Linkwitz eq circuit

[The Dudster]

Hello all.

I am in the process of building a set of Linkwitz's Phoenix speakers. I just need some dosh so that I can buy the Seas 8" Excel drivers and I'll be on a roll. All ICE powered, 500ASP's.
My problem is with the Thor sub. I have four 12XLS woofers for the dipole woofer, but I also have 8(!) Seas L21RNX/P woofers laying around from another project. I was thinking of putting 4 of these babies in each sub, sort of a Thor clone. The problem comes when I need to calculate the box size. I guess the Q should be 0,7. Is that all I have to do? Dimension the box for four drivers with a Q of 0,7. Then I calculate the Linkwitz EQ based on the f0 of the drivers in the box and the Q of 0,7? I think I can go all the way down to 20Hz or so, since 4 x 8" woofers have almost the same cone area as a 15" woofer. They are also long stroke, with a P-P of 14mm.

Hope anybody can shed some ligth on this.

 

[cocolino]

The problem comes when I need to calculate the box size. I guess the Q should be 0,7.

Q doesn`t necessarily need to be 0,7. Within restrictions (check the math of the formulars) Q can be almost anything.

That`s the beauty of the Linkwitz EQ - You can put a driver in a small enclosure (high-Q, high-fc) and with the LR-EQ You can make a low-Q/ low- fc driver out of it (of course within the mechanical and electrical restrictions of the driver).

Is that all I have to do? Dimension the box for four drivers with a Q of 0,7. Then I calculate the Linkwitz EQ based on the f0 of the drivers in the box and the Q of 0,7? I think I can go all the way down to 20Hz or so, since 4 x 8" woofers have almost the same cone area as a 15" woofer. They are also long stroke, with a P-P of 14mm.

Also the target Q doesn`t need to be 0,7.
Actually I found that a lower Q than 0,7 is almost always better sounding (depends of the room and target fc). Don`t forget that at very low frequencies there is room gain. A box with linear free field response down to very low frequencies (as with Q= 0,7) would yield to amplification of those frequencies in a listening room due to room gain.
Besides more linear in-room response and better pulse response, lowering the Q below 0,7 has also another nice side effect - power capability increases and excursion is lower .

 

[The Dudster]

Hohum, I guess I am up for some trial and error then.
Thanks for the pointers.

 

[Ron E]

That`s the beauty of the Linkwitz EQ - You can put a driver in a small enclosure (high-Q, high-fc) and with the LR-EQ You can make a low-Q/ low- fc driver out of it (of course within the mechanical and electrical restrictions of the driver).

Besides more linear in-room response and better pulse response, lowering the Q below 0,7 has also another nice side effect - power capability increases and excursion is lower .

The pulse response advantage of <0.7 Qtc systems is overstated.
Power handling decreases with low Q systems because the compliance is higher in the compiance controlled region below resonance - so teh driver moves more with the same input.

Hohum, I guess I am up for some trial and error then.
Thanks for the pointers.

Without knowing the driver parameters and your goals for the system, you aren't going to get much help. How much power do you have? How big a box can you stand? If it were me, for starters I would make a box for them that had a Q of 1 or so and then tune the LT to the approximate lowest modal frequency of the room and shoot for a Qtc of 0.7 or so. You can experiment from there.

 

[Pan]

Dudster,

http://www.diyaudio.com/forums/showthread.php?s=&threadid=43145

Check out if interested.

/Peter

 

[The Dudster]

Originally posted by cocolino

Ok, so I guess Q=0.7 is ok then?

The speaker paramaters are these:

Fs = 23Hz
VAS = 58 litres
QMS = 3,98
QES = 0,33
QTS = 0,30

If I input these values into LSPCAD Lite and optimize the closed box for Q=0,7 I end up with a box of 18,6 litres. Since I have four units that should be 74,4 litres. I will build two cabinets since I have 8 woofers, four in each. If I also increase the size of the cabinet sligthly to compensate for the volume displaced by the speaker units, it should be around 80 litres. That will be approx 43x43x43cm internal size. I know cubes are bad, but the 1/4 wavelength of the frequencies 50-20Hz are so long, 10-17 metres, that it shouldn't be a problem. Or am I wrong? Each sub will be powered by it's own 250W/8ohm ICE power amp. The same applies for the dual XLS phoenix woofers that I am building as part of my Linkwitz Phoenix system. I live in a apartment, so I can't imagine a need for more power than this.

Any comments about the stuff above?