An Introductory Guide to Subwoofers 
Introductory Q & A

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So, what exactly is a sub-woofer?

When what we now call subwoofers were 'invented', the so called full-range loudspeakers all contained woofers. Since the idea was to have a driver that played frequencies lower than those being reproduced by the woofers, the term sub-woofer came about.

In modern terms a subwoofer is a loudspeaker optimized to reproduce low bass. Usually these are frequencies 80Hz and lower. 

For those that don't know, Hz = Hertz = cycles/second = the number of times the cone moves in and out in one second. ________________________________________________________________________________________________

How many kinds of subwoofers are there?

There are many different alignments, (an alignment is term used to describe the driver mounted in box of a specific design). The chart below from John Murray's website shows a cross-section of the standard so-called "box" alignments. To this we need to add, horns, transmission lines, IB subwoofers, etc.....

To see a full sized version of this chart click HERE (note this is a .doc file)

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How do I know which design is the best for me?

Unless you're a DIY builder, the choice of alignments is usually limited to a few standard designs (alignments). Those are sealed, vented, (note that ports and passive radiators are both examples of a vented design), and bandpass.

The choice is which is "best" is a function of your tastes, the size of your room, the desired output levels, and the size of your wallet.

As with any other loudspeaker, an in-home audition is the only definitive way of knowing  how a particular sub will perform in your listening room.
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Ported/sealed what's the difference?

A ported box contains a tube (port) that tunes the box to a specific frequency. (think about blowing air across the top of a pop bottle). That's in essence what happens with a ported design. The port boosts the output at the tuning frequency. 

A sealed box also has a tuning frequency. It's primarily based on the size of the box.
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Ported/sealed which is better?

Neither is inherently 'better'. There are good ported subs, and bad ported subs, just like there are good and bad sealed designs.
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Ported/passive radiator which is better?

Technically these are the same alignment. Unfortunately there are more poor sounding PR based designs than ported ones. If one does transient response or impulse testing, the PR based subs tend to have 'ringing' or 'overshoot'. This means the bass won't be as tight or well defined. The primary benefit to using PRs is that the box can be small and still have boost at the tuning frequency, since there's no port to occupy space inside the box. 
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What's all this about "boom"? I don't want a boomy sub....

Okay then, pay attention to where you place it in the room. Boom is primarily a function of the interaction of the sub and the room. More on this later.......
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What's all this about small subwoofers  = 'fast' bass?

 Decades ago, the technology was such that the impulse response (transient response) for larger diameter drivers was not as good as that from smaller diameter drivers. With modern technology this is no longer the case. But this concept is now used to market subs using smaller drivers, since people erroneously assume that a smaller driver will move 'faster' than a larger one. This is false if the large driver is properly designed.
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So, what size subwoofer should I buy?

There's the old phrase "there's no replacement for displacement". Reproducing low bass is all about moving air,  lots of air. The easiest way to do that is with a larger driver or drivers that displace (move) a lot of air. So with regard to subwoofers, bigger will always play both louder and lower. So buy the largest sub you can afford, or that your spouse is willing to live with ..... ;^)
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My friend has this tiny cub sub, and it plays really loud, and it ......

There's what's called "Hoffman's Iron Law". Basically it states, a loudspeaker perform 2 of the following 3 functions. It can be sensitive (play loud with little power), it can have a small box, it can play low frequencies. So chose the 2 you want, the 3rd you have no control over.

Given high amounts of power and TONS of EQ very small subs can be forced to play at reasonably high SPLs. The primary trade off for this is poor sound quality.
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Musical sub vs. HT sub, what's the difference?

Like many things a major component of this is marketing driven. Any quality sub will work equally well for music and HT. If someone wants a bit more "punch" for HT, dial in a bit of boost around 40Hz. More on EQ later in this guide
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I heard someone mention the term "room gain". What's "room gain"?

Room gain (aka cabin gain) is a boost in the lowest frequencies, that's a result of the interaction of the sub and the boundaries of the room (wall/ceiling/floor = boundaries). To see a frequency response plot showing an example of room gain click HERE. The plot is the nearfield output of the sub. The magenta plot is the sub operating in a room. It's pretty clear how much the room boosts the lowest frequencies.
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What is calibrating a subwoofer?

For some reason with the advent of multi-channel sound, the powers that be decided they needed to make the world more complicated. So they coined the term "calibrate". In reality when someone is 'calibrating' a sub, they're simply setting it's output level relative to the output level of the other channels.
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What's Schroeder frequency, Sabine units, comb filtering, RT60 ......?

This is an elementary tutorial, eventually we'll get into more advanced thinking. Right now we're sticking with just the basics
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