Missing Link - Box Tuning

[Zilch]

O.K., who posted the link to a page (Altec Tech Info, I think) describing placing a 1 kOhm resistor in series with the woofer to measure impedance as a method of box tuning, and where the heckie-darn is it?

I wanted to discuss this, but did I bookmark it?

NOOoooo....

[Robh3606]

I didn't see the post but you have BassBox Pro?? When you measure FS same set up if I remember right.

http://alteclansingunofficial.nlenet...ers/TL_226.pdf

This it??


http://alteclansingunofficial.nlenet...rs/TL_264A.pdf

This cause it's a gem.

Rob

[Zilch]

That's the method, but the page I'm looking for showed measured impedance curves as examples for optimal box tuning.

I believe the point it made was that "optimal" meant that both peaks were the same height, a new concept to me, the basis and implications of which I wanted to discuss further....

[moldyoldy]

Sounds like this, pp13-16

http://www.lansingheritage.org/html/...enc-manual.htm

[Zilch]

Thanks, Moldy. That's IT!

First, I don't see a difference between figs. 12A and 12B. 12A should not have the little second "hump" at 190 Hz, I presume? Otherwise, where is the leak shown there?

Figure 13 and associated discussion indicates that for a given driver and box, "optimum" tuning may be achieved by making the impedance peaks symmetrical about Fmin?

In the example cited, 25 Hz (Fmin) is too low, 42 Hz is too high, and 33 Hz is optimal.

What tuning does this produce?

I'm gonna have to pay more attention to the impedance curves generated in BB6P, looks like.

Any insight on the basis of this approach to tuning? AES reference(s)?

[moldyoldy]

I read the stuff, and even once in awhile understand bits and pieces, but am far from qualified to disscuss it.

One I've repeatedly try to 'grok' is;

http://www.dbkeele.com/PDF/Keele%20(...estatement.pdf

with little success, though you may find it more enlightening than I did.

[Zilch]

***Zilch konks Moldy with a 79-page root locus plot...***

[moldyoldy]

So that's what it is. Ain't the first time it's konked me.

[Zilch]

O.K., so I follow the Keele footnote 6 to the Ashley and Swan reference [8] for "an excellent way to measure box resonance frequency."

Ashley illustrates phase zero-crossing as an accurate means to measure the vented box frequencies Flo, Fhi, and Fmin, but does not translate them to box tuning.

Fine, that's the WT2 method, and Fsb is calculated:

Fsb = Flo * Fhi/Fmin

and termed "Vented box tuning frequency. Compare this to design."

4507 with 2235H, two ports closed measures:

Flo = 11.50 Hz
Fhi = 47.96 Hz
Fmin = 27.60 Hz

WT2 therefore reports Fsb = 19.98 Hz

Thiele definition of Fsb is "effective value of Fs when mounted in an enclosure,"

as distinct from Fb, "resonant frequency of vented enclosure."

There is NO way these boxes are tuned to 20 Hz as measured by WT2.

BB6P says Fb = 27.49 Hz, i.e., WT2's measured Fmin, essentially.

So, do I conclude WT2's Fsb is NOT the box tuning frequency? :dont-know

[I've had this problem before, unresolved.]

Note: CLIO gets basically the same impedance curve as WT2....

[4313B]

What is the Fc of the box with all ports closed tight.

BTW - Besides Loudspeaker Enclosures Their Design and Use I also posted JBL Enclosure Information Manual which has tuning methods as well.

[4313B]

What is the Fc of the box with all ports closed tight.

[4313B]

Ok.

[Zilch]

Fc = resonance frequency of closed-box system.

Fc = 40.12 Hz, top.

Also, vented system results, bottom.

[4313B]

sqrt(Fh^2 + Fl^2 - Fc^2) = 28.9 Hz so I would say both BB6P and WT2 are "correct" at ~ 27.5 Hz.

I checked WT2 awhile back against known volumes/tunings and I do believe Fmin in WT2 is Fb of the system. I can't remember WT2's actual variable labels offhand.

[Zilch]

Thank you, Giskard, that's what I was thinking, too: WT2 doesn't report Fb as such; it apparently uses Fmin as Fb to calculate Fsb and Ha.

The measurements are good; it's just a matter of definition and interpretation.

There's lots of information on how to tune boxes.

They presume I know how to measure box tuning once it's done.

[Well, there's what I know, now....]

Fb = sqrt(Fh^2 + Fl^2 - Fc^2)