditions and are rarely, if ever, duplicated in real life situations.