The Importance Of Subwoofers In 20 Hz - 20 kHz Playback
Dear Gary,
Congratulations on your terrific magazine. It is a vanguard of bright white light in this too often quagmire landscape of sound and picture that we all love and work in. In reference to the Home Theater Cruise® Recap article in the March 2006 issue, I would like to add a different point of view not presented by my Nashville colleagues and the other esteemed panelists. That is, the view of a professional who designs and custom-tunes recording studios and home screening rooms on a day-to-day basis. Although there is much to be discussed here, I will limit my comments to the following.
The successful use of subwoofers in a studio or screening room is based on one thing and one thing only––accurate 20 Hz to 20 kHz frequency response––period. A simplified description of the proper procedure entails shooting an individual loudspeaker with highly accurate analyzing software from the listening position and documenting it to see what its frequency response is in the room after careful placement relative to the listening position, boundary effects, and axial modes of the room. If the loudspeaker shows wide dips in the low-end, which is very common even in the best of acoustic environments or is not a full-range loudspeaker, then the subwoofer can complete the spectral picture by correspondingly filling in the holes and/or extending the low end. The subwoofer is then shot and results documented, and then the loudspeaker and the subwoofer are shot together so that phase and levels can be determined. Once this is achieved, 360-degree all-pass phase filters and high-resolution parametric equalization are carefully implemented to correct frequency/ phase anomalies. (Yes, your loudspeakers are not producing an accurate frequency response because the room has caused phase shift in your loudspeakers, and surprise, surprise––good parametric equalization corrects phase shift. Yeah!) This is repeated for the rest of the loudspeakers in the system whether stereo or 5/7.1. Generally the loudspeakers will be run full-range in both scenarios. The choice of low pass filter setting for the subwoofer will be determined by what is needed to complete the loudspeaker’s needs in the room. However, high passing the loudspeakers can be valuable for the same reasons. Rolling off a loudspeaker, even one that is full-range, can occasionally be desirable to avoid cancellations in the low end if it is fighting with the subwoofer. Depending on the acoustic symmetry of the room and its relative position to the listener and subwoofer system (always two for studios, not for stereo but for phase control and ideally four for screening rooms), each loudspeaker will require its own individual phase and frequency consideration. The result of course, is that you now have two (stereo) or seven (7.1) accurate 20 Hz to 20 kHz loudspeakers. Any other argument about bass redirection or use or non-use of the LFE channel in a mix becomes quite frankly nonsense once this concept is grasped and experienced. This protocol, which seems so obvious and basic, continues to be ignored by the pro audio and home theatre community––until they hear it for themselves and have their own aural epiphany This same system will be used both in the studio for mixing and at home for listening.
Carl Tatz, Nashville, Tennessee
mailto:carl@carltatzdesign.com
Editor-In-Chief and Publisher Gary Reber Comments:
Thank you for providing more clarity to the argument for full-range 20 Hz to 20 kHz frequency response, as the optimum choice, in both stereo and multichannel playback systems. Your explanation is the approach that we have used at Widescreen Review for years and have written about extensively, especially in relation to the building and fine tuning of our Reference Holosonic™ Spherical Surround™ Home Theatre Laboratory. It is an approach that is used in nearly all of our reference systems, as diagramed in the magazine and on our Web site.
Thank you for pointing out that all measurements and fine tuning are based on the relationship of the listening position (or “sweet spot” as I have termed that position) relative to the loudspeakers positioned in the room.
I was quite surprised that the mixing engineers on the “Setting A New Surround Sound Standard” panel were not more appreciative and pro-active in terms of desiring a true full-range playback response and acknowledging that there is always an optimum listening position in the room when setting up a stereo or multichannel system.
You can E-mail Widescreen Review @ mailto:editorgary@widescreenreview.com