The tweeter is also very important for the total loudspeaker system. A bad tweeter will make the loudspeaker sound horrible independent of the rest of the drivers. In this tutorial I will use Morel MDT32-S (datasheet), a tweeter that according to the datasheet has a very flat and smotth frequnecy response. It can also be used to play very low frequencies (such as 2000 Hz) which might be important since we are using midrange drivers that are pretty hard to deal with. We will also take a look at Vifa D27TG-05-06 (datasheet), a silk dome tweeter and Peerles 115DT-26-72-SF (datasheet), another dome tweeter which has been around for a couple of years.
The problem with the datasheets for almost all tweeters is that the frequency response are often measures with the tweeter mounted on an infinite baffle. This will lead to a very flat frequency reponse without the dips you will see on measurements with the tweeters mounted in an enclosure. The dips are a result from standing waves formed between the cone or edge of the tweeter and the edge of the enclosure. This will usually result in a dip in the frequency response from 2-4kHz. You can compensate for this in the crossover network but if you are out of luck it can be very difficult to accomplish a flat total frequnecy response. Fortunately you can use GSpeakers to test if your system can handle this. Of course you can not know exactely how the frequnecy response will be in you enclosure but you can at least look at how the filter you end up with will handle such a dip. More about this in the filter section.
Even more problems
Another problem might be the sensitivity of a driver. For example, you would not want to use a woofer that is more sensitive then the midrange. In this tutorial the Vifa woofer has 91 dB/W/m sensitivity. However, since its frequency response magnitude increases with frequency this might, or might not, be a problem. Chanses are that it will be problems together with the 87 dB Seas midrange but it might work like a charm with the other Seas midrange, which has a sensitivity of 89dB/W/m. In the filter you can compensate for a to high sensitivity but you should not damp a woofer that is used to play the lowest frequencies. For our system this means that we may damp the midrange and the tweeter but we should not damp the woofer.
However, a tweeter with slighly lower sensitivity than the midrange is probably not a problem. This is something which you can simulate very well with GSpeakers.