Page 48 - The Master Handbook Of Acoustics

CHAPTER
2

Sound Levels and the Decibel

he decibel is as commonly used in audio circles as the minute or the
Tmile 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 mole-
cules 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.

Ratios vs. Differences

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.
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