S.E.X.y speakers

[myrun]

Hi B'heads,

Well, I have the 166 fostex drivers and the .25 cf cabinets from Parts Express.  I don't have the subwoofers and am not planning on buying them for budget reasons.  I understand that the 166s in the sealed cab will really only reach down to 125hz. 

Could I move the driver up a little in the cabinet to fit a port down below?  Is this a worthwhile endeavor to try to get the response down below 100hz?  If so, what diameter of port tube should I order and what length should I trim it to?

Sorry for knowing nothing about speaker design, but since I have all the goods except for subs, can I do any better than sealed response with a port?  Thanks in advance folks.

Regards,
Mario

 

[Paul Joppa]

Not in those boxes. The vented alignment gives almost exactly the same response in the 0.25CF box; mostly it just lets the cone flop around more below the vent frequency.

In a larger box, 0.5 to 1.0 cubic feet, you can get some decent bass, down to 50 or 60Hz with a vent. But the increased excursion leads to audible distortion at any useful loudness. That was my first design for this speaker; we just didn't like the distortion. (A non-audiophile friend bought the prototypes and loves them - go figure.)

By the way, all these designs assume an SET amp with damping factor of 2 to 3. With a high-feedback solid state amp, you can get down to 125Hz with a vent but the sealed box only goes down to 180Hz or so.

 

[myrun]

Thanks Paul for the quick response!

I'll have to re-think my decision not to buy a couple relatively cheap subs but in any event won't try to shoe-horn a port w/ 166s in .25cf boxes...I suppose thats one less hole to cut in the baffle! 

But just one more stupid question to suffer:  If I save my 166s for another day and bigger box and instead use these .25cf boxes for a 4ish" single driver ala fostex or tang bang, is it indeed that case that I might entertain a port if driver Q < .4 but not to consider it for driver Q > .4?  Unfortunately, I'm barely starting to learn just a little about speaker/box compatability....that is, just enough to be dangerous.

Best regards,
Mario

PS: I really enjoyed SEX amp camp many summers ago in SF and would be willing to go up the coast to your neck of the woods for another......

 

[Paul Joppa]

I use a shareware program called "Boxplot" - it's quite old, and there are many more modern alternatives. Get one and learn to use it, then you can play around and learn a lot!

Speakers of lower efficiency can go deeper in the same box. That's sometimes called "Hoffman's Iron Law".

Q is a more complex issue that it seems. First off, QT is the "total" Q, usually dominated by the electrical QE, with the mechanical damping QM a much smaller player. But QE is based on an infinite damping factor. When using SET amps with no feedback, QE is increased by 30% to 50% - and so is QT. This makes a big difference in box design.

The mathematically simplest ported boxes are for a speaker with QT = 0.38. But you can tune the box and get good enough results if the Q is in the range of 0.25 to 0.5 - the higher QT will require a larger box and will go deeper. A large QT indicates a stiff suspension, which will usually be less linear. Such a system often has a more constricted-sounding bass. The bass can extend to around 0.4 times the speaker resonance, divided by QT.

For sealed boxes, a QT of 0.5 or less is desired. You can only extend the bass to 0.7 times the speaker resonance, divided by QT; many people prefer to keep it a bit higher. The box will be smaller than the ported box.

Be aware that cone volume displacement (diaphragm area times peak displacement xmax) must increase rapidly as you go deeper. As I said, getting the FE166 down to 50Hz (for example) exposed this weakness; cutting it off at 125Hz eliminated that problem. Small drivers usually have small displacement limits as well.

 

[myrun]

Thanks again Paul,

I'll check out some box design shared software...I understand what you've written enough to understand that ported v sealed box design choices involve much more than a total Q factor litmus test.  I'll complement this software-based learning by purchasing another set of baffles for $20 so as to experiment with both sealed and ported using just one pair of boxes.  There is a bevy of relatively cheap 4"ers to try and am looking forward to much fun with this.

Regards,
Mario

 

[Wanderer]

I may have an oppertunity to aquire some Fostex 206e (not the new "EN"). I note that Parts Express has some cheapsy sub-woofers on special. 

Any thoughts if a S.E.X.y-ish speaker could be made using the 206?

Kevin   

 

[btrancho]

Kevin,

I have built the Half Changs here using the 206e and they were marvelous.

You can see my build thread in the DIY Audio forum

The Chang family is really designed for the Fostex 207, but the BSC described in the thread worked quite well.  The 206e is an 8" driver so your box will be much larger than the one described in this thread, but properly designed, you might get away without having to use a sub woofer.  These Changs easily go down into the 40Hz-45Hz range.

Search the DIY Audio Full Range forum and you'll probably find some other designs for the 206e.  I believe other members of this forum have DIY 206e speakers, as well.

 

[Paul Joppa]

I may have an oppertunity to aquire some Fostex 206e (not the new "EN"). I note that Parts Express has some cheapsy sub-woofers on special.   

Any thoughts if a S.E.X.y-ish speaker could be made using the 206?

Kevin     

Sure! Looks like with an SET amp, a 0.5 cubic foot sealed box will work, crossover at about 85Hz.