The trend toward small, easily concealed main speakers has fueled an increase in the use of subwoofers. Compact main speakers, and in-wall or in-ceiling speakers, are often severely lacking bass with most that can only hit as high as 120 Hz. Large, costly floor-standing speakers often have plenty of bass, but they may be cosmetically unacceptable. Even so, most of them cannot reach frequencies below 32 Hz without having a discernable low frequency roll-off. Also, the speaker room placement that results in the best stereo imaging is almost never the best placement area for reproducing deep bass. Because of that, in order to have a full frequency bandwidth reproduced faithfully in your system you will always use a subwoofer.
A subwoofer is a dedicated low-frequency speaker cabinet, usually self-amplified, that fills in the deep bass that is missing from most main speakers. Subwoofers are specialized and do not (and should never) reproduce midrange or high frequencies although exceptions are certainly there such as Bose bass module (notice that Bose do not call it "subwoofer" but "bass module" instead) which tend to go upwards to and beyond 120 Hz. They are designed to reproduce the deepest notes in the musical spectrum and low frequency energy provided by music recordings and (most importantly) home theatre soundtracks. Most main speakers are passive (unamplified).
On the other hand, most subwoofers include a power amplifier, creating the raw power needed to cleanly generate deep bass without robbing excessive amplifier power from your receiver. An ideal subwoofer usually handles frequencies below 80 Hz or so...the lower the cutoff point, the better. A quality subwoofer not only faithfully reproduces the deep bass notes with authority but also the LFE (Low Frequency Effect, the .1 in 5.1 / 7.1 system) with oomph and gusto. Its presence in the system also allows the receiver/amplifier to reproduce mid-bass and mid-range notes more cleanly. The burden of reproducing deep bass, the most difficult and power hungry frequencies to reproduce, is lifted. Your amplifier and main speakers work more efficiently, with far less distortion. All kinds of instruments, including voices, can be heard more clearly.
Why should you add at least one great subwoofer to your system?
1. Deep bass adds the emotion, the physical feeling of presence and excitement, to a good music or movie performance! One reason that inexpensive speakers sound "just OK" is that they cannot reproduce deep bass very well.
2. Subwoofers help solve problems in speaker set-up and placement. It's difficult to set the main speakers where they are able to provide realistic imaging, and set them in the best place for bass. A separate subwoofer can be placed where the bass sounds best, with the main speakers set up to provide the best image. In-wall and in-ceiling speakers can be placed where they work best cosmetically, without concern for their bass reproducing ability, when the system has at least one great subwoofer.
3. High-quality subwoofers are engineered to generate the massive cone movement required by low bass reproduction. Main speaker drivers are often taxed to their limits, and beyond, by the demands of deep bass, and audible distortion results. A good subwoofer is specifically designed to generate deep bass. Whether you like jazz, classical, pop or rock, a good system can be made better with a good subwoofer. Regardless of your musical preference or taste in movies, a good subwoofer with professional-grade technician to properly set it up can turn a good listening experience great.
Types of subwoofers
There are two main types of subwoofers: sealed and ported, and the former may utilize passive radiator driver(s).
1. Sealed boxes: For deep, fast, precise bass
A sealed box is an airtight enclosure housing your subwoofer. A sealed box is best for any music that demands tight, accurate bass. Expect flat response (not excessively boom-y), deep bass extension, and excellent power handling. Since a sealed enclosure tends to require more power than a ported box, they tend to use a built-in amplifier with ample wattage for optimum performance.
2. Ported boxes: For "bigger" bass
Ported boxes use a port to reinforce low bass response. You'll get more output than you would from a sealed box at any given amplifier wattage. Some people prefer the sound of ported boxes for rock, heavy metal, or any hard-driving music. Ported boxes can deliver deeper bass than sealed boxes, though they need to be much larger than sealed enclosures to accomplish that.
As previously stated, either ported or sealed subwoofer may employ passive radiator driver(s). A passive radiator speaker is a simple device that increases the low frequency response of a speaker system (in this article's context: a subwoofer). When used properly, a passive radiator can give a speaker system the comparable performance characteristics of a much larger speaker driver in a smaller enclosure; that's the point, in a nutshell.
A lot of air needs to be moved in order to produce audible bass frequencies. Traditionally, when it comes to bass production, a larger woofer meant louder, clearer bass. Passive radiator technology has broken this tradition. From then front, a passive radiator looks like a normal speaker driver, but on the backside, it had all of its "guts" removed. A passive radiator is a speaker without the magnet, and electronic structure attached to it; it is just the cone, suspension, and frame. Really though, a normal speaker can also be used as a passive radiator, the "guts" would just be superfluous and bringing up manufacturing cost for no reason whatsoever.
Therefore essentially, a passive radiator is a reactionary device as the name suggests. When a driver (such as a subwoofer) is mounted to a sealed speaker box (enclosure), the physical forward/back movement of the speaker affects the internal air pressure of the enclosure. When a passive radiator is mounted to the same speaker box, the internal air pressure fluctuations (caused by the movement of the driving speaker) causes the passive radiator to begin moving forward/back as if it was also a driven speaker. When the passive radiator moves, it creates sound frequencies just as a normal driver does. In fact, passive radiator systems can have the same sonic output as larger speaker cabinets, but in a much smaller size.
Subwoofer Myths, Confusion, and Delusions
Now that we understand a little bit about bass management, it's time to dispel a few myths and examine common areas of confusion that relate to bass management, the LFE channel, and bass reproduction.
1. Low bass is NOT directional. If I had a penny for every time someone has told me they can hear bass directionality down to 20 Hz, I would not be writing this article as I will be driving up and down to my favourite Dollar Store while driving in my Lamborghini Aventador Tri Colore Special Edition. Yes, we can hear the overtones of bass instruments above 120 Hz, and those overtones should definitely be played by the main speakers correctly located in the room for proper imaging. However, we cannot-I repeat cannot-localize bass below about 80 Hz. This is a simple, basic, entry-level thing of something called psychoacoustics.
The most important thing we can do with non-directional bass frequencies is to produce them in locations in a room that don't favour strong coupling with standing-wave resonances. These resonances turn a bass punch into an event that takes two seconds to decay. Now that is bad, that is mushy, slow and sometimes can (literally) be puke inducing. Attempting to play stereo bass in a listening room without regard to 2-second-long bass resonances, which completely swamp any sense of separation, is either pure ignorance or pure idiocy! That is NOT only my theory; other psychoacousticians (if there is such word) who had done the tiniest bit of scientific field testing will draw identical own conclusions.
2. Full range speakers are NOT an excuse for eliminating bass management. In order to render bass management unnecessary, each full range speaker must have at least a 15-inch woofer (or dual 12-inch woofers) with 200 watts of power driving it. Anything smaller will simply unable to do a flat full frequency range down to 20 Hz. Also realize that each one of these elephantine speakers must be carefully placed so its interaction with the bass resonances of the room is optimized. As if the task of placing full-spectrum speakers in a room is not daunting enough, imagine the near-impossibility to place five speakers (or more) in a room so that each speaker identically loads bass resonances. The result of improper loading is grossly uneven bass. Some speakers will be cranking out thick, boomy and inarticulate bass, while others seem downright anemic and lacking.
The right way to handle this dual problem is to implement bass management and strategically place subwoofers in the room for optimized resonance loading. Research has shown that some locations in a room yield predictably good results after all is said, done, and installed. One of the best layouts is four subwoofers in a cross pattern
3. The LFE channel is NOT a "subwoofer channel." In fact, there is no such thing as "subwoofer channel". The LFE signal should be thought of as a path for super-loud extra-bass that would otherwise overload the main channels. Sound designers use this path when the main channels just can't put out enough bass to rock the house. Remember that in (properly set up) movie theatres the volume control is fixed at a reference level. At that volume setting, the peak sound pressure level from the recorded medium should be 105dB in the listening area. In the mid-frequency range, 105dB is good and loud, but in the deep bass region, it just isn't enough to get the impact we all expect from big A-list titles when the hero characters wielding limitless firepower.
To get real chest-pounding bass, we need to get up to 115dB. The main channels are missing 10dB of headroom, and that's where the LFE channel comes to the rescue. With 10dB of extra headroom, it can really get a person's body bouncing around in the seat. LFE is only used during high-octane action with lots of bass; during the rest of a movie, the LFE channel has no content. The LFE channel should only be fed directly to subwoofers in any systems. Even for an exceedingly large home theatre with gigantic main speakers that could theoretically run the LFE channel should still be using a dedicated subwoofer for the LFE channel.
4. There should be ONE, only ONE, and nothing but ONE audio connection between an AV controller and powered subwoofers. Some AV controllers offer an LFE-only output in addition to the subwoofer output, and some subwoofers offer multiple line-level inputs. Controllers with both LFE-only outputs and subwoofer outputs may tempt us to connect a separate subwoofer to each output and run one subwoofer for the LFE channel and another one for summed main-channel bass. This is an exceptionally bad idea in most cases, because we need both subwoofers playing the same thing, working together to cancel bass resonances. The most effective use of two subwoofers is to have them play the sum of the bass from the main channels and the occasional LFE hit. Bass character stays consistent throughout the movie, and the subwoofers are used to their full potential.
This brings us to subwoofers with multiple line-level inputs. The concept of a subwoofer with multiple inputs that have various filtering and summing functions makes no sense in light of our discussion of bass management. In a multichannel audio system with bass management, a subwoofer needs one input, a polarity switch, a power switch, and that's all folks! Those other things-diverse inputs with misleading names, a stereo summing input, a low-pass filter, a volume control, etc.- are only useful in a stereo system without bass management and just add to the cost and complexity of a subwoofer. It is the job of a custom installer, an audio calibrator or end users to search out the right input for the bass-managed feed from a controller, and that's almost always an extremely daunting task. This is the reason why most two-channel systems do not use subwoofer. It's never about the "purity of sound" or whatever excuse (ehm...I mean, "reasoning") audiophiles want to use, it's about the complexity and its near-impossibility for the DIY audiophile without enough knowledge and proper tools to set a subwoofer properly. There is no such thing as an ideal placement of subwoofer unless you can literally put your head inside the subwoofer cabinet itself. I'm not joking. The best placement for a subwoofer in any audio set up, two-channel or otherwise, is in your listening position, at your ear level. Therefore any other place other than having your head inside a subwoofer will require highly accurate distance, phase, time alignment and EQ.
5. There are NO clear rules governing the bandwidth of the LFE channel. The production statutes applied to LFE channels are so varied that determining the upper LFE cut-off frequency for a playback system is often an exercise in futility. Some LFE channels contain no content above 50Hz, while others (usually due to an error in mastering) are full range! The generally accepted safe approach (and as per THX recommendation too) on the playback end is to low-pass-filter the LFE channel at 80 Hz. However, some AV controllers, albeit seldom, do not apply a low-pass filter to the LFE channel at all, meaning that highly directional bass on some recordings could potentially be produced by the subwoofers. Beware of these controllers, as there is usually no way to add an external low-pass filter without serious repercussions to the main-channel bass.
6. The ratio of the LFE level relative to the level of the bass from main channels should NOT be adjusted in the AV controller. The ratio should be such that LFE signals are 10dB louder than signals of equivalent level in any other channel. A few early DTS music releases contained LFE channels that were 10dB louder than the industry standard. For this reason, some controllers include a DTS music mode that reduces the LFE channel by 10dB. There is no other logical reason to adjust the level of the LFE channel separately from the main channel bass. Doing so irreparably alters the mix intended by the sound engineer.
7. Although there is a need for subwoofer for music applications, there is NO NEED for an LFE channel in the vast majority of music applications. There continue to be multichannel music recordings released with content in the LFE channel when the bass in the main channels isn't even close to overload. Inexplicably, some music-recording engineers think that they must put something into the LFE channel so that end users will hear sound coming from their subwoofers. This is wrong wrong wrong wrong WRONG!!! There is no LFE in music (unless it's being used as effect). YES you will still need a subwoofer to deal with the lower frequency region when the two-channel system can not reproduce a straight 20 Hz to 20 kHz spectrum, but NO you do not need a subwoofer to deal with LFE.
Frankly, that's terrible logic because the subwoofers in bass-managed systems (which represent the overwhelming majority) receive the LFE channel and the sum of the main-channel bass. Users and installers of multichannel systems don't really need to worry about a music recording with an LFE channel as long as they set up their systems correctly with bass management. At times, however, bass-managed playback systems dig up bass that recording engineers didn't hear because their monitoring systems weren't bass-managed and their monitor speakers weren't full-range. This unmonitored bass sounds plain weird and there's absolutely nothing users can do about it without reconfiguring their systems every time they switch discs. Alas, having been working within the recording industry for more than a couple decades, I keep finding that almost all mixing suites do not have a properly set up (let alone bass-managed) monitoring system. Sigh.
I do not know how to end this article, I can write and write and keep writing for weeks and this topic will never end. Even to bring my relatively limited knowledge in psychoacoustics on to a 10-page story (from the original draft of 40 pages) took me two weeks to do so. One thing for sure, however, all equipment will need at least a subwoofer with up to (ideally) four subwoofers (one in each side, but never in any corner). To set it up to make your system better? Unfortunately you can not trust your ears. It requires measuring equipment, parametric EQ, time, patience, and skill as setting up a subwoofer is not a matter of what you like but a matter of right or wrong relative to your current set up.
This article originally appeared at David Susilo Uncensored.
David Susilo is an ISF, THX Certified Professional, CEDIA Certified Instructor, CEA and HAA member with a PhD in Psychoacoustics who has been doing theatre design internationally (both home and commercial) for more than 20 years. He has more than 4,000 projects under his belt.