What’s in a decibel?

Sound happens in the atmosphere. The rarefaction and compression of air molecules, the minute rising and lowering of air pressure, causes our human eardrums to sympathetically vibrate and pass along nerve impulses to our brains. Audio microphones work pretty much the same way. They’re called transducers and convert sound pressure waves into electrical energy, which is passed along to preamplifiers and ultimately to amplifiers and speakers.

Volume or amplitude is measure in a standard unit called a decibel, named after Alexander Graham Bell (1847-1922), the inventor of the telephone and an early researcher in audio. The original unit was actually the Bel but subsequent acousticians needed something less coarse and divided the Bel into 10 subdivisions thus the decibel or its abbreviation the dB became the standard that is still in current use.

But decibels are not absolute measures of air pressure; they correspond to a ratio between two amplitude levels. When designated with an accompanying SPL for example 120 dB SPL, the amplitude or volume level becomes an absolute value.

Our ears can detect sounds that are very quiet and they respond to very loud sounds as well (although extended exposure to very loud sounds can be dangerous). The quietest sounds exist near what’s known as the threshold of hearing. In SPL terms, this threshold is referred to as 0 dB SPL. Every other amplitude level then becomes anchored to that reference.


So we have a range of values for human hearing that has 0 dB SPL at the lowest end and 140 dB SPL at or near the high end or the threshold of pain. This continuum is not linear, however. The decibel scale is logarithmic, meaning each doubling of the change in air pressure or volts when we’re talking about electrical systems results in 6 dB of additional sound level.

This logarithmic measurement system allows us to keep the numbers manageable. For example, a change of 20 dB is equal to 10 times more air pressure or voltage, 40 dB is 100 times more and 60 dB translates to 1000 times more. If we didn’t use this system the difference from the softest sound to the loudest would be 10 to 12 power or 10 followed by 12 zeros! Let’s stick with the logarithmic scale for amplitude levels.


Mark Waldrep, aka Dr. AIX, has been producing and engineering music for over 40 years. He learned electronics as a teenager from his HAM radio father while learning to play the guitar. Mark received the first doctorate in music composition from UCLA in 1986 for a "binaural" electronic music composition. Other advanced degrees include an MS in computer science, an MFA/MA in music, BM in music and a BA in art. As an engineer and producer, Mark has worked on projects for the Rolling Stones, 311, Tool, KISS, Blink 182, Blues Traveler, Britney Spears, the San Francisco Symphony, The Dover Quartet, Willie Nelson, Paul Williams, The Allman Brothers, Bad Company and many more. Dr. Waldrep has been an innovator when it comes to multimedia and music. He created the first enhanced CDs in the 90s, the first DVD-Videos released in the U.S., the first web-connected DVD, the first DVD-Audio title, the first music Blu-ray disc and the first 3D Music Album. Additionally, he launched the first High Definition Music Download site in 2007 called iTrax.com. A frequency speaker at audio events, author of numerous articles, Dr. Waldrep is currently writing a book on the production and reproduction of high-end music called, "High-End Audio: A Practical Guide to Production and Playback". The book should be completed in the fall of 2013.

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