Decibel levels are not additive. That is, if a machine is operating at 100 dB and a second machine is switched on at 80 dB, the noise level in the room remains 100 dB, not 180 dB (the sound intensity increases, but not the actual noise level).
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‘But it does me no injury for my neighbour to say there are twenty gods or no gods. It neither picks my pocket nor breaks my leg.’ - Thomas Jefferson