Generally speaking, "normal" people ask me about subs--be they teenagers about car subs, folks that want some oomph for their music/HT systems or bands that need to get subwoofers for their gigs. All of them have the same things in common, they all do! So I ask these simple questions:
1. How large/heavy can the subwoofers be for your needs?
2. How many subwoofers can you put in the room/car/gig?
3. How much money is in your budget?
4. How much SPL do you require?
5. How low in frequency do you desire? Spec it at -3dB and -6dB to fill out the spec.
If they say they want "tens" and they demand 16Hz -6dB--they can get that but it severely limits options. Something like a CSS SDX10 can be forced to do that ported but you'll need a rather large quantity to get the SPL up and it can get expensive and they will be quite large for a "ten". If you want Bose bass module small or as small as posssible--get sealed ten (SDX 10 or UM10) and keep adding them with EQ to get the SPL and frequency response you desire. Sure, you might need four, eight or 32 of them but at least you can keep adding until you get what you want. Start blowing breakers? Add more electrical lines--you can get what you want without having a clue by just adding more! I call it the "government money" option, keep throwig money at it untill the problem goes away!
Most people learned about "subwoofers" from the Bose School of the AM-5-they teach you only need one and it can be thrown anywhere and hidden with a flower pot on the top. I am talking real world here, not forums--you know, real people that ask me things in person. It is difficult to deprogram the Bose graduates of bass and they are everywhere!
The easiest way to derail them is to offer THX and Dolby recommendations which are two or more subs and to mention that three (Earl Geddes alignment) or FOUR subs (Harmon International) are recommended for larger rooms with multiple seats. It does not matter if it is music in mono, 2 channel, quadraphonic or 5 to 13 channel HT systems--bass is bass and there are rules. The first "audiophile" system I saw as a kid had TWO subwoofers and they were diagonal from each other and not aethetically pleasing. The guy had measuring gear, books on acoustics and so on and I soon learned the method behind the madness.
Once they get that, then I ask about "how low do you want to go" and inform them that the difference in size between 16Hz and 32Hz is the box will be four times larger. That wakes them up! Everyone wants 120dB at 16Hz--I know I do but the math gets insane. Luckily, it is much easier to explain with calculators on the internet. Plug in your SPL demands in dB, plug in your frequency demands then plug in the Sd (surface area of the driver) and press go. You will get the stroke (Xmax) required to move that amount of air. Sometimes you'll see something like 300 mm or more which is impossible so keep adding to the quantity until the Xmax reaches what drivers can actually do.
Most people's eyes start to glaze over once the evil math shows up--so I offer this: Tell me what you want for extension in frequency be it 16Hz, 20Hz or 25Hz and purchase whatever subwoofer that is in the size/weight/price you want to deal with. Be aware that, generally speaking if you see 10, 12, 15, 18, 21, 24 or..you can get an 80 inch sub from China--generally the larger diameter the more air it can pump so you'll have higher output. Also, because life tends to suck that way--the lower you go the higher in SPL you need to actually hear it--don't ignore our very insensitive hearing!
Over the years, I tend to push people into at least 12 inch subwoofers for several reasons. They are the most produced size subwoofer drivers so have a lower cost due to economy of scale. Generally speaking,they can go low in frequency at enough SPL that you can hear it. For the most part, people can deal with the size of them (4 cubic feet or so) if they really care about audio. If they are not sure about frequency response (most people don't hang around discussing audio) I offer THX specifications for sub bass or 20Hz at -6dB or IMAX which is 20Hz at -3dB. This is very easy to understand and removes my opinion from the equation so they can't shoot the messenger!
If they want the subwoofers for music, I offer up that the 88 key piano has a low key of 27.5 Hz so might as wel cover the piano--blame the piano--not me!
For the most part, a single 12 will get them the frequency response but it can't do the SPL in most typical living rooms. This is a GOOD thing because most of the people I deal with have friends and family so don't sit alone in a single chair and need good bass response across multiple seats. They get the SPL they want while the rest of the family gets the coverage so they can hear it also by using two or more subs. Say the "bug bites" them and they want MORE! Keep adding subs, three of them and read this configuration from Earl Geddes or four of them and here is a Harmon "white paper". Generally speaking, once a person hits say four 12" subwoofers in their living room and still wants more.., they can use all of them for nearfield "tactical" subwoofers and go nuts with 15/18/21/24 and so on. At least if they know what frequency they want, the system then can be scaled up without throwing everything away and starting over. For real world when you are either related to the people and can't hide from them--they wlll complain about you "getting it wrong" for life... I tend to use THX, IMAX or the 88 key piano for specifications so they make the decision, not me and if the system scales they can always add more.
The great thing about forums is I have no responsibility and if you blow your budget on something that won't work--no skin off my butt!
Plenty of information a simple Google search away from THX, Dolby, IMAX and the Acoustic Engineering Society--if you don't want to bother learing about that, more power to you. I do strongly advise to at least have a familiarity with the recommendations from the folks that set the standards though, saves a ton of time in the long run.
In summation, it the most generic terms possible... start off with a 10/12 inch sub that does THX standards for frequency response and keep adding them until you get the SPL you desire. For music, go for the low note on the 88 keys at a minimum and for PA systems.. 27.5 Hz at -6dB (high passed a 25Hz) should do it for moden dance music. If you desire 8 Hz for cannon fire, pipe organs or special effects... sealed, infinite baffle or LLT and keep stacking until you either get there, divorce happens, the dwelling burns down or you run out of space
In the end, this is supposed to be fun and is just a hobby--a first world silly little hobby at that. For clarity, I use 12 and 15 inch subs in sealed, ported, push-pull slot loaded and eventually a tapped horn. Smaller for cars, medium for houses and larger for PA/party systems works for me. Enjoy your new subs!