AES Section Meeting Reports

Pacific Northwest - March 30, 2016

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The PNW Section presented its March meeting on "The decibel revisited", a review and tutorial, for both beginners and those of us who think we understand. It was held March 30, 2016 at Shoreline Community College in Shoreline, WA. About 14 AES members and 16 other attended.

Our presenter was Mark Rogers, PE, and current AES PNW Section Committee member. Mark is Director of the AV Department at the Greenbusch Group, a Seattle engineering consulting firm. He is a designer of audio/visual systems, including sound reinforcement, audio reproduction, video projection and displays, videoconferencing and audioconferencing, and related control systems. Typical projects include corporate boardrooms, convention centers, universities and hospitals. He has been designing and installing AV for over 45 years, and also teaches classes and seminars on AV technology. He is a registered Professional Engineer (Washington and Idaho) and earned his BSEE at the University of Idaho. He is a past Vice Chair and Committee member of the PNW AES Section and has presented several topics to the section.

Section Chair Chris Deckard opened the meeting with news, and had the audience introduce themselves.

Mark began by describing his firm, the Greenbusch Group, before launching into the Decibel, revisited (We have presented similar meeting on the dB before. It never gets old). What is a decibel? Several attendees gave answers. Mark went through several definitions from some books, some not so good. Rane's is good. In the end, to be accurate, its a power ratio. The dB can quantify the strength of many phenomenon like sound, light, radio waves, and more; its based on human hearing and the logarithmic aspect allows working on a huge range with reasonable math.

He covered some history. The phone company started using their Standard Cable Mile (SCM) measurement in early telephone days. Harvey Fletcher's research started using the "SU" (Sensation Unit) about that time. W. H. Martin of Bell Labs used the "TU" (Transmission Unit) instead of the similar SCM/SU, and by 1929 was calling the TU the deci-Bel, after A. G. Bell.

Mark used some quiz questions and True/False "correct usage or incorrect usage" examples to illustrate just how pervasion incorrect usage of decibels is, even among reference books and manufacturer's data.

Yes, a decibel is 1/10 of a Bel. Yes, with power the formula is 10 log(p1/p2) and with voltage/current and equal impedances, the formula is 20log(v1/v2). Mark showed/derived the 10/20 difference, usually glossed over by others. While the dB itself is dimensionless, when you establish a reference, you do use it as a dimension. An example is 0dB SPL, defined as 20uPascals. And there are more agreed upon dB references than you can shake a stick at, such as dBm, dBA, dB SPL, dbv, dBV, and many others.

One pet peeve is that Pro/Consumer I/O level names of the +4/-10dB is not 14dB different, it's about 12.8, because +4 is usually +4dBU and -10 is usually -10dBV (different references).

Mark discussed how much louder is 10dB or 3dB. Twice? What does that mean? And twice the power is not exactly 3dB. He also introduced the concept of decilogs (excludes impedance).

After an intermission, Mark discussed the Fletcher-Munson curves, the classic experiment that derived equal loudness curves of a set of human subjects. Human hearing is not flat and varies with level. There was an extended discussion of Fletcher's experiments, and newer versions of the experiment still agree well.

How much loudness change is perceptible? It varies. Many of us audio people can hear 1dB difference under good conditions quite easily. Mark played pink noise at various preset levels to hear 3, 6, 10 & 20 dB differences. This brought a discussion of the concepts of "twice as loud" and so on.

There was a discussion of weighting curves, A-wtd, phons, and standardized listening levels for mixing, cinema and home theater.

Mark had a story of noisy industrial lamps/ballasts, and problems seeing similar numbers and curves on instruments, but hearing massive differences. A recorded example by Bob Smith was played.

Lastly, Mark recommended some books and took Q&A.

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