For those interested in audio and / or video equipment, the term subwoofer is most likely a well recognized term.  It denotes the speaker component designed to deliver the heavy bass output of audio / video sound systems as well as automobile audio systems.  Much of this guide is intended for active subwoofers.  Active subs are subwoofers with built-in amplifiers.  All they need is a low level input from any output labled 'subwoofer', 'super woofer', 'line out', etc. 

Back in the '80's, subwoofers were hard to come by.  Why?  Back then, stereo and quadraphonic systems generally relied on either one or two pairs of large floor standing speaker systems to deliver the audio.  It was not at all uncommon to own a stereo system with two pairs of large speakers connected to it, surrounding the listener in glorious stereophonic sound.  At one time, I owned a pair of decent front speaker systems featuring 15 inch woofers for the bass output, as well as a pair of low-end rear speaker systems, each containing two 12 inch woofers.  The bass was great.  My parents yelled at me to 'turn-it-down' all the time.  My point:  who needed a subwoofer?

Nowadays, most audio systems are designed in a home theater fashion, featuring several mini speakers which face or surround the listener, and a single bass driving subwoofer.  This type of audio system has changed the sound dramatically.

As you can plainly see, the bass output that once was delivered by several large bass drivers is now expected to be produced by a single subwoofer.  Also, it should be noted that the upper bass and lower midrange is also reproduced by most floor standing speaker's woofers, whereas in a home theater set-up, the upper bass and midrange are delivered by a tiny driver maybe 2-3 inches in diameter.  That's because the floor standing speakers will have internal crossover networks which allow frequencies well above the bass range to be played by the respective speaker's woofer.  In a modern home theater set-up, the sub may have a set or adjustable crossover which only allows bass frequencies exclusively to be reproduced by the sub.  Everything else goes to the satelite speakers.

Subwoofers come in all shapes and sizes.  One can buy a 15 inch sub, meaning a subwoofer cabinet containing one bass driver measuring 15 inches in diameter.  Conversely, there are many subs on the market which may feature a subwoofer as small as 3 inches!  Obviously, not all subs are alike.  Unfortunately for the consumer, there is no standard in the audio industry regarding subwoofers.  A manufacturer can just about call any bass driver a sub if they choose. 

In the original idea for a home theater set-up, the subwoofer was supposed to play only the deepest bass.  That is because the deepest bass frequencies are non-directional.  A very low frequency bass tone cannot be pinpointed.  The ear is tricked, so to speak.  It is an acoustic illusion of sorts.  The ear hears the upper bass, midrange and treble output from the satelite speakers and thinks the bass is coming from those speakers as well, because the deep bass notes' source cannot be pinpointed.

Needless to say, over time, audio and video manufacturers have misused and abuse the subwoofer term. The following are some examples.  They will help you learn how to rate a subwoofer for yourself.

In my example above, I mentioned a 15 inch subwoofer.  For most purposes, that is the biggest sub the average consumer can buy.  Of course, one might be able to set-up an array of subs, but that is not the norm for most people.  A 15 inch sub can reproduce quite a bit of bass.  The diaphram is large and may be able to move back and forth a great deal, thus moving an air mass capable of delivering very low bass tones at louder sound pressure levels (SPL). 

On the other hand, the so-called 3 inch subwoofer is a very different story.  Most likely, it can only deliver bass harmonics which can trick the ear.  This acoustic principle was employed back in the 80's with the invention of the mini speaker, such as Radio Shack's Minimus 7.  It employed a very high quality 4 inch bass driver which could move back and forth a great deal, and featured a low resonant frequency.  When installed in a mini aluminum diecast sealed cabinet, it was capable of delivering bass harmonics that gave the illusion of deeper bass, but without the shaking of the walls and floor, hence the term illusion.

Why can't a small bass driver deliver the bass of a large bass driver?  Because simple mechanical principles cannot be broken.  For instance, a bass drum might have a diaphram measuring 30 inches in diameter.  Therefore, it's total surface area might be around 700 square inches.  In contrast, the 15 inch sub I mentioned may have a cone radiating area of only 110 square inches.  And the measely 3 inch sub.  A tiny 7 square inches.  In order for the 15 inch sub to move the same amount of air, it's cone will have to move back and forth about 7 times as much as the bass drum, in order to push the same air mass.  The tiny 3 inch sub however, would have to move 100 times as far back and forth! 

Besides the above, the resonant frequency now plays a huge roll.  Every conventional speaker has a resonant frequency.  Simply put, below the resonant frequency, the output of the speaker drops dramatically and becomes useless.  A well designed 15 inch subwoofer might have a resonant frequency of 20-25 Hz, meaning it has the capability of reaching down to the lowest audible bass tone, generally considered to be a 20 Hz tone.  Below that point, humans generally 'feel' the bass rather than hear the tones.

What about the 3 inch driver?  It may have a resonant frequency of 100-200 Hz.  Besides its impossible task of physically moving the amount of air to create a realistic low bass note, its resonant frequency only allows this little driver to touch the 3rd or 4th octave, which is way above the 1st octave of audible sound (deep bass). 

Yet, small subs are popular.  Obviously, they are less expensive.  They are much smaller and easier to place or hide. 

A large subwoofer generally means a large box.  The box is heavy and solid.  A light weight box denotes an inferior design.  The larger the box, the bigger must be each side.  In order to make each side as solid, the sides must be thicker or more dense.  If not, the sides will vibrate due to the bass frequencies eminating from the rear of the subwoofer speaker.  This vibration may be outright audible and annoying, or may be more subtle, refered to as intermodulation distortion.  Such distortion can be caused when a surface vibrates, therefore moving the air around it, thus producing unwanted sounds of its own.  Any sound not intended by the playback source is a distortion.  Physically, small subs have an advantage here.  Small surfaces are generally much tighter and 'feel' much sturdier. 

There are front firing subs, down firing subs, side firing subs and rear firing subs.  This simply denotes the placement of the subwoofer driver.  For instance, down firing means the subwoofer speaker is facing the floor.  Why the different positions?  A front firing sub faces the listener.  If it reproduces any notes above the lowest bass tones, those notes may be audible, thus a distortion.  All the other sub positions are designed partly to mask the upper bass notes, by aiming them away from the listener.  In that way, only the deepest bass output of the sub might be heard, which is the purpose of the sub after all.

Then there are bandpass subs.  These are subwoofers of which the bass driver usually cannot be seen.  The actual bass speaker is loaded completely inside the cabinet, rather than attached to a side of the cabinet.  Only a single hole or port is visible.  This type of sub allows only the bass eminating from the rear of the speaker to emerge from the port.  A bandpass sub can work well because only the deepest bass frequencies find their way out of the port.  Because the bandpass sub physically limits its output to only the lowest bass tones, there is no need for a crossover network.

Most subs feature a crossover network.  The crossover is a point at which the sub's output will be severely reduced.  So, if the sub has a crossover of 100 Hz, then all frequencies above 100 Hz will be muted.  This feature enhances the bass properties of the subwoofer.

Many decent subs feature adjustable crossovers.  Simply put, the listener can adjust the crossover frequency from around 40-50 Hz to as high as 200 Hz, or more.  By adjusting the crossover, one can improve the bass output and match the sub to the surrounding satelite speakers. 

Did you notice that sub crossovers can go well above 100 Hz?  Herein lies one of the problems with home theater systems.  Many times, the satelite speakers are very small, maybe 1-4 inches in diameter.  These small drivers cannot possible handle the amplifier power required to deliver bass tones, nor are they designed to deliver bass notes.  Therefore, manufacturers design the subs to be able to reach frequencies of 100-250 Hz.  While adjusting the frequency of the sub higher may reduce distortion and eliminate holes in the sound, the higher frequencies may become directional, thus defeating the whole idea of a subwoofer. 

Holes in the sound are caused by speakers which do not crossover correctly.  For instance, in a conventional floor standing speaker, if the woofer is crossed over at 1000 Hz, but the midrange speaker can only deliver frequencies down to 1500 Hz, then it is quite possible there will be a hole in the sound.  A noticable dip that does not sound even.  This is also a type of distortion.  A well designed crossover should be smooth and fluid like, maintaining a steady even output at all frequencies.  By the way, holes in the sound are one reason why audiophiles want less speakers rather than more.  A perfect speaker would be one which can deliver all frequencies at the same output.

Many subs can sound hollow.  Hollow is an annoying sound.  It can be caused by an inferior bass speaker, cheap construction of the cabinet, lack of acoustic insulation, etc.  Sometimes, the shape of the cabinet may induce a hollow uneven sound.  There really is no way to tell other than to audition the subwoofer.  If it sounds hollow, it will probably always sound hollow.  A hi-fi enthusiast can go to a lot of trouble and expense investing in expensive equalizers and such gadgets in an attempt to smooth out the bass.  Better to just buy a sub that sounds right in the first place.

Deep smooth bass is what a sub should provide.  Bass notes should be clear and distinct.  Not muddied or confused.  The listener's floors should rumble as if there was an earthquake.  That's a real sub. 

On the other hand, I currently own a sub employing a driver only slightly larger than 5 inches.  It will not rumble the floor, but it will output bass tones well below those attainable by the TV to which it is connected.  The overall sound is still large.  Movies and music are much more impressive.  And, the sub fits nicely on my bottom audio shelf, meaning no unsightly large box. 

If you own a sub that seems to be crossed over too high, you can buy inline low pass crossovers.  These are small components with a RCA plug at one end and a RCA jack at the other.  The plug end is inserted into the subwoofer output or line-output of your amplifer or receiver or other such component.  Then, the cable from the subwoofer is connected to the back end of the inline crossover.  These crossovers effectively filter out the higher frequencies, thus changing the overall characteristics of your sub.  You can buy such inline crossovers which will allow only tones below 50 Hz to pass, or, just about any other frequency reaching well into the midrange area of audible sound.

But, all the above is worthless if you have a passive sub.  A passive subwoofer is one which has no internal amplifier.  It is simply a speaker.  It must be driven by the amplifier or receiver in your system just as any other speaker.  Personally, I don't like passive subs as much.  Active subs are much more sophisticated and if they have all the proper built-in controls, they can mesh well with many different audio systems. 

Many subs come with a volume level adjustment as well. This control simply adjusts the output or loudness of the sub.  You usually adjust it once, then you change the output of the system by the system's main volume controls. 

Some home theater systems feature adjustable  bass output to the subwoofer.  The amp or receiver can adjust the crossover limit to the subwoofer.  This feature will help mesh an active sub without a built-in crossover to the rest of the audio system.

But now, back to size, as the title of this guide refers.  If your satelite speakers are very small and tinny sounding, you may require a sub which is not too big, and is able to reach higher into the upper octaves, in order to prevent holes in the sound.  If the sub is too big, it may overpower the other speakers and the resulting sound can be muddy or dull.  If you know the SPL rating of your surround speakers, then you might be able to find a sub with an SPL which is of similar rating.  For instance, if your satelite speakers are rated 90 dB SPL at 1w (watt), 1m (meter), than you don't want a sub which is rated 100 dB at 1w (watt), 1m.  A 10 dB difference means you'll need 10 times as much wattage to your satelite speakers to deliver the same loudness level delivered by the sub.  So, try to keep it close.  Even a 3 dB difference means you will need twice the wattage to equal the same output sensation.

The sub has to also fit physically into your space.  Some subs are tall and narrow, a tower design as they are called.  Some are cubed in shape.  Some even are designed to hide under a couch or chair, but the furniture's legs must be fairly high, so measure the distance from the floor to the bottom of your furniture before buying such a sub.  Some subs are wireless.  That makes placement a little easier, but may introduce other problems such as interfernence from kitchen appliances, telephones, etc, which may induce noise and distortion into the subwoofer signal.

Large subs will be the most difficult to hide.  They are heavy and difficult to move.  They can be downright ugly to look at. 

Medium sized subs may have smaller transducers only 8-10 inches in diameter, but may be able to reproduce a fair amount of low bass tones.  Their cabinets may be smaller and narrower and able to squeeze into spaces between a shelf and a wall, or a chair and a couch. 

Which side the subwoofer speaker faces can make a huge difference.  If you intend to place the speaker into a tight corner, make certain the speaker driver will not face a wall.  The side with the subwoofer speaker requires around a foot of space from any wall to operate properly and generate the bass frequencies. 

Large subs may disturb your neighbors.  Their bass output can be quite loud.  Such low bass tones will travel through walls and closed windows.  keep that in mind.

Active subs are rated in watts.  More means louder.  But, like all other modern audio and video equipment, the term watts is not as meaningful, in my opinion.  Many subs may state a power output of, say, 250 watts.  When you bring home and unpack this sub, you find its power output is a peak rating, which is useless.  This same sub may have a continuous or RMS output of only 150 watts, or less.  And still, there may be no mention of what frequencies or frequency is referenced.  A sub with an output of 150 watts at 100 Hz may not be able to deliver nearly that many watts at 40 Hz, where the rumbling effect begins to be slightly felt.  A proper sub power output rating will be a stated output in watts, from a low end frequency of maybe 20-30 Hz up to a frequency of maybe 100-150 Hz. 

The bottom line.  Like any speaker, it is the consumer's hearing which must be the determining factor.  If the sub sounds good to you, then that is all you need to know.  Then, the buyer must factor in the aesthetics, the space in their room, and the proper match to the rest of the buyer's audio system.  Obviously, a huge sub and a surround sound with tiny speakers may not sound balanced.  But, a large sub will give the listener the feeling of actually being in a theater.  Some people love heavy bass, while others desire more midrange and detail.  A person's hearing capabilities, both genetic and educated, make a difference.  One who is accustomed to the small speakers of their TV will appreciate just about any sub out there.  A younger listener may need much more bass.  An older one may just be irritated by the vibrations of a big subwoofer.  Speakers are a very personal thing.  What sounds good to you, the listener, is what's important.  Not what the salesperson tells you.  Subs are a form of speakers, so the rule applies.  Buy what sounds good to you.  When you get the sub home, experiment with placement.  Moving the position of the sub by a foot can make a real difference.  That involves sound wave cancelations, standing waves and other room acoustics too involved for this guide.