TABLE OF CONTENTS The decibel is as commonly used in audio circles as the minute or the mile is in general usage. A superficial understanding of the decibel can hinder the study of the science of sound and be a barrier in the proper use and development of its many applications. The goal of this chapter is to show the need for the decibel concept and how decibels can be applied in many different ways.
Levels in decibels make it easy to handle the extremely wide range of sensitivity in human hearing. The threshold of hearing matches the ultimate lower limit of perceptible sound in air, the noise of air molecules beating a tattoo on the eardrum. The sensitivity of normal human eyes also matches the ultimate limit by responding to one or a very few photons of light. From these threshold responses to the most feeble stimuli, the ear and eye are also capable of handling high intensities of sound and light. A level in decibels is a convenient way of handling the billion-fold range of sound pressures to which the ear is sensitive without getting bogged down in long strings of zeros.
Imagine a sound source set up in a room completely protected from interfering noise. (The term sound-proof is avoided because there will be much sound in it.) The sound source is adjusted for a weak sound with a sound pressure of 1 unit, and its loudness is carefully noted. When the sound pressure...
