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March 2000 How Important Is the LFE Subwoofer to Home Theater?This, And Other Questions About Home-Theater Bass Years ago, I was convinced that main left and right speakers that went down to 25Hz, or which had subwoofers that went down that low, were all you needed for good home-theater sound. Then Dolby Digital came to town with 5.1 sound that had much more complex bass-management issues. With today’s 5.1 sound incorporating an LFE channel, I have swung the other way. You really can’t get all the thrills of home-theater surround without a good LFE sub…even if you already have a pair of subs on your main left and right speakers. Who’s on first? There are a number of reasons for the LFE sub being a key to good home-theater bass. First there is Dolby Labs themselves. In an interview with Dolby Labs personnel, I honed in on the issues around bass management in Dolby Digital 5.1 decoding equipment. I knew that many products with Dolby Digital decoders were not following Dolby Lab’s rules for bass management, yet they were still able to apply the Dolby Digital logo to their products. I formed some questions around this subject and learned the disappointing truth. Dolby Labs does not insist that their "rules" or "standards" have to be met in order for the manufacturer to apply the Dolby logo to their products. Dolby Labs admitted that many (I would say most) products are not managing bass properly. Dolby Labs indicated that they will explain to the manufacturer what the problems are with bass management in each product Dolby Labs evaluates, but Dolby Labs will not stop the company from selling the product as is, nor will Dolby Labs withhold permission to apply the Dolby Digital logo. The situation sounded to me like Dolby Labs would tell the customer (manufacturer) where the problems were and to take care of it in the next generation product. However, when the next generation product shows up, if the bass-management problems still exist (or new ones appear), Dolby Labs performs the same "notification" service but still does not require compliance with any bass-management standards. There is a small penalty for using an LFE subwoofer though. When you turn LFE on something else happens in the decoder circuitry that causes the midrange and highs to loose air and to sound slightly squashed, gray and dry compared to what you hear when LFE is off. This maddening situation may not exist in 100% of surround processors, but it has been present in all that I have heard so far. The problem arises from the fact that LFE on/off is not just turning LFE on or off. The bass management circuit takes a little bass from the main speakers and off loads it to the LFE sub whenever LFE is on -- even if you have selected "large" main speakers. This is actually not a bad idea, but it needs to be executed in a better way than it is currently done. I find the effect bad enough that I generally prefer to drive the LFE subwoofer from the analog outputs of the DVD player rather than from the LFE output of the receiver or processor. This won't wok for DTS discs unless you happen to have one of the few DVD players with on-board DTS decoding. Another bass management problem can come when you have LFE off. The LFE bass should be redirected to large main speakers at -6 dB so that the 2 main speakers produce bass at the intended level. Many processors fail to reduce the bass level when LFE is off which results in just about double the intended bass loudness in the room from your large main speakers. What’s on second? So you’re at home with no LFE subwoofer. You go to the setup menu on your Dolby Digital receiver or surround processor. You select "No LFE subwoofer" on the menu. You expect that the bass will go to your large main speakers, which you have identified as "large" to the setup menu. You sit back thinking you are getting all the bass; all the sound that was originally in the movie save for that last octave from 20Hz to 40Hz that your main speakers just can’t do. Shouldn’t be that big of a loss, right? Wrong! Many if not most receivers and surround processors, just throw away the LFE bass when you select "no LFE subwoofer." This means there is sound that is thrown away that your large main speakers never have even a chance of reproducing. Other bass management problems include what to do with deep bass in the surround speakers. Most setup menus let you select "large" surround speakers or "small" surround speakers. If you select "large", deep bass is sent back to the surround speakers. If you select "small", the bass that would have gone there is supposed to go to the LFE subwoofer, according to Dolby Labs specifications. Unfortunately, many (most?) receivers or surround processors just dump that bass into nothingness. Even if you do have an LFE subwoofer, the bass that is intended for those rear channels might just evaporate somewhere inside the surround processing circuit, never appearing at the LFE sub. If you have small main speakers, there may be a setup menu choice to identify these speakers as "small." If you do that, Dolby Labs says that bass should be redirected to the LFE channel. Again, unfortunately for us, many (most?) receivers and processors do not perform that redirection, and the bass that should go to the main speakers also evaporates inside the processor somewhere. The main problem with lack of control of bass management is that consumers don’t really know that errors are being made. They sit at home in their home theater thinking that they are getting some hot sound. Yet they could be missing all or most of the fun of movies because it is easier for manufacturers to ignore proper bass management than it is to do the bass management correctly. "I don’t know" is on third! So what’s the home-theater enthusiast to do? How do you tell who is and who is not managing bass properly? My rule of thumb is: Assume the processor is not following the rules and that you will lose bass unless you have large main speakers, a large center-channel speaker, an LFE sub and large surround speakers. Try to equip your home theater that way and you’ll get all the bass regardless of whether the processor/receiver manufacturer did their job right. Unfortunately, this is a very expensive proposition. Good full-range front speakers are probably going to cost at least $1500, with $2000 not being outrageous. A good full-range center channel is almost impossible to find, but count on $800 to $1000. The pair of full-range surround speakers…$1500 or so again. Add a good LFE subwoofer for at least $800 and you are looking at a minimum investment of $4600. That’s enough to take the breath away from the majority of consumers. The reality of your budget may require that you get a smaller set of speakers. How can you do that and still end up with a minimum of bass loss? I don’t know yet. There is no easy way to tell if a processor is distributing bass properly or not. Perhaps one day we will have a 5.1 test DVD or CD which exercises all channels with deep bass information so we can find out if all of it gets properly redirected to the LFE subwoofer (if you have smaller left/center/right & surround speakers). You can tell, if you live with the surround processor or receiver for a few weeks and do a number of experiments. But just going into a store and selecting a product that does manage bass correctly…I wish I could help. If I find a way to do this in the future, I will definitely publish it here. So how much am I missing without the LFE subwoofer? Having just watched The Mummy with and without an LFE sub, all I can say is "hahaha ahahahaha hahahahahaha ahahhhahahah." Uh, sorry. You really need the LFE sub. The foundation, shock and solidity of many sounds come from the LFE channel. With no LFE sub and lame bass management, you end up with a shadow of the intended effect. You get emasculated, uninteresting sound. You may never realize to what degree the sound is gutted by not having the LFE sub until the first time you install one. The listening experience jumps up a good 3 or 4 steps with a decent LFE sub. Where do you get a decent LFE sub? Well, that’s the question isn’t it? Subwoofers are probably the hardest single speaker or speaker type to make successfully. Anybody can put a big bass driver in a box and end up with a satisfying boom…at least for the indiscriminate listeners. But we want quality bass with that home theater, please! Where to look? Cheap woofers almost always mean really bad bass. Good bass requires big subwoofer size. Little-bitty cube subs may please the spouse, but they can be so light in weight that they jump across the carpet during big bass transients. Furthermore, the switching power supplies used to save space and weight inject rather large amounts of medium to low frequency noise on the power line. You may believe that your little cube subwoofer sounds great, but until you hear a really good subwoofer, you won’t realize how limited the small cube is. Small drivers with lots of extension and a switching amplifier don’t sound good when compared directly to a large sub with a large driver area and a conventional amp/power supply. To sound really good, your LFE sub needs at least 3 adjustments. The 4th is optional depending on the subwoofer:
Number 4 is optional because some LFE subs compensate internally, based on the roll-off present in the LFE signal from the receiver or processor, which is perfectly OK. If you accept an LFE sub lacking these adjustments, you won’t be satisfied with the sub for the long term, unless you are very, very lucky and it just happens to work well in your room/system without the adjustments. To get the most of a sub like this, set it up properly and connect it to the surround processor. Play a movie or music DVD or laserdisc with significant bass energy in it. Start with the subwoofer settings as follows: level set to "0"; phase set to "0"; normal polarity. - Swap polarity first. If the bass gets louder, leave the polarity swapped permanently. If there is no change in level, put the polarity back to "normal." - Next, adjust the phase control through its entire adjustment range and stop where the bass is the loudest. A helper is a big assist with this setting. - Next, set the gain control (perhaps in combination with the internal gain control in the receiver or processor). In the receiver or processor, you may want to explore what the maximum LFE volume setting is and set that level control to no less than 75% of the maximum LFE level setting. If this is too loud, reduce the level/gain control on the subwoofer to achieve an appropriate loudness level. That’s it. Your subwoofer will be optimally setup using this relatively simple procedure. If you don’t trust your ear to tell you when the bass level hits its highest level, you may want to invest in a sound-pressure level meter. Radio Shack sells two models, a $35 analog unit and a $60 digital unit that is a little more convenient to use, but no more accurate. Set the meter to "C" weighting and "slow" response. Set the scale to 70dB if you have the analog model, and you are all set to measure bass (or other speaker) sound pressure levels. Hold the meter angled upward at about forty-five degrees with the microphone on the end at the same height/position as the ears of someone sitting in the sweet spot. When the sub reaches 70dB and all the other channels are set to reproduce 70dB using the setup menu in the receiver/processor, you are ready for a great movie experience. A sound pressure level meter will add considerably to your surround level setup abilities and will probably convince you of its worth the first time you use it to correctly set surround levels in your system. Isn’t the LFE channel just a lot of ill-defined rumbling and explosion-type bass? That’s what I used to think. But in the last few years, filmmakers have turned the LFE channel into something else. Car doors slamming get a little "whump" from the LFE channel in better soundtracks. This gives impressive body and heft to the door. You can even "feel" the difference between the door of a BMW 7-series and that of a Chevy Lumina if the Foley artists did their job well. Apartment doors, large indoor or outdoor spaces, truck noises, Harley noises…they all get better. Even mundane noises like lawnmowers, garage-door openers, and blows to the head, get fleshed-out to a startling degree. It all goes back to the quality of the soundtrack and the attention to detail. Not every movie does this much in the LFE channel. We recently watched Election and the LFE channel sounded muted for the entire flick. On the other hand, the aforementioned The Mummy was a tour-de-force of proper use of the LFE channel. There was detail, texture, solidity, and conviction in the LFE channel. The bottom line…so to speak There is an excellent chance that you aren’t getting all the bass you could or should be getting from your home theater. You may not have the sub setup correctly or you may not think a sub is really needed. You may not have understood how bass works for surround and center-channel speakers. Finally, the equipment itself may not manage bass correctly. The information here should arm you with the basics to understand and win the Dolby Digital bass wars. GO TO
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