Musicians talk freely about decibels, but very few of them really understand what the term means. Many of them know that an increase in volume of one decibel is actually a very significant gain, but they don't really know why. It's worth discussing briefly, not least because it might save your hearing if you know just how loud 118dB SPL might actually be!
Decibel is a term most often used as a measure of sound pressure, though its true meaning is far more subtle. It actually describes a ratio - that is, the relationship between two different quantities - so there is actually no such thing as a definite decibel; it depends on how it is being used.
Decibels are used in electronics and optics, but it is their use in acoustics and sound pressure in particular that concerns us here - dB SPL. When measuring sound pressure, a decibel shows the relationship between the sound pressure and the specified measure of zero. Usually, zero is taken to represent the point at which a sound is inaudible to a human ear, which means that strictly speaking, one person's decibel may be another person's zero, depending on the quality of your hearing.
In fact, the zero from which most decibel measures are taken is usually set at around 20uPa, the normal lower end of the human audible range. Decibels make it easier to measure sound pressure because they are a logarithmic measure. If a simple measure was used, the figures would run into trillions - the decibel measure allows a more manageable way of expressing changes in sound pressure.
So how loud is loud? As we have seen, 130dB SPL is generally reckoned to be the pain threshold. Note however, that much lower levels can still be damaging, especially if they are sustained. Research in the field known as psycho-physics has shown that an increase of 10dB appears to equate to a doubling of volume for most people. So be careful when working with decibels - they are not a simple measure, and they jump much higher and more quickly than you may think.