We do not hear all sounds in the same way. In order to give an indication of how loud sound may appear to a listener, it is often expressed in terms of dBA rather than just decibels (dB). The ‘A’ means that an ‘A weighting’ (rather than B, C or D weightings for example; the Z (zero) weighting is linear i.e. none) has been applied to the frequency spectrum as an approximation for the way in which the human hearing system responds to sound.
A level of 0dBA equates to around the threshold of hearing for someone with good hearing. As the sound-pressure level increases, the level in dB also increases. However, because it is a logarithmic ratio, this can cope with the very wide range of pressure levels encountered in the natural world to which our hearing system is adapted. It is possible to have a level less than 0dBA, although this will generally be inaudible even to someone with good hearing.
Although an increase of 3dB equates to a doubling of sound energy, a change of 3dBA is generally regarded as ‘just noticeable’ in most cases. A good example is that if one machine is operating and another that is exactly the same is switched on, the noise level does not appear to double but only seems to be slightly louder, despite the fact that double the sound energy is now being produced.