Need help with Bipole calculations...

[roadsterdriver]

In my quest to design and build a destop hifi setup, I need a little help, or possibly a lot of help.

Math has never been my forte.

I'm trying to calculate Vb, but it seems for sealed enclosures I need either Qtc or the constant and Vas. (then I also need to know how to calculate Vb for two drivers in the same box)

How do I find the constant or Qtc?

Any help is appreciated.

Thanks!

I am considering a Fostex full range 3" paper cone driver: http://www.fostexinternational.com/d.../fe87erev2.pdf

[GM]

Maximally flat (minimal under-damped with 12 dB/octave roll-off) = 0.707 Qtc, critically damped (transient 'perfect' with 6 dB/octave roll-off) = 0.5 Qtc.

From this we see that as Qtc goes up/down the system becomes either increasingly under-damped ('loose'/'flabby'/resonant sounding due to slower decay rate) or over-damped ('tight'/'fast' due to its inability to reach the transient's peak before decaying at a too fast rate with the amp's slew rate being the limiting factor).

Bottom line, what Qtc you use is mostly a personal preference one since we all hear the same, yet not so much, with 0.707 Qtc being the default calculation in most cab design programs and a good one for most near-field apps, especially if mating to a sub. If in doubt and/or response measurements aren't used though, I recommend designing to a lower Qtc and reduce net Vb as required to get the best blend through the XO's BW.

For more than one driver in a common space, sum their Vas, so for a basic calculator that only allows inputting one driver you can either do this for input or design for one and multiply the net Vb by the number of drivers, and if vented, increase the number of vents to match for a close enough tuning to the desired Fb.

GM

[roadsterdriver]

Yes! Excellent. That's exactly what I needed to get to the next step. Thanks.

So, if I got the calculations right that means a sealed bipole box with the above speaker would have a Vb of 380 cubic inches and basically be too big for the desktop.

I would either have to go down to smaller cone size or use a single driver[or maybe both!!!].

But, the single drivers ata this size have an EBP that indicates ported boxes, which I think will be bigger.

Does a bipole speaker configuration still have to conform to the EBP model for sealed boxes? i.e. >90 EBP to be compatible with a sealed box?

[Just in case you haven't noticed yet, I have no idea what I am talking about. So, if I say something that doesn't make sense, fell free to enlighten me.]

[roadsterdriver]

OK, different driver:

http://www.madisound.com/catalog/PDF...vers/FF85K.pdf

Can someone check my math please?



Qts .47 Qtc .707 = constant 1.263

Vas 2.14 (two drivers) constant 1.263 = vB 103 cubic inches?

Do I have it all wrong?

and...how do you decide which drivers are appropriate for bipole boxes?

Thanks...

[roadsterdriver]

Never mind FF85K has an EBP way to high for sealed box...

The Fostex FE 87E has an EBP of 116

Is an EBP of 116 plausible for a sealed box?

[roadsterdriver]

Yes, I am having a conversation with myself! Probably because nobody is awake but me.

Number crunching and draconian space limitations have led me to this as possibly my only option for a desktop bipole...

TB 2X3" full range
http://www.tb-speaker.com/detail/1208_03/w23-1286sc.htm

Vb 3.75 (so it will fit on desk in an appropriately ported box)
F3 107Hz
Fb 120
Freq response flat above 130Hz

Am I on the right track?

[MJK]

Number crunching and draconian space limitations have led me to this as possibly my only option for a desktop bipole...

Why are you pursuing a bipole for a desktop application?

If space is a concern and you are listening in the near field, why not take the cost of four drivers and buy two better drivers while at the same time probably reducing enclosure size?

There will probably be a wall in close proximity behind the rear facing bipole driver, how are you taking this into acount?

What am I missing?

[roadsterdriver]

Obviously I need to study more. However, I was thinking that I will as much of my listening from my big comfy chair that is about 6-7 feet away from the mains as I will from my desk chair.

I thought bipoles would fill the room nicely and reduce the tiny sweet spot that single driver speakers might produce.

I was thinking the second driver would face up not back.

[roadsterdriver]

Am i totally messed up in my thinking on the bipole issue?

Am here to learn!

Thanks!

[GM]

Originally Posted by roadsterdriver

But, the single drivers ata this size have an EBP that indicates ported boxes, which I think will be bigger.

Does a bipole speaker configuration still have to conform to the EBP model for sealed boxes? i.e. >90 EBP to be compatible with a sealed box?

[Just in case you haven't noticed yet, I have no idea what I am talking about. So, if I say something that doesn't make sense, fell free to enlighten me.]

For sealed response except with a bit more gain BW, low tune it with a vent and the only size increase is whatever it adds.

I never had EBP to 'guide' me during my active building career and yet managed to make many above average performing systems that don't come close to meeting its guidelines, so for me it's a worthless, design limiting criteria.

GM