Port tuning

[sebackman]

Port tuning help needed

I have just set up my M2 wannabe’s (from the Compact Monitor thread) and started measuring. The cabinets are about 70 liter with two 69mm ports on the back side. They are flared on one side so far.

Woofer is LE1400H with:
Fs = 28,8 Hz
VAS = 171 L
QTS = 0,22

When measuring :
Dual 69mm ports about 24cm length gives a system Fs = 30 Hz
No ports, just two open holes of 75mm gives a system Fs = 52HZ

What would be a god frequency to aim for when tuning the ports?

Historically I have always set the port tuning to the FS of the woofer but there is very little info below 30Hz in ordinary music so maybe it should be higher. Or lower....

Original M2 uses 2216Nd with a mechanical Fs = 36Hz and a measured of Fs = 39Hz. The system seem to be tuned to 27Hz which is way lower. Is it the fact that the cabinet is much larger and QTS high that makes that possible.
2216Nd:
Fs = 36 Hz
QTS = 0,44
VAS = 140 L

Kind regards
//RoB

[johnlcnm]

RoB,

My memory of ported base alignments dates back almost 40 years ago, so take my dribbling for what it's worth. A QTS of .383 will give a flat 4th order Butterworth alignment in an box of equivalent driver VAS. A forth order alignment with a high QTS like .44 will yield something that has ripple in the pass band no matter the size of the box. With active filtering the ripple can be eliminated. I think the importance of the box frequency will be a reduction of distortion in the octave at the box frequency, i.e. the loading of the driver to the air. So, my guess would be tune to the 27cps and take any ripple out with the DSP. Years ago Speaker Builder published loads of non optimal alignments. To bad I pitched all of those long ago. Look for the Weems book Building Speakers on line, if you can still find it.

Regards,

John

[sebackman]

Hi johnlcnm,

I did just that and it looks pretty nice. The cabinet is too small for the LE1400H so the curve falls off already from 100Hz but in such a nice way that it can easily be fixed with the DSP.

Kind regards
//RoB

[baldrick]

I'm going to try LE1400 in a pair of 250TI instead of LE14-1, have no idea how it will work

[johnlcnm]

Seabackman,

I think most commercial alignments are a compromise. Some design at a 50% of optimum box volume and allow a couple of dB ripple around the rolloff octave. The room effects pretty much swamp those rippled responses out. A sixth order alignment takes advantage of the ported box frequency, and that is where a second order section prior to the power amplifier applies a boost in the response to bring the bottom end up. Mechanical stress is less at that frequency as the driver is loaded with the box resonance. With a DSP you can make that happen. I am using two Crown HD 5000s to bi-amp a pair of 4333Bs. Measured and EQ'd both speakers. IMO they sound fabulous. I can't believe how three dimensional the sound stage is. The old JBL horn driven speakers really take to equalization!

Regards

John

[Ruediger]

Have a look at table I in the Thiele paper, alignments 2 and 3.

Alignment 2 requires a Qt of 0.209, out of reach.
Alignment 3 requires a Qt of 0.259, can be achieved by using a passive crossover where the low-pass inductor has the proper value to set Qt as needed.

Interpolate between the two alignments, for a Qt of 0.22.
Approximate values are:
f3 / fs = 2.0. Result: f3 = 2 * fs = 58 Hz (ca.).
Vas / Vab = 6.0. Result: Vab = Vas / 6 = 30 ltr (ca.).

My approximations are quick & dirty, You should invest some time and do it properly.

Are You sure the TS params are correct?

Ruediger