DIY loudspeakers part 2: What's all this stuff mean, anyway?

In a standard pair of speakers, there are three parts: The box, the crossover, and the drivers.

Speaker drivers are the objects in the box that make noise - woofers and tweeters. Due to their complex construction, these are almost always purchased pre-manufactured, unlike the box and crossover. While some drivers are definitely of better quality than others, it's important to pick the right driver for the right application - for example, if you want to use a tweeter that only works at high frequencies, it's necessary to use a woofer that can cover the midrange.

A speaker driver's performance is measured in one of several ways, some of which are meaningless, while others are no more useful unless you have a degree in acoustics. However, we'lll be ignoring them in favor of the two most useful: the response graph and total harmonic distortion (THD) graph.

This graph courtsey of Zaphaudio.com - visit it!

This is a response graph for a Seas 27TBFC/G, a very popular, high-performance and moderately priced ($35) tweeter popular for DIY use. A sound spectrum sweep - similar to playing all the keys on the piano from the low end to the high - was done, and the Sound Pressure Level, or SPL, was measured at a set distance. (SPL is essentially a very accurate measure of how loud something is.)
The purpose of a SPL chart is to show how well a driver works as a whole. Because this is a tweeter, the SPL - in other words, how loud it is - decreases rapidly below 2khz, as it is not intended to be used at low frequencies. Above that, it produces more or less the same volume level up to above 20khz. (The very high level of response above 20khz is due to resonance, but you can ignore it - it's largely inaudiable.)

The other important measure of a driver's performance is the harmonic distortion graph. Harmonic distortion is exactly what it sounds like - how much a loudspeaker driver distorts from the audio signal input to it. Expensive drivers generally have much lower distortion than cheap drivers, but all drivers distort differently at different levels - even the best tweeter will not work very well at low frequencies.

Yet another awesome graph from Zaphaudio.com

The above is a THD graph for a tweeter, the Dayton ND20TA ($5). As you can see above, there are several forms of distortion, but it's easy to see that the higher the frequency the tweeter is used at, the less it will distort. As a result, it's strongly reccomended not to use this tweeter below about 5khz.

Tomrrow: Crosssovers, and how they will eat your babies.