I've attached a graph comparing an intriguing looking Beyma 12K200 woofer to a JBL 2206H woofer in a Pi-Aligned cabinet. The T/S parms for the Beyma unit are:

Fs = 35Hz
Re = 6.2 ohms
Qms = 11.17
Qes = 0.229
Qts = 0.225
Vas = 160 liters
Cms = 383 um / N
Rms = 1 kg / s
n0 = 2.9%
Sd = 0.053 m^2
Xmax = 4.5 mm
Le = 0.8 mH

I also compared the Beyma unit in a "shelved" type alignment you mentioned in the previous thread. I think the Beyma compares well to the JBL, though it's distortion figures and build quality are certainly not going to match the JBL. Comments?

I guess my main questions are:

How realistically will these graphs represent the true frequency response? And, which sort of alignment is likely to give best in room response? And if the answer to the previous question is "it depends on the room" is it worthwhile trying to tailor the alignment to the room?

Anyone feel free to chime in.

Thanks,

Jeff Mai

Honestly, I've found that Thiele/Small models do a great job of predicting response. Most of the audio community approves of their work, so to use it to develop these graphs and show predicted response is probably quite accurate. It is better than measurements for most hobbyists and small shops, since it is difficult to obtain reliable measurements.

Thanks again!

In a bass-reflex system, you have five frequencies of interest - Fs, Fo, Fb, Fh and Fl. Fs is the woofer's free air resonance, Fo is the mechanical resonance that's been shifted by putting the woofer in the box and Fb is the resonant frequency of the box, it's Helmholtz frequency. Fh is the upper frequency of highest impedance, sometimes called the upper resonant frequency, and Fl is the lower frequency of highest impedance, the lower resonant frequency.

The upper resonant frequency (Fh) is usually nearly coincident with the enclosed woofer's resonant frequency (Fo). They actually aren't the same, but they are near enough that the port damps the woofer's resonant frequency by a great deal. So for the octave above woofer resonance (approximately Fh) down to resonance (Fo), the port is tightly coupled with the woofer, damping its motion and making the system more rigid. Distortion is reduced because excursion is limited. Then, as frequency drops near Fl, the port begins to augment system output. In this mode, the port isn't limiting motion but it is increasing output so that the woofer doesn't have to move as much. You can find the formulas that define the relationships in the post called "Measure impedance."

I probably shoulda figured it out from your post, but I'm not sure I 'get it'... :-(

Let us know what you think.