How to choose the right speaker project - for your ears

Being a newbie in diy-loudspeakers, the #1 question obviously is this: What am I going to build? What will suit my needs, my system, my room, my wife, etc. HiFi magazines are full of mails posing the same problem, because we can't hear them all and in particular when we're dealing with diy, it takes a lot of practice to "read" a project and get some ideas on how it will perform. Actually it may take many, many sheets of MDF and experience with a huge number of drivers. I could have bought a pair of extremely expensive speakers for the money I've invested in diy loudspeakers - but it wouldn't have been the same fun!
Those of us having made a large number of speakers will know the excitement of setting up our finished speakers and thinking this is the best thing since sliced bread. After a couple of months, post honeymoon, we may start thinking this really wasn't the stroke of genius we thought it was - and start thinking about the next project.

If I really had to help you - I would have to know you personally, your spouse, your room, your equipment, your musical taste, how loud you play, etc., to make any suggestions. And even then you might think my suggestion wasn't exactly what you had in mind.
Always consider size, cost, complexity and your current skills in loudspeaker building before engaging.

My suggestions below are only pointing to my own constructions, but these are only a very few examples found on the web. There may be a lot of other constructions out there that may suit your needs and money better.

Size (matters):

This may seem an odd place to start, but nevertheless: You must have an idea about what size speaker you're aiming at. What is the size of your room and do you have a wife who thinks loudspeakers are ugly and should be stuck away on a shelf in the upper corners of your living room? You need to address these issues first. No point in dreaming about a 100 liter floorstander if it means divorce.
What I have experienced from mails coming in is that people may have chosen a (small) low-efficiency speaker and "compensated" by buying a powerful amp, and they find out the speaker really can't play that loud all the same...
Well, it doesn't work that way. We must look at the size of the main driver, and if we're dealing with 55-75 square centimeters of membrane area (4" drivers) it simply cannot move a lot of air. You may have heard a speaker with a 10-12" bass driver and we're talking 300-500 cm^2 membrane area. Up to ten times more radiating area compared to a small mini-mini!
Even a 6" driver, typically making some 135-145 cm^2, won't move a lot of air in a vented or closed enclosure. A high-efficiency 6" in a huge rear-loaded horn may be another story, but few people build these things.
Generally people have too high expectations from their diy projects and buying high-quality small drivers won't compensate for the lack of radiating area. So, size matters!

Really small, ~4 liter:
If it must be really, really small you may take a look at Eekels' minis or similar designs. A 4 liter "bookshelf", which, by the way, do better on a pair of solid stands. This is probably as small as it gets - if size is a no-no and it still has to be decent hifi. You may add a small 1 cu. f./30 L sub somewhere in the corner to get some deep bass if need be, but there are limits to how loud a 4" driver will go if you also think of throwing a Saturday night party. Usually very small speakers also have very low efficiency, so 30-40 watts may be minimum and never expect 4" drivers to throw a huge soundstage in a 40 m^2 livingroom. They can't.

Small, ~7-10 liter:
If you have room for a little more, the W15/OWI may be an option. 7 liter net volume and we get more bass from the 5" driver compared to the 4" driver. Still you won't get it all from this speaker unless you invest in - or build - a pair of solid stands and get it out on the floor. Efficiency is still very low and a decent amp is required. No rock parties either from this speaker.
If 10 liter net volume is OK, there are more options. The SEAS 5INCH projects describe a number of speakers in this category where the CA15RLY provides a lot of good sound for the money.
If you want the best bass possible from a 10 litre speaker I suggest taking a look at the Ellam or Ellam-XT. So, why more bass from this speaker compared to the SEAS-5INCH speakers? All the SEAS 5-inch speakers have a 75 cm^2 membrane area and have higher efficiency (except for the W15CY001) compared to the ScanSpeak's 95 cm^2 membrane area. The price to pay for more bass from this design is reduced efficiency. Nothing comes for free when we're dealing with loudspeakers. Remember, when we reduce efficiency by 3 dB, you need twice the amount of power to render the same sound level - and don't be sure your drivers like it at all.

Medium size:
So, what's a medium size speaker? Let's say 10-20 litre and either a standmount or a small floorstander. I have a serious lack of this size standmounts in my portfolio. Why? I never could figure out why people would buy a 20 litre standmount + a pair of heavy stands - and occupy the same floor space as a pair of true floorstanders would do. It's gotta be something visually. Apparently we "see" an e.g. 15 litre standmount as a smaller speaker compared to 25 litre true floorstander despite occupying the same space in our living room. With the floorstander we may have 10-15 litre more cabinet volume and for the same money we can significantly increased sensitivity and bass extension, both things so much needed. Standmounts apparently have higher WAF compared to a similar sized floorstanders. Dealing with commercial speakers, a given standmount may have just the sound you like - and that's another story. No objections here. If you find the one-and-only among the huge number of speakers available in this category, then go for it. The only standmount I have in this range is the AT-SW, which is a serious investment, I admit, but I don't think you'll be disappointed.
The ScanSpeak CSM range provide a more affordable range on constructions and size can be varied from 13 liter to ~22 liter. And the STUDIO 101 is worth a look in this range. Standmount or bookshelf? If 22-23 liters goes, the Ellam25 is to my ears a knockout in terms of cost, performance and size.

20-40 liter bookshelf/standmount/floorstander:
Usually we're dealing with a 6-6½-7-8 inch basic driver here and we may even try a 2½-way in this category depending on bass driver parametres. Quite a range here: SP95, SP38, Ekta, TJL2W, TJL3W, SEAS CNO, SEAS CNO25, 3W-Classic, SP98-09, etc. The 3W-Classic is special in this context delivering some 90 dB sensitivity and an easy load on valve amps.
The ScanSpeak drivers will deliver deeper bass compared to the 18W- SEAS drivers. The SS 18W/8531 driver is bigger and seems build for bass, where the SEAS drivers may deliver better midrange. The SEAS CNO is my favourite despite not delivering the deep bass of the 18W ScanSpeak drivers, but combined with the excellent SEAS T29 tweeters I find it more lively and the lack of colouration is almost as good as the magnesium drivers. A 2½-way SEAS 18WNX001 has been finished: SEAS CNO25.

40-80 liter:
Quite a range here too: Acapella SEas, Acapella NEXT, Ekta Grande, Zahra, Cyclop, Jazzman, Mark21 and Jensen, etc. We're entering a range of speakers that will deliver a significant soundstage. The EktaG, Jazzman and Jensen may deliver the deepest bass of them all followed by the ATR25 (unfortunately discontinued). The Acapella NEXT and ATR25 are my favourites here, not needing a massive power-amp like the Ekta Grande. The sensitivity of the Ekta Grande is around 90 dB/2.8 volts but like most SS constructions, it takes a powerful workhorse to deliver its best. The Acapella NEXT, Zahra and ATR25 are all speakers that can deliver the best from only 50 watts. The constructions listed here will set you back some 1,500-3,000 US $ and don't be stingy on the quality of crossover components. Use good baked coils and super caps where needed, in particular for mid and tweeter.
BTW: I have had reports back on the Ekta Grande saying it's simply too bass-heavy for their rooms. So don't put the EG into a smallish 12-18 square meter room. It can (and should) handle a lot more space.

80-160 liter:
I've only made the TQWT and DTQWT in this volume range and they are in the high-efficiency category of speakers (read below). I've never built a conventional "low-efficiency" speaker this size - and probably never will. Not that it wouldn't be great fun, but the trouble of getting rid of it again is enough for not engaging. I have never made any profit from making finished speakers for sale. A few times I get a price equal to the cost of the components, but all the working hours I have to let go for free. The number of good second hand speakers here in Denmark is humongous and often speakers are sold for ridiculously low prices.
I have a lot of requests from people who want to discuss their dream speaker, double 10" bass drivers, etc., and I've promised to make the crossover, but when they start realising the amount of work going into cabinets, overall costs, etc., I usually don't hear any more. Please think it through before writing.

Open baffles:
If you have the room and the patience for proper set-up of one of these monster panels, take a look at OB9 and OB7. I dare guarantee the OB9 will deliver a live feeling that none of the other speakers mentioned here will ever do. WAF is ultra low, so take care!

Cost

Next: Take a deep look into your pockets and decide what amount of money you want to spend on your speaker adventure. Be prepared it will cost more than you think when you scan driver specs and think: Well, a pair of drivers - can't be that serious. You need at lot of components before the project is completed and you may even have to buy additional tools to finish your cabs.
So, is it going to be 500, 1,000, 2,000 or 4,000 US $?? For 2000 US $ you can build a pair of speakers that will compete with some serious high-end commercial speakers at 5-10 times the building cost - that is - if you choose the right combination of drivers and have someone design a proper crossover.
Sometimes commercial speakers are priced deliberately high, because we may think that a cheaper product may not be as good as an expensive one.
Bear in mind also that diy speakers will never have the re-sale value of some well-known commercial speakers. I've seen used ProAc Response 2.5 speakers sold for horrific prices, alone due to the ProAc badge.
Speakers are also a show-off thing, something people may want to own to impress friends - like cars or whatever paraphernalia we buy to create attention. B&O score high here in Denmark for show-off. People may think you're filthy rich (and well, you have to be well off to buy serious B&O stuff!) if you own a whole bunch of B&O gear, where "hardcore" hifi gear like huge horn, fancy turntables and glowing valves may only produce sympathy for the poor spouse having to live with these ugly monsters.

Bass extension

People have very different expectations when it comes to overall bass performance. We may all be acquanted with earthquake subs at hifi shows and would like our home speakers to shake our guts as well, but we won't experience anything alike from a modest 22 liter/6" woofer floorstander. I can't be done.
To clarify your requirements on bass extension, visit a hifi store and listen to a range of speakers based on 4", 5", 6", 2 x 6" and bigger bass drivers if available and try to imagine how this will perform in your own listening environment.
Another thing is that you may think that in order to produce decent bass, you need speakers with a linear response down to 30 Hz. Now, 30 Hz would be nice - if the room allows - but I wish it was possible to demonstrate to all first-time diy'ers what it sounds like when we compare a 6" bass driver with -3dB @ 30 Hz to a 12" bass driver with -3 dB @ "only" 50 Hz. Most people would probably agree that bass can mean a lot of things.

Efficiency

Efficiency: My favourite subject, but I won't get into details here, but refer to my pages on high-efficiency speakers listed here:
High Efficiency Speakers intro, part I, discussion
High-Efficiency speakers, part II,

Obviously you have to pick a speaker project providing a system sensitivity that your amplifier can cope with. If you have a fine little 10 wpc class A amplifier, a 2.5 clone may not be the best choice, although I've seen exactly this set-up, but this person never did play very loud.
I see a lot of people building 8 wpc 300B amplifiers, but apparently they often run these amps on 85-88 dB speakers! I don't say you can't enjoy the midrange of a 300B amp on these speakers, but they may never reveal all the qualities of your amp.

Most modern speakers have very low efficiency compared to the speakers of the Fifties and Sixties. Low efficiency drivers came around due to the availability of high-wattage solid state amplifiers and the need for small speakers providing decent bass extension, e.g. the "acoustic suspension" (closed box) speakers from Acoustic Research. To do this we need a membrane with high mass and a magnet with low strength and to some people this means "modern hifi" = dull and unexciting reproduction. To others it doesn't matter - and taste cannot be argued.