Early acousticians came up with a simple method of comparing two sounds. A sound that was perceived to be twice as loud as another was said to be one Bel greater in sound level. The Bel was used as a unit of comparison. It is not a unit of measure. Its namesake, Alexander Grahm Bell, was a pioneer in the science of audiology (the study of human hearing). It soon became apparent this unit of comparison was not very useful in describing the difference between similar sounds. A small unit of comparison, the decibel, was established. One decibel (1 dB) is one-tenth of a Bel. Since a decibel is one-tenth of a Bel, then 10 decibels (10 dB) would equal one Bel. In other words, a sound that is twice as loud as another sound could be described as being 10 decibels (10 dB) louder. By definition, one decibel (1 dB) represents the smallest change in volume a human ear can perceive. The average ear, however, can only detect a 3 dB change.

Figure 1
Comparison of sounds and their average sound pressure level (SPL) - shown in decibels.

Take a look at Figure 1. This chart shows several common sounds and their average sound pressure level (SPL). Notice the threshold of hearing is defined as 0 dB SPL. However, sound still exists at 0 dB. Remember the dB (decibel) is a unit of comparison, not a unit of measure. For example, leaves rustling is given an average value of 10 dB SPL. That means the sound of leaves rustling is 10 dB louder than the threshold of hearing (0 dB SPL), or twice as loud. Commercial background music is approximately 60 dB SPL and a very quiet radio at home is 40 dB SPL. A sound that is 50 dB SPL would be twice as loud as 40 dB SPL and 60 dB SPL would be twice as loud as 50 dB SPL.