Help with 3-way crossover frequencies

[grjr]

Hello, I'm working on a stereo project with some classmates and I have the job of building an active crossover for a 3-way speaker system. My classmate has already purchased these speakers from parts express and designed the box for them. The speakers are as follows:

12" Woofer (GW-12PC-50-8)
32 - 4,000 Hz
89.7 dB (1W/1m)
Vas: 2.79 ft^3 (79.0 L)
Fs: 32 Hz
Qts: 0.49

5" sealed mid (GM-65/8)
800 - 8,000 Hz
89 dB (1W/1m)
Fs: 470 Hz

1 1/8" dome tweeter (DC28F-8)
2,000-20,000 Hz
89 dB (1W/1m)
Fs: 907 Hz

Enclosure Specs:
Vb= 5.4 ft^3 (152.9 L)
Fb= 26.1 Hz
F3= 23.9 Hz
Duct: 3" dia. 5.2" length

The problem I have is how do I go about selecting the proper crossover frequencies? I think I read somewhere that each speaker should ideally only have to reproduce a decade of frequency content. I'm guessing the mid is going to pose a problem with its frequency response not going low enough. I'm planning on using LR4 filters but have never designed or used them before so any tips would be appreciated. Thanks!

[infinia]

Originally posted by grjr

12" Woofer (GW-12PC-50-8)
32 - 4,000 Hz
89.7 dB (1W/1m)
Vas: 2.79 ft^3 (79.0 L)
Fs: 32 Hz
Qts: 0.49


Enclosure Specs:
Vb= 5.4 ft^3 (152.9 L)
Fb= 26.1 Hz
F3= 23.9 Hz
Duct: 3" dia. 5.2" length

Hi
Please provide links to all the drivers you have purchased. You really ought to have a "paper design incl. Xovers" before you hit the buy button. You will get better advice and more options to move. It's like pouring the foundation of a house before deciding on the house plans.
1) Are you using a passive or active Xover?
2) Amplifier make?
3) Room and placement of the speakers?
4) You're an EE student with buddies right?
5) How did you design your enclosure?

Problem 1) Your woofer Qts will be greater than 0.5 with series passive losses. This means the woofer alignment is not ammenable to a bass reflex box design.

Problem 2) Your BR design has Vb = 2*Vas? Your enclosure is 4 times the normal volume for BR. Should be about 1/2* Vas or so.

Problem 3) Final Sens. of your finished speaker will be low around 85-86 dB/watt.

[grjr]

woofer
midrange
tweeter

1) Are you using a passive or active Xover?
I will be using an active crossover, the crossover will be designed by myself

2) Amplifier make?
The amplifiers are being designed by us as well, they are only good for 65W RMS

3) Room and placement of the speakers?
The stereo is not for a permanent installation, so it will be placed wherever we happen to be.

4) You're an EE student with buddies right?
Yes, I'm an EE student in my last semester of school, some buddies yes

5) How did you design your enclosure?
The enclosure was designed by my partner, I don't have her specific methods but I verified the design with an excel worksheet from diysubwoofers.org
excel worksheet

I should say that the stereo we are building is not expected to be perfect, it is simply a prototype to be used as a learning experience. If we can get away with simply changing out the midrange for another that would be ideal.

[tinitus]

That mid doesnt mate very well with woofer...too high xo point(highpass)

Guys, a mid that cost 5USD...do you know something that we need to know

[HK26147]

grjr
I agree with your comment about the 5" ( the Fs is too high ), a 12" woofer can start beaming at approx 900Hz.
Generally speaking I set an active x/o frequency ( 24db or greater ) at 2x driver's fs ( no lower )

[PeteMcK]

Unless you have frequency response graphs, any crossover points you choose will be hit & miss....

[infinia]

So not sure what you want... just 2 xover frequencies?
I still think your enclosure is an issue. Group delay is off the map and woofer excursion limits show you will need a HPF.

edit> will you triamp or bi

[grjr]

It will be a triamp setup, and I plan to use 24 dB/octave filters.
I know the xo frequencies will be hit or miss without the frequency response plots, but for the sake of simplicity and money (poor student syndrome) I was planning on assuming a flat response in the frequency bands given. I'm looking for help on deciding where to put the xo frequencies at.

Any recommendations on a different mid that isn't expensive and is 8 ohms? Thanks guys.

[HK26147]

Let's face it for 5 bucks you can't expect a lot...
I would suggest you keep a look out for deals like this closeout:
http://www.parts-express.com/pe/show...number=299-842
Unfortunately sold out.
Or spend some more and get a better driver.
You can also use a 5" woofer and place it into a sub chamber, this expands your range of candidates greatly.
Many 5" drivers labeled woofers have very good extended response.

[infinia]

Originally posted by grjr
It will be a triamp setup, Any recommendations on a different mid that isn't expensive and is 8 ohms? Thanks guys.

Mmmm.... it's got it's own amp, then 8 or 4 ohms should be non matter at this price point.