Why revisit a topic that everyone knows all about? This was soon answered at the PNW Section's February 2003 meeting, where over 35 attendees gathered at Opus 4 Studios in Bothell, WA. The most knowledgeable person can get lazy and misuse and abuse the poor decibel, often leading to more confusion with the less knowledgeable. Even manufacturers have been known to not use the decibel correctly. Therefore, the PNW Section felt a rigorous review was an excellent idea. Hopefully, the facts would be reestablished, and the fallacies exorcised. Mark Rogers, a registered engineer for consulting firm The Greenbusch Group Inc. of Seattle, and a PNW Committee member, presented the tutorial.
Mark "Powerpointed" the group through a review of the correct definition of the decibel, its history, and correct usage. Even the standard references such as the IEEE dictionary do not make it easy to form a concise and accurate definition.
Mark described the early telephone history. Prior to 1923, telcos used a "standard cable mile" (SCM) characteristic to figure signal losses in telephone wiring. in 1923, W. H. Martin of Bell Labs defined the Transmission Unit (TU) to replace the SCM as well as Harvey Fletcher's SU which he defined as a minimum perceptable change in sound level. By 1929, Martin called the TU the decibel, as one tenth of a unit named for Alexander Graham Bell.
It was noted that we use the decibel to quantify the strength of many things such as electricity, sound, light and radio waves. It is based on human hearing, and is logarithmic so as to accomodate large ranges of levels. But first and foremost, the decibel always describes a ratio of power. How then, do people use dBs for voltage? Mark wrote out the simple algebra that, of course, zeros a term of the equation if the impedances of the two powers are the same, something that is usually omitted in texts, giving the familiar 20log(v1/v2) equation. A review of common decibel forms and their proper usage included dBm, dBu, dBV, dB-SPL, dB-A and others. Most people still can't use or write many of these correctly, and they have sometimes changed over the years. It was reminded that the difference between "+4" and "-10" audio levels was not 14 dB, but about 12.8 dB, as +4 usually is referenced to 0.775 volt and -10 usually is referenced to 1.0 volt.
Acoustic decibels were discussed, and a quiz included many examples of true/false decibel questions and correct/incorrect usage scenarios. Demonstration equipment was used to show how changes of 1, 3, and 10 decibels sounded.
The evening was a reminder that even the simplest thing we use daily can be reviewed every so often to keep us honest and accurate, and that there are always those learning the profession that will benefit from reviews of the basics.
Reported by Gary Louie, AES PNW Secretary