The first thing to get into your head is the actual part list, so to speak, of a typical subwoofer.

Subwoofers often have different connection types than normal speakers. Most accept line-level connections, but there is the possibility that your subwoofer will accept a speaker-level connection.

Once you’ve got your head around the parts involved, there are several terms that just defy categorization. Still, you’ll want to understand these to the same degree that you have your head wrapped around plate amps and line-level connections.

When it comes to high-output subs, there simply is no replacement for displacement. What is meant by displacement? Displacement is the volume of air the driver can displace. Take the effective radiating surface of the driver (usually abbreviated in driver specs as Sd) and multiply it by the distance the driver can linearly move its cone (usually listed as the driver’s Xmax). This determines the volume of a cylinder that will be mapped out by the cone’s full range of motion. This is the Vd, or volume displacement, of a driver. The higher the volume of air that the driver can displace, the better.

Table 6-1 supplies the approximate Sd values of the most common subwoofer driver sizes.

As you can see, a 10-inch driver would have to move more than three times as far as an 18-inch driver to displace the same amount of air. Generally, bigger drivers can move more air. However, if the smaller driver can move more than three times as far as the 18-inch driver, the smaller driver works out just fine. In the end, don’t make assumptions about a particular sub based on the size of the driver or any other one factor. Listen to it and trust your ears.

How many times have you heard your buddy brag about the wattage on his newest speaker system? Probably a lot more times than he’s explained how that wattage actually affects the sound produced! High wattage doesn’t always translate directly to good sound, and you need to be clear on that before you spend thousands of dollars on a sub just because of its power rating.

Power with subs is something that is often very much misunderstood. Unlike Vd, more power isn’t necessarily better. There are large, efficient subs that can do significantly more with 100 watts of power than a little 14-inch sealed cube with far more than 2,000 watts. How many watts a sub needs depends on a number of factors; the most important of these are the sensitivity of the driver and what enclosure it is in.

Another point not commonly known about the relationship between a driver and power is that there are two types of power-handling limits. There is a thermal limit and a mechanical limit.

The bottom line is that you shouldn’t pay much attention to the wattage rating on a sub. The driver and enclosure combination might or might not be able to use that power in the first place. All that really matters are the frequency response of the sub, whether the power it has allows it to play as loudly as you need, and whether the distortion levels the subwoofer produces at those levels are acceptable.

One other item related to a subwoofer’s power that often is very much misunderstood is the volume dial on the plate amp. The position of this dial is not an indication of what portion of the subwoofer’s capabilities you are utilizing. It also is not an absolute volume control. For example, turning the dial to one-third of its maximum setting does not mean you have two-thirds of the sub’s potential left. This dial is a gain control, and it controls how much the input signal is boosted before it is amplified. This means it is entirely possible that if the input signal is hot enough, with the dial in the one-third position, the plate amp could be putting out all it can. You should use the volume dial on a sub plate amp for rough level matching to your other speakers, and nothing more.

A number of different types of enclosure designs are in common use for subwoofers. Each type of design and the variations within each design are called an alignment. They all have their pluses and minuses, and if you’re serious about getting the right subwoofer, you’ll need to be at least generally familiar with each.

When talking about subs, there is one law that everyone should know. This law has many variations in many other fields, and all center on the fact that to get one thing, almost invariably you have to trade something else. This most definitely is true of subwoofers. Hoffman’s Iron Law simply states that out of enclosure size (how big the box is), extension (how low a frequency the sub is capable of playing), and efficiency (how much power the sub needs to get to a volume you want) you can pick any two to control, but the third will be dictated to you. This means that if you want a small enclosure and good extension, you are going to need gobs of power. If you want great extension needing very little power to get loud, you are going to need a massive enclosure. Always keep this in mind when looking at subs: don’t expect the tiny, one-cubic-foot subs to produce mind-blowing, loud, low, clean bass. No sub manufacturer has figured out a way to beat physics. Loud, low, clean bass requires big boxes with big drivers (or lots of medium-size ones). Small sub-power requirements are simply too high to achieve the same results. It’s just not possible to build a driver that can take the kind of power a small box needs to produce loud, low, clean, effortless bass. There’s just no way around it.

In the continuing evolution of subwoofer speakers, the ability to play low (and lower-than-low) frequencies has become a blessing and a curse. It is a blessing because lower frequencies provide that room-shattering, earth-shaking bass that makes you believe a plane really is flying overhead, or that 10,000 horses really are galloping by. On the other hand, with increased frequency range, there is a tendency for the brainiacs to over-analyze the use of subwoofers and confuse the typical consumer.

As a prime example of this confusion, a number of fallacies seem to persist on the Internet about subwoofers. One of the silliest of these is the assumption that if a room’s dimensions aren’t large enough to contain a full wavelength of a frequency, the frequency can’t be played in the room. In other words, a subwoofer in a small room is not capable of playing those super-low, long-wavelength frequencies. Of course, this is false, and really rather absurd. Any frequency can be produced in any size room.

To prove this, a little math is in order. The speed of sound is roughly 1,130 feet per second. Based on this, Table 6-2 shows some typical low frequencies and their resultant wavelengths.

As you can see if frequencies could play only within rooms as large as their wavelength, home theater would be a terrible disaster! How many homes have a theater room (or any room!) more than 70 feet long or wide? If this myth were a reality, very few rooms would be able to produce bass to the last octave, car stereos would seem anemic, and headphones simply couldn’t work.

Sound is just varying air pressure over time. As a sound wave passes by you, the pressure changes and you hear the sound. The driver of your speaker will start the wave propagating, and once it passes your ear, you will perceive the wave as sound. Not having enough space for the entire wave to exist in a straight line doesn’t matter; these pressure waves will bounce off walls, no matter what their frequency. So, in the case of higher frequencies that have very short wavelengths, many full wavelengths will pass by your ear before you start to hear the first reflections. But with waves longer than room dimensions (such as bass frequencies) it’s possible that the beginning of the wave will pass your ears twice before the end of the wave reaches it once. This doesn’t mean you won’t be able to hear the sound; however, it does mean you won’t be able to localize the source of the sound (there is more to localization than this, but this is one of the factors that makes it difficult to localize low frequencies).

Where you place a subwoofer in your room is just as important to the sound of your system as where you place your speakers. Oddly enough, though, one factor that you won’t have to worry about is the direction that your subwoofer is facing.

Subwoofers in modern home theaters are playing extremely low frequencies. Because these wavelengths are so long, the orientation of the driver or the port won’t have any effect on them. Compare this to the tweeter on a typical bookshelf speaker; it will produce wavelengths that are only a fraction of an inch long! If you change the orientation of the tweeter even a small amount on the enclosure, it will have an effect on the sound. The shortest wavelength a subwoofer will produce, though, will be many times the dimensions of its enclosure. This means that, relative to the wavelengths being produced, any effect a change in orientation will have on the wavelength’s perception is negligible. The driver can fire up, down, to the front, to the back, or at an angle, and it won’t matter. The same is true of the subwoofer’s port.

In fact, you don’t even need to worry about the surface the subwoofer rests on. Many people with carpet in their theater rooms have taken great lengths to get their sub off that carpet, with the idea that sound is somehow being muffled. Carpet would need to be ridiculously thick (on the order of two to three feet) to have any real effect on bass frequencies.

Downward-firing subs also aren’t more likely to disturb downstairs neighbors than front-firing subs. Always remember the extreme length of a subwoofer’s wavelength. A front-firing sub is just as effective at propagating these pressure waves to the floor as to the back wall, and downward-firing subs are just as effective at propagating these pressure waves to the back wall as to the floor. Without modifying the construction of the surface, there is only one way to reduce the level of sound that penetrates: increase the distance of the source to the surface. The change in distance between a front-firing and downward-firing sub is insignificant in this situation.

The one caveat to this is that the driver and port do need sufficient clearance to the nearest surface to prevent compression problems. This means that optimally, you want to have your subwoofer’s sound-producing faces out a bit from any surface. As long as you stay four inches or more from any obstruction, you should have nothing to worry about.

Although a room won’t limit how low a subwoofer can play, it will have a rather large impact on a subwoofer’s low-end response. There is a phenomenon known as room gain (in the car audio world it is called cabin gain, and the size difference is why car stereos can get so ridiculously loud). When wavelengths are longer than the room’s dimensions, the room boosts those wavelengths. The smaller the room and the lower the frequency, the higher this boost. This means it is possible for a sub to sound simply amazing in a small room but absolutely pathetic in a big room. This results from the sub not having the headroom left to counteract the drop in room gain from the larger room.

The other major effect a room will have on a subwoofer is the standing wave patterns it will produce. A typical room will have three axial room modes (a room mode is the particular audio frequency a standing wave will form at in a room): one for the height, one for the length, and one for the width. At the exact frequency a perfect standing wave will form, one of two things will happen: the frequency will be reinforced, doubling its output, or the frequency will be completely cancelled out. As you move off the exact frequency, the amount of reinforcement or canceling will decrease. Reinforcement is called a peak in the subwoofer’s response, while cancellation is called a null.

When you place a sub in the corner of a room, you will maximally excite the three main axial room modes (there also are tangential and oblique room modes and multiples of all three, but we won’t worry about those in this discussion). You can roughly calculate what those frequencies will be by dividing 565 by each room dimension.

Say your room is 24 x 18 x 8 feet. With your sub placed in the corner, you’ll have room modes at roughly 24, 32, and 70 Hz (you’ll also have a full-wavelength room mode of the 8-foot dimension at 35 Hz). If you had some extensive schooling in physics and acoustic theory (or access to the right computer software), you could get a pretty good picture of how all the different frequencies will interact with your room when produced from different locations. The problem is that the majority of us don’t have access to the software, and fewer of us have the knowledge to work it all out!

The bottom line is that you have to experiment to find the best location in a room for a subwoofer. This location will be the one that causes the most even distribution of standing waves in your room. The more even the distribution, the more even your subwoofer’s frequency response will be, and the better it will sound. See “Optimize Subwoofer Placement” [Hack #52] later in this chapter for one of the best and easiest methods to correctly place a subwoofer in a room.

I hope now you feel you have a decent understanding of what a subwoofer is all about. So, now I’ll look at actually buying a sub. First, think about a few things before you actually go demoing [Hack #2] :

There are two ways to look at question one. Many enthusiasts ask, “How much money can I spend?” when a better question might be “How much money should I spend?” I recommend allocating 50% to 100% of what you spent on the other five speakers combined, if possible. If you think this is excessive, keep in mind that a subwoofer should be handling the low bass of every speaker in your system. You definitely don’t want the subwoofer to be the bottleneck in your system, especially for home theater viewing.

You need to consider the second and third questions in conjunction. The second question almost always is answered with “I want to do full Dolby reference level.” What most people don’t realize is how few subwoofers can actually do full Dolby reference level, and what it will cost to get one of those few subwoofer(s) that can.

Full Dolby reference level in a typical home theater setup will require the subwoofer to be capable of putting out 121dB to the listening position all the way down to 20 Hz. Considering that most subwoofers in the $750 to $1,000 price range have difficulty passing 105dB at 20 Hz when measured 2m from the speaker in a medium- to large-size room, and that with this output capability it would take eight of those subs to hit 121dB at 4m, the financial reality of trying to achieve Dolby reference scares most folks away. The third question also has a big effect on how hard it is to meet the output goal you have for your system. If your room is 1,500 cubic feet, you’ll be able to get away with a lot less subwoofer than if your room is 8,000 cubic feet. Keep this in mind when demoing subwoofers. If the demo room is bigger than your room, expect the sub to be more capable at home; if the demo room is smaller, the subwoofer won’t be as loud at home. Most good audio stores [Hack #4] will let you have an in-home demo. Take advantage of this.

There is also some good news related to questions 2 and 3: most people don’t realize just how loud Dolby reference level is. Most people will be more than happy at 10–15dB below Dolby’s reference level; I know I am. Use an SPL meter [Hack #63] to determine what level you like to watch at and make sure the sub you decide on is capable of reaching the levels you want.

With this information in hand, start demoing some subwoofers. Check out the subwoofer(s) that fit within your budget and find the ones that will meet your output goals in your room. Then concentrate on the final and most important factor: do you like how it sounds? Preferably, make this decision in your own home theater, with your main speakers and components.

—Dustin Bartlett

The first hurdle you need to get over is a rather unfortunate naming convention used in receivers. The receiver will have speaker settings called “small” and “large.” The knee-jerk reaction to these names is to set your tower speakers (or your big, capable bookshelf speakers) to “large.”

You need to get over this reaction; even the most capable tower speakers out there can’t produce low bass as well as a good sub. Setting all your speakers to “small” has two advantages, and both are worth far more than seeing “large” scroll across your receiver’s display. The first advantage is that the “small” setting relieves your speakers and amp from the difficult task of producing the lowest few octaves of a soundtrack. An easier load is placed on your amp, and you’ll find the midrange of your speakers improves when the last few octaves don’t need to be produced.

The second major advantage goes back to how bass frequencies interact with a room. Finding the correct placement for one speaker to produce an even bass sound in your room is much easier than finding the correct combination of five, six, or seven bass sources; the speakers that now are producing bass in addition to your sub. To further complicate matters, the best location for those sources in relation to bass production rarely will match up with the best place to produce the midrange and high frequencies.

The last thing to consider on this front is that a crossover isn’t a brick wall (remember the section on crossovers in the primer [Hack #48] ?). The rule of thumb is that your speakers should be capable of a flat frequency response to one octave below the crossover frequency. So, if you set your receiver to an 80-Hz crossover, your speaker should be capable of solid output down to 40 Hz. Because most good tower speakers are flat to just below 40 Hz (manufacturers’ claims on frequency response almost always are exaggerated), this ends up working well. So, dig out your receiver’s manual and figure out how to do the following:

Never touch these settings again!

Now you’ll need to make a line-level connection between your receiver’s subwoofer pre-out and your subwoofer’s line-level input. The cable just needs to be a standard audio interconnect with RCA connectors on the end; you don’t need a special subwoofer cable. Any decently shielded audio interconnect will do.

Some plate amps have a left and right input instead of just one line-level input. When using your receiver’s bass management features with a sub such as this, you have two options. The first is to plug the cable into one of the left or right inputs (you don’t have to use both); the second is to purchase a Y-adapter cable and plug your receiver’s subwoofer pre-out into both. Either method works equally well on 99% of plate amps. On a select few, the input signal to the sub isn’t strong enough unless you use the Y-cable. For most setups start without a Y-cable and if you get the output you’re after from the sub, leave it alone. If you aren’t getting the output you expect from the sub, you should try the Y-adapter. These are the only connections (besides plugging in the power) that you’ll need to make to the subwoofer.

With everything hooked up and the receiver configured, you have one thing left to do. The plate amp on your sub likely will have its own crossover, but because you are using your receiver’s bass management, you’ll want to defeat this “feature.” Some subs have a switch to do this, but others don’t. If your subwoofer has a switch, simply set it to the “off” position. If not, turn the crossover dial up as high as it will go, as this is the next best thing. Finally, make sure your sub is optimally placed [Hack #52] , and you’re ready to go.

What about all the other connections on the subwoofer’s amp? If your receiver has a subwoofer pre-out and bass management (and almost all do these days), you should ignore all of these additional connections. If your receiver doesn’t have these features, you should consult your subwoofer’s manual for alternative connection options. The only time you should consider not using your receiver’s bass management is if you are setting up a two-channel system, and the subwoofer’s plate amp has two dials for the crossover—one that controls a high-pass filter and one that controls a low-pass filter. If this is the case, you’ve gone far enough into the world of two-channel reproduction that you’ll know what to do with the two filter controls, or your dealer will have no problems giving you some advice (these are home theater hacks, remember?).

One final item I’d like to comment on with this topic relates to receivers that allow you to choose different crossover points for different speakers. If your receiver has this feature, don’t use it; set all speakers to the same crossover point. To understand why, consider a simple example: assume you set your surrounds to an 80-Hz crossover, your center channel to a 60-Hz crossover, and your mains to a 40-Hz crossover. Each speaker will receive the appropriate high-pass information—surrounds 80 Hz and up, center channel 60 Hz and up, and mains 40 Hz and up. The subwoofer, though, will receive a signal that will be a summation of the surrounds', center’s, and mains’ signals, which then will be low-passed at 40 Hz. Do you see the problem? The main speakers will be the only properly produced channels! The center channel will have a hole between 40 Hz and 60 Hz in its response, and the surrounds will have a hole between 40 Hz and 80 Hz in their response.

—Dustin Bartlett, Brett McLaughlin

A subwoofer for home theater tends to have the biggest impact if placed in a front corner with the longest unbroken walls on either side. This gives the sound the full length of the walls to reflect off of and reinforces the lowest possible frequencies.

However, many music lovers sometimes find the corner placement to have too much enhancement. What sounds great for movie special effects might overemphasize sounds for music.

To find a happy medium between powerful bass and maintain a good musical balance, you can put the subwoofer at one-third (or even three-fifths) down the length of the longest wall. This causes the subwoofer to “see” two smaller walls on either side. Although the sub no longer reinforces the lowest frequency, it still has the smaller wall sections to reflect from. This can create a more level response. However, even this is not a perfect solution in every case. The room walls and ceiling will reflect some sounds more than others. The interaction of all these reflections in the room can cause huge swings in sound depending upon where you sit. Moving your head six inches one way, or the sub six inches another way, can have a dramatic effect. So, instead of just accepting a rule of thumb, you should test out different locations.

One of the more popular techniques for sub placement is called the “Six-Pack Method.” Here is what you need:

Attach the F-to-RCA adapters to the ends of the coaxial cable to make a long subwoofer cable.

Take your subwoofer and put it right in the middle of your primary seating location (yes, move the couch out of the way). Use your homemade subwoofer cable to connect the sub to your receiver. Disconnect all your other speakers, and find a bass-heavy chapter on a favorite DVD. Use the A-B Repeat feature of your DVD player to play the chapter repeatedly.

Take the six pack and crawl into the corner that meets the longest wall. Sitting in that corner, pay attention to the sound of the subwoofer as it plays the DVD. Then, begin to move along that long wall, working your way around the room, listening to the sub’s response. At some locations the sound will be boomy and rough, and at other spots the sound will be smoother and sometimes even quiet. Using a can, mark the location where the sound is smooth. Try and pick three or four spots like this where the subwoofer would fit.

When you have several spots picked out, put the subwoofer in the first location you’ve marked with a can. Replace the couch, reconnect all the speakers, and sit down to listen. By the law of reflections, the sound in your primary seating location now should be as smooth as the sound where the can was. Now chug the can you just moved.

If the subwoofer (along with the rest of the system) sounds great, you are done. If not, try one of the other can locations for the subwoofer and repeat the process, making sure to drink the whole can each time. Eventually you will find the best location for the subwoofer—or be so caffeinated, sugared up, or inebriated that you won’t care.

—Robert McElfresh

The number-one cause of subwoofer/speaker hum is the coaxial cable connecting your cable or satellite receiver to your provider (either through an inground run to a cable box or through a satellite dish). Here is how you test for this:

Did the hum stop or reduce by a large amount? If so, the cable you disconnected is the source of the noise. If this is the problem, you have several ways to fix the issue permanently (if not, jump ahead to the next section):

Here is why this trick works: the first transformer converts your 75-ohm coax into a 300-ohm antenna connector. The second transformer converts the 300-ohm back to a 75-ohm connector. The humming, which usually is at around 60 Hz, can’t pass through these conversions.

The second cause of hum is called a ground loop, and it almost always shows up right after you bring home a brand-new, self-powered subwoofer, or perhaps an external amplifier.

Take a look at all the plugs on the power cords on your home theater equipment. In most systems, the receiver (or amplifier) has a three-prong power plug, but most of your other devices have only two-prong plugs. This is not by accident; the device with the three-prong power plug is grounded. This means that device “owns” the ground. As long as no other power device has a three-prong plug, everything works well.

When you bring home a self-powered subwoofer and plug it in, though, you might notice it has a three-prong plug; this is for safety reasons. However, when you connect an RCA cable from your receiver to your subwoofer and turn everything on, you suddenly notice a loud hum.

The external amplifier in your subwoofer is now fighting with the amplifier in your receiver for possession of the ground. Both devices want to define 0.00 volts. But because the wiring in the two amplifiers to your household AC ground is different, one device is really using 0.001 volt and the other device uses something closer to 0.003 volts. The subwoofer cable connects the two, and the fighting begins.

You have to stop these two devices from trying to own the ground, or get them to not “see” each other. First, make your system produce the humming noise. Disconnect the single RCA cable between your receiver and subwoofer. Did the noise stop? If so, you have a ground loop issue.

The proper, safest way to solve this problem is to buy a special subwoofer cable with little arrows on the wire to show the signal direction (see Figure 6-1).

Figure 6-1. Subwoofer cable with directional arrow

Unfortunately, many people have been ridiculed when asking about these sorts of cables at their local electronics store: “But cables don’t really have a direction. My expert friend at work laughed at me when I asked about this!” Yes, your friend is right. Cables don’t have a direction, but these little arrows indicate that this cable will prevent or solve your ground loop problem.

Remember when I said the hum started when you connected the RCA cable? That RCA cable really contains two wires: the center wire and something called the shield. The center wire carries the audio signal, but the shield tries to define 0.00 volts. The shield is the wire that lets the two different components (the receiver or amplifier, and the subwoofer) see each other’s ground, and causes the fight. What if you took your subwoofer cable and disconnected the shielding from just one end? Wouldn’t that solve the problem? Yes, it would. This is exactly what a subwoofer cable with little arrows does. The shield is not connected at both ends. The shield has to be connected at one end, for connecting to your receiver or amp, so you should run the cable so that the arrows show the flow from the receiver to the subwoofer.

—Robert McElfresh