There are many sources in the field of acoustic engineering that may be useful. In terms of contrast, here is some general guidance:
A way of predicting a human reaction to a new noise environment is based on how the noise compares to the existing environment to which one has adapted: the so-called “ambient noise” level. In general, the more a new noise exceeds the previously existing ambient noise level, the less acceptable the new noise will be judged by those hearing it. With regard to increases in A-weighted noise level, the following relationships occur:
• except in carefully controlled laboratory experiments, a change of 1 dBA cannot be perceived;
• outside of the laboratory, a 3-dBA change is considered a just-perceivable difference;
• a change in level of at least 5 dBA is required before any noticeable change in human response would be expected; and
• a 10-dBA change is subjectively heard as approximately a doubling in loudness, and can cause adverse response.
These relationships occur in part because of the logarithmic nature of sound and the decibel system. The human ear perceives sound in a non-linear fashion, hence the decibel scale was developed. Because the decibel scale is based on logarithms, two noise sources do not combine in a simple additive fashion, rather logarithmically. For example, if two identical noise sources produce noise levels of 50 dBA, the combined sound level would be 53 dBA, not 100 dBA.
There is also a useful table for typical dBA levels here:
http://www.dot.ca.gov/ser/downloads/noise/Final_Noise_Study_Report.pdf
