1. To get 35 dB noise (at a certain distance and so on) you would need about 1,26 34dB fans, not taking possible interference into account. Thus, about a "quarter-fan" extra...
2. From 10 fans, each emitting 34 dB of sound, you would actually get 44 dB of sound (possible interference neglected). This would generally appear as slightly more than "doubling the volume", as the ear interprets it. Once again - interference neglected. This is a numerically bad example, though, since you might think that 20 fans would increase the decibel value to 54, etc. This is not true. Here are some better examples:
2 fans -> Delta-decibel = 10·log2 = 3.01 dB
5 fans -> Delta-decibel = 10·log5 = 6.99 dB
10 fans -> Delta-decibel = 10·log10 = 10.00 dB
20 fans -> Delta-decibel = 10·log20 = 13.01 dB
Thus,
Epyon is correct in the sense that doubling the sound
intensity gives a 3.01 increase in decibels. The term
volume (instead of intensity) is not strictly correct, though - volume is a subjective measure. Thus the frequency-adjusted dBA scale, with frequencies around 2000 Hz having the highest impact on the perceived volume. Also, and more important here: for the ear to interpret a volume increase as a doubling of "loudness" it takes about 8-10 dB. A change of 3.01 dB (two fans instead of one) is barely perceptible to the human ear, although the sound intensity is increased by a factor of 2.
Did I make myself clear?
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