It begins like this...
Q: I have drivers W + X + Y + Z. Can I build a kick-ass speaker from them that will blow away all the brands at the HiFi store?
A: Can you build a speaker from them, probably. Will it rival the stuff you hear in the HiFi store, no.
Q: Why not? My friend showed me the Parts Express/Madisound/Meniscus/Zalytron website where they sell the same drivers, and they don't cost nearly as much money as the speakers in the store. What else is a speaker but some drivers and a box? If I can get those same drivers for much cheaper, and build a nice box for them, then didn't I just save myself a whole bunch of money?
A: I get the feeling that you believe you are building a supercar. You found out the distributors for the monster engine used in the supercar, and now all you have to do is build a body around it and you have just built your own supercar, at a fraction of the price. But speakers don't really work like that (neither does a supercar, really). You can have the best-engineered, best-performing drivers on the planet, but unless you use the right crossover with those drivers, the end result can sound disappointingly bad.
Q: Well, I have other speakers in my house. You know, stuff you'd get at a big box store. I know those drivers aren't very good. But what if I swapped those primo X, Y, and Z drivers into the cabinets and used the crossovers on those speakers? Wouldn't that be an improvement?
A: You mean like a tuner car? Unfortunately, it doesn't work like that, either. Chances are, the expensive drivers will sound worse than the original ones (Unless you think they sound better because you paid so much money for them). Reason being: the crossover in that store-bought cabinet was designed specifically to get a good performance out of those cheap drivers. Any other drivers you try to use will probably not sound as good as the original.
Q: What if I upgrade the crossover components in my store-bought cabinet BEFORE I put in the expensive X, Y, and Z drivers?
A: No difference. It will sound just as bad. A capacitor acts like a capacitor, a resistor acts like a resistor, an inductor acts like an inductor. While there are some subtle differences in audio quality between cheap and expensive crossover components, the differences only become noticeable when everything else is in place.
Q: What about the pre-made crossovers I saw on the Parts Express/Madisound website. Could I just use those?
A: Yes, you could. Will your new speakers sound like those expensive ones in the HiFi stores? No.
Q: But they'll still sound better than something from a big box store?
A: Better? Hard to say. Depending on which drivers you chose, and how well the textbook crossover works with them, they could sound all right. With a good ear and some tweaking, you can probably even get rock music to sound good with them, which can be fun. You see, pre-made crossovers use "textbook" values, which are based on "ideal" drivers, which, of course, do not exist. However, the textbook values can sometimes get you in the ballpark. A textbook crossover is certainly better than no crossover at all.
Q: What's the purpose of a crossover, anyway?
A: Drivers all have certain frequencies they play well. Large woofers play bass well, but do not do treble well. Tweeters can play treble well, but cannot play bass at all. And some drivers just do midrange well, and can't play very high or low.
Q: Why don't they just design one driver that can play all frequencies well?
A: It's coming, along with the cure for plate tectonics. In all seriousness, there are some unusual transducer designs out there that claim to cover the audible frequency range. But these suffer from various other problems: laser-beam-like directivity, super-low sensitivity, or they are just plain impractical. So in the meantime, we're basically perfecting the same electromagnetic-voice-coil-attached-to-a-vibrating-material that was invented by Alexander Graham Bell over 100 years ago. It's not to say such a transducer will never be invented; but these cone, dome (and occasionally ribbon) loudspeakers are what we have to work with now, so we're making the best of it.
Q: So back to these "textbook" crossovers. Theoretically, I could build a cabinet, buy some nice drivers, and use textbook crossovers and make a speaker?
A: Yes, you could. And no that's not a sin, and the audio gods will not smote you. In fact, with careful use of variable L-pads or resistors, you could actually make your speakers make music that would sound all right.
Q: Great! So why not just do that? How is that any worse than what the speaker companies do?
A: Well, it comes down to a difference of two different concepts: "Sound Reinforcement" or "a speaker as a system."
Q: What do you mean "Sound Reinforcement?"
A: "Sound Reinforcement" is basic playback of sound. For example, if you're walking through the mall, or swimming at the pool, or even dancing at a club, chances are there's music being played back through speakers somewhere. Yes, you can tell what song is playing, you can even sing along. But does it sound like real performers playing? Of course not. Often there's too much bass, or not enough, or the midrange is honky, or the treble is muffled and so on. And even if the treble, midrange, and bass were all at proportionate volumes to one another, the playback of the song still does not sound like real instruments or performers. The song is just plain louder. I'm not saying "Sound Reinforcement" is a bad thing; in fact it's very necessary wherever people like background music, or whenever a PA system is used for a live event. Plus, some music isn't recorded all that well, and the best way to do it justice is to just crank it loud!
Q: So, you're saying if I use textbook crossovers and L-pads, the best I'll get is "Sound Reinforcement"? What's so much better about "a speaker as a system"?
A: When a speaker is properly designed--that is, the cabinet, drivers, and crossover all work together to create a range of sound that is uniform in volume, and the drivers integrate with one another in terms of phase and/or time alignment--listening to a recording is a very different experience. You can hear all the nuances and subtleties of a recording, from the singer's inflection, to the guitarist's strumming pattern, to the actual acoustics of the room the song was recorded in. When a speaker works as a cohesive system, you are experiencing true playback of a song, and suddenly you're not just "playing music" but you're specifically re-creating a performance in your living room.
Q: I'm not sure. That sort of sounds hoity-toity.
A: It's probably not for everyone. Most people just like some background music, and don't really care how true the reproduction is to the actual recordings. But if your goal was--as you originally stated--to build something to rival speakers at a HiFi store, then you have to know that those speakers are designed "as a system." And it takes time, practice, a good ear, and a reasonable comfort level with math and electrical engineering to to be able to design the cabinet and crossover of a high-quality speaker, whether for DIY or retail purposes.
Q: But why not just buy some drivers and start tinkering?
A: You could. But there is a lot of electrical and acoustic physics theory involved, and without a grasp of that, you wouldn't really even know what you were tinkering with or why. So, to get a background in theory, you should read "Speakerbuilding 201" or a book by David Weems. Then when you start feeling comfortable with the basics, you can try "The Loudspeaker Design Cookbook." Then you can start tinkering, and studying how and why other designers do the things they do.
Q: That sounds like it would take a long time. I want to build my dream speaker NOW.
A: Then go hang around the Parts Express board or the HTGuide board and ask around for a proven design that fits your needs and budget. Building a speaker--even someone else's design--is not necessarily an easy thing to do. So really, building someone else's design is a good way to learn the ins-and-outs of building speakers; and in the end, you'll have something you really enjoy listening to. And yes, it probably will sound as good as the stuff in the HiFi store, at a fraction of the cost.