The decibel (dB) is an unusual unit of measurement. Firstly, it doesn't measure the absolute value of a quantity, like the metre or the second, but the ratio of two values. That's why it's used for very different quantities, but above all for measuring sound. Secondly, the decibel doesn't use a linear but a logarithmic scale. When the measured value doubles, the dB value only increases by three, i.e. by a fixed amount. This point is very important when comparing projectors.
The average operating noise of a projector is in the range of 19–30 decibels (dB). This difference of 11 dB corresponds roughly to a doubling of perceived loudness – the rule of thumb is around 10 dB per doubling. And how loud is this projector noise? The threshold of hearing is defined as 0 dB – it's the zero point of the scale, but depends very much on the frequency, i.e. the pitch, and on individual hearing ability. That's why, in a quiet room, you may still notice something even from a projector noise of 19 dB.
