There is no strictly accurate convertion, however as a rule of thumb, subtract between 6 to 7 dB
i.e. 47dBA is approx NC40 ish
The best method however is to use a sound meter and record the spectrum and plot on an NC curve chart.
The human ear detects an increase of 10dB as a doubling of perceived sound level i.e. it sounds twice as loud
3dB increase is a doubling of the power or pressure level but the human ear only picks this up as a slight increase in perceived noise level.
The simple test is that two vacuum cleaners when switched on, each being say 75dBA do not give you 150dBA but 78dBA and they do not sound twice as loud.
4 vacuums would give 78dBA and 8 vacuums 81dBA (which would then be about twice as loud.
after all that, your 35NC would be 35+6 or7=42dBA
(Its not dB but dBA weighting)
dB could be at any frequency, dBA is a weighted average..
