Dr. AIX's POSTS — 05 March 2015

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There was no such thing as bass management when I was putting together my first stereo system. The woofers in my Electro Voice speakers were expected to handle the low end of my vinyl LPs without the assistance of an additional subwoofer dedicated to the lowest portion of the frequency spectrum. It wasn’t until the 1980s that companies like M& K (Miller and Kreisel) and Triad Speaker started pushing for separate low bass speakers…the subwoofer. Today, most home theater systems and many music systems employ a subwoofer. But making sure your subwoofer is getting the right signal AND outputting the right amplituce of bass depends on whether you’re enjoying Hollywood blockbuster movies in your theater or listening to surround music.

We can thank George Lucas for upgrading the quality of sound reproduction in movie theaters and at home. His THX standards required theaters meet certain performance specifications, speaker arrangements, and power. The home version of the THX standard expects each speaker to output 105 dB of bass in the sweet spot. Adopting the THX standard meant you could count on getting the same kick in the low end as you experienced in the theater. However, getting that level of performance out of the average home theater speaker was asking a bit much and so the low end from all the main speakers was redirected to a dedicated subwoofer. This strategy worked. The main speakers took care of the midrange and high frequency materials and the sub dealt with the lowest portion of the spectrum…generally from about 80 HZ on down.

The whole issue of bass management was finalized when the audio specifications of the DVD-Video format were being considered by the DVD Forum in 1995. Both Dolby and DTS developed 5.1 surround sound encoding and decoding systems for the new format. Dolby won that battle as a required format and DTS was given optional status. The new 5.1 channel digital formats included an additional channel called the LFE channel. This Low Frequency Enhancement channel was supposed to handle the high amplitude, low frequency sound effects found in the movies. When an earthquake was destroying LA on the screen, the subwoofer speaker would produce the needed 115 dB of bass to match the visual.

This took care of the needs for the Hollywood moviemakers but it didn’t address the needs of music mixers. Just how are we supposed to use the additional .1 channel? It turns out that many music mixers don’t send anything to the sub. I listened to the Daniel Barenboim DVD-Audio disc of the Beethoven Symphony #9 last weekend and noticed that the engineer decided not to use the sub at all. I can remember getting an email from a potential customer lamenting that fact. He wanted to know if my Beethoven Symphony No. 6 used the sub. I assured him that it did. He was pissed that his fancy home theater rig with its impressive subwoofer wasn’t involved when playing back music. He felt cheated somehow.

Bass management is employed in most modern A/V Receivers. The system filters the signals that are going to the main speakers using a LPF (Low Pass Filter) coupled with a HPF (High Pass Filter) and peak limiter for the subwoofer output. The bass frequencies from the main speakers are redirected to the subwoofer.

To be continued.

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About Author

Dr. AIX

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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