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.

(9) Readers Comments

  1. What a totally confusing predicament this is turning out to be. What is Joe Consumer with a 5.1 AVR HT system supposed to do when he wants to use his system to watch movies and listen to some music and get correct bass levels for all. I hope this has a simpler answer than what I’ve grasped so far.

  2. I tend to credit George Lucas with recognizing there were issues with theater sound and hiring the right guy to find the solution. The real “father” of THX, both in theaters and homes, is Tomlinson Holman, the TH being a nod to his initials, the X indicating “crossover” (x-over) as well as eXperiment, and the whole moniker a reference to Lucas’s first scifi film, THX1138.

    Holman also gets credit for naming 5.1 at a SMPTE conference on multichannel digital audio in film, the .1 being “marketing rounding”. Dolby and DTS worked out the codec to pack it all on film and optical media.

    I haven’t seen an LFE peak limiter in an AVR in a decade, even THX dropped the requirement, though I personally use an external device for that purpose. That peak limiter function would be best placed in the powered sub amp and respond based on real XMax parameters, but that’s not done.

    At the IAMM conference in LA in 1997, producers were invited to submit multichannel demo material produced in their choice of channel plans, either 5.1 with diffuse surrounds, 5.1 with direct surrounds. The multichannel mixes were demonstrated against the 2 channel stereo mixes. In the submitted multichannel mixes, the LFE channel was mostly used, except for several classical submissions that frankly wouldn’t have demanded it anway, though classical seems to be where the LFE gap is. There were several submissions in other genres that included plenty of LFE useage. The live demo done at the conference included LFE usage.

    • Thanks Jim…I’m on the hunt.

    • Ah yes, I remember going to the release of the first Star Wars film and hearing that Starfighter fly from the back to the front of the theater, seemingly over our heads. My jaw was on the floor along with everyone else. As we left the theater people couldn’t stop talking about the films sound effects.
      Thanks George and Tom, not only for a great film but for bringing Joe Average’s attention to great sound, in many cases for the first time in their lives!

  3. Looking forward to demystifying what, when, why and how much to send to the LFE/.1

  4. You can tell that customer that if he wants to hear real bass from a DVD-Audio disc, get the DVD-Audio of “Soul Rebels” by Bob Marley and the Wailers. They defiantly did not leave out the bass, listening to it as I write this comment, plenty of bass that will show off his systems bass handling.

  5. Dr. AIX,

    If you are using five B&W 801’s, why would you use LFE? Wouldn’t five 15″ woofers top a single sub of just about any size? Certainly the woofers in my full range stereo system can blow out candles in the room (possibly a slight exaggeration, but only slight).

    • There is energy below the B&Ws and it makes a subtle difference.

  6. Modern AVRs allow listeners to send however much BASS they desire to the subwoofer, regardless whether there’s a native subwoofer channel recorded onto the disc or not.

    My Yamaha RX-A1000 even has a mode called “extra bass” or somesuch that allows simultaneous full-range bass AND a low-pass to the sub: DOUBLE TROUBLE!

    So boost the subwoofer a full15 dBs thru the AVR, twist the subwoofer knob to 11 and break that mutha off — and now you’re pumping bass sufficient to induce vomiting all ’round the neighborhood. Hope you know a good lawyer!

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