A crash course in the decibel scale...
The decibel scale is logarithmic at +3dba increments. This basically means that 10db is not 10x louder than 0db (and 0db does NOT mean perfect silence!). Because of the nature of the DB, +3dB is actually twice as loud, I say +3 because 3db is twice as loud as 0db and 6db is twice as loud as 3db, etc, etc. Secondly, the dB is
not a measurments, but a comparison. One fan being 10db louder than another fan simply means that the quieter fan could be considered 0 db (the base of reference) and the louder fan is measured as 10db louder than the base of reference. For simplification, the reference of a singular sound is silence (given a db of 0) so when fans are rated at say, 30 db, they are being measured against total silence, so it is actually 3^10 times louder than silence. The other thing to remember is that decibel ratings do not add together beacause the scale is logarithmic. Two objects with a loudness of 55dba for example, will have a combined loudness of ~58db (remember, +3db is twice as loud, so two objects of the same loudness are twice as loud as one - but that doesn't mean 110db!). It gets incredibly more complicated from there, but just keep that in mind when you're talking about loudness as measured in dB. Just as a side note, the decibel doesn't actually measure loudness - it measures the pressure/power ratio
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