Sound, Loudness, Noise, Sones, Phons and Decibels
Sound is a huge subject. One engineering challenge is to minimize noise, to keep things quiet, make things less loud. This could fill up a book. Here are a few observations, followed by some good links.
Noise is annoying sound, not necessarily loudness.
Limiting our noise problems to reducing loudness of a continuous noise source, there is much to consider.
Quantifying loudness and predicting a reduction in loudness due to an engineering fix is difficult, and quite a specialized field.
Decibels is our most common measure of sound, but it is not a very good measure of what our ears perceive as loudness. Sones is a very good measure of what our ears perceive as loudness, but there are no physical instruments that measure sones.
Loudness does not add and subtract intuitively. For example, two roaring jet engines sound about 1.1 times as loud as one roaring jet engine.
The different frequencies of sound has everything to do with the ear's perception of loudness, and actions to take to reduce the loudness.
Sound links:
A learning map of Sound subjects, from which the following pages are taken:
http://230nsc1.phy-astr.gsu.edu/hbase/sound/dbcon.html
Shows conversion table between Sones and Phons:
http://230nsc1.phy-astr.gsu.edu/hbase/sound/phon.html
Shows equal loudness curves in terms of Phons, Decibles and Frequency:
http://230nsc1.phy-astr.gsu.edu/hbase/sound/eqloud.html
Together with the sones to phons conversion, all four - Sones, Phons, Decibles and frequency can be related.
Several mini-essays on this page give insight into "Adding Loudness"
http://230nsc1.phy-astr.gsu.edu/hbase/sound/loud.html
One corollary is that two fans of similar pitch sound 1.1 times as loud as one fan, not 2 times as loud.
Another interesting chart showing relationships among Sones, decibles and frequency:
http://feynman.stanford.edu/people/Oliver_www/singhtml/node36.html
Community noise study by WHO:
http://www.nonoise.org/library/whonoise/whonoise.htm#7.9.6.4
chart of sound intensities:
http://www.hunter.cuny.edu/health/coeh/publications/over_head/2healthawoh/sld002.htm
chart from
http://emc2.acu.edu/acoustics/askill/chapter4.html