Take a look at this infographic to know all about the decibel (dB), the unit used to measure the intensity of a sound. The logarithmic decibel scale measures sound base on human hearing.
Many regulatory noise limits are specified in terms of A-weighted decibels, or dBA, which is adjusted so that low and very high frequencies of sound are given less weight – as perceived by human ears. Zero decibels (0 dB) is the quietest sound audible to a healthy human hear.
From there, every 3 dB increase represents twice the sound intensity, or acoustic power.
Loudness and intensity are different. Whereas a 10dB increase equtes to 10x in intensity, is is perceived to be only twice as loud.
If one vacuum cleaner is measured to be 70dB, then ten vacuum cleaners has an acoustic power of 80db (not 700dB!), and they would sound only twice as loud as one vacuum cleaner.
You would need 100 vacuum cleaners to reach a noise level of 90dB, and together they would sound only four times as loud as a single vacuum cleaner.