The University of New South Wales
This website provides a definition of a decibel and has examples of how logarithms are used. There is also a review of logarithms at the bottom of the page.
The Engineering ToolBox website
This website provides a definition of the equations used to calculate decibels, converting between the exponential and logarithmic form.
The Sonic Research Studio, School of Communication, Simon Fraser University
This page is another description of decibels. At the bottom of the page is a table providing examples of different decibel levels.