I'm pretty new to this too, and I'm still trying to get people to give me guidelines that help me separate the 'impossible' speaker combinations from the 'maybes'.

There are actually two models of that tweeter -

Vifa D27TG-05 1" Silk Dome Tweeter

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=264-524
Frequency range: 2,500-30,000 Hz

Fs: 1000 Hz

Vifa D27TG-35 1" Silk Dome Tweeter

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=264-526
Frequency range: 1,500-30,000 Hz

Fs: 650 Hz

Vifa P17SJ-00 6-1/2" Shielded Woofer

http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=297-308&CFID=20705100&CFTOKEN=45209297
Frequency range: 35-5,000 Hz

On the surface, that would seem good. We have tweeters that go down to 2,500hz or 1,500hz and a woofer that goes up to 5,000hz. It would seem that is enough room for us to work with.

Now look at the spec sheets for these speakers -

Vifa D27TG-05 1" Silk Dome Tweeter SPECS

http://www.partsexpress.com/pdf/264-524.pdf
Vifa D27TG-35 1" Silk Dome Tweeter SPECS

http://www.partsexpress.com/pdf/264-526.pdf
Vifa P17SJ-00 6-1/2" Shielded Woofer SPECS

http://www.partsexpress.com/pdf/297-308.pdf
Look at the frequency response graphs.

Notice on the woofer there are three colors; I'm guessing the Blue is the on-axis 0 degrees response, green is 30 degrees off-axis, and Red is 60 degrees off-axis. Off-axis means off the center line, off center, or to the side by the number of degree specified.

Now we see that what we thought was 5000hz, is really 2khz. Now, I suppose if you don't mind the woofer starting to beam or narrow the field of sound coming from it, we could maybe squeeze this up to 3k, but that would be pushing it. I would be reluctant to crossover at above 2k.

Now the tweeters. If we look at the model that ends in '-05', and we look at the bottom line on the frequency response graph, we see a line with a small hill on the left, a valley in the middle, and a gradual slope on the right. That is the impedance graph. It shows how the speaker impedance changes with frequency.

I've been told as a general rule that you want to crossover at 2 octaves or more above the resonance frequency (Fs), that little hill on the left.

For the '-05' model that is 1khz; one octave up it 2khz, two octaves up is 4,000hz. if you look at the frequency response graph alone, it would seem that you could use anything 1.5khz or above. But our rule of thumb says nothing less than 4khz, thought the spec sheet recommend and gives you the values necessary for a 3.5khz crossover.

For the '-35' model the resonance frequency(Fs) is 650hz; one octave up is 1300hz, two octave up is 2600hz. As a rule of thumb that is the lowest, the tweeter should be crossed over. Again, the specs sheet gives you the values for a 3500hz crossover.

Now back to the woofer. Again, looking at the frequency response graph. I decided from looking at it, that the best highest crossover would be 2000hz. But that could possible be squeeze up to 2500hz. At 2500hz, 30 degrees off center the sound level is down about 2db to 3db, at 60 degrees off center the sound is down about 7db to 8db. It's tolerable at 2000hz, but very marginal at 2500hz.

So, of the components we have looked at, only the woofer and the D27TG-35 tweeter have a chance of working together. And, only a crossover of 2000hz to at the most 2500hz have a chance of working well. At this stage I am assuming a crossover with 12db slopes, a 2nd order crossover; the most common type.

I suppose you could try to raise the crossover to something like 2.2khz. Every thing we do is a compromise, it is just a question of whether this is a compromise you are willing to make.

As a side note: You will notice on the right of the suggested crossover on the spec sheet, a series combination of Rc, Cc, and Lc. These values are flattening the slope on the right side of the impedance graph. Regardless of where you crossover, you can still use these values. This is called a Zobel Network, and you will probably have to create on for your woofer too. But it's not that hard. I don't have the formula readily available, but it is calculated using Le, which is the inductance of the voice coil of the speaker. If you search 'Zobel network' in Google, tons of information will come up.

For specifics of crossover design, I would recommend searching the internet for free speaker design software. There are several of them out there. WinISD is a relatively popular one. SpeakerWorks is another. They will take all the T/S parameters of your speaker and model them for different box sizes and designs and help with crossover design.

In this case, if you follow what I said, you are pushing both the woofers and tweeters to the limits of their truly workable range.

That is about the limit of what I know. There are a lot more details to consider, and hopefully someone will jump in here, and help you with those, and will also correct any mistakes I might have made.

You can certainly pick two reasonable speaker off the shelf, made a reasonable determination of a crossover point, add an off the shelf crossover plus the Zobel networks, and have an OK set of speakers. The question is, is 'OK' good enough? If you want good to great speakers, it means investing some time and money, and a lot of work and study, to get it right. How far you are wiling to take it, is up to you.

Steve/bluewizard