ML TQWT with Vifa PL21WN-20-08

q-rz

Member
2005-06-30 1:30 pm
Vienna
hi,

this is my first post here so please forgive me if i do something wrong :)

this week i bought two vifa pl21wn midbasses and two vifa d27tg0506 tweeters to start a 2-way project. i've never built a mass-loaded tqwt enclosure so i decided to give it a try. i would like to ask for your help on some questions.

1) i did some simulations with mj king's mathcad worksheets, and found something that i cannot understand: setting very low So and Sl areas (0.5sd) seems to make no significant difference in the bass response. i even tried to set mad values such as 0.1sd for both small and large end, but it changed only slightly. i cannot imagine i could build such a small enclosure without loosing the lower frequencies.

2) does folding the line affect the response?

3) is there a minimal recommended port diameter? for me seems my 6.6cm too small (that's only my opinion - i don't know why), but if i make it greater, the port has to be very long and the response above 200-300hz gets more peaks that i cannot damp without loosing bass.

thank you very much and sorry for my bad english :)
 
Greets!

Well, I understand your English much better than you would understand my Hungarian since I don't know any. Note that when asking for this kind of help, in the future please include either a link to the driver's specs or post them. BTW, the specs I found makes it a woofer, not a midbass driver.

1) Something is very wrong, but without knowing what specs you are using........

2) It can, but as long as you do not dramatically change the cross sectional area (CSA) through the bend it is not a problem.

3) Yes. I can not find the formula at the moment to calculate the minimum diameter, but just use one of the freeware box programs to find it.

Anyway, here are the specs I found:

Fs = 28 Hz
Vas = 113L
Re = 5.7 ohms
Qms = 1.6
Qes = 0.52
Qts = 0.392
Le = 0.59 mH/1 kHz
Sd = 175.8 cm^2
Xmax = 4 mm
SPL sens = 88.64 dB/2.83V/m
Pe = 80 W

calc'd specs:

BL = 4.9109 N/A
Cms = 2.5835e-03
Mms = 12.5061 g

To keep it simple I designed a T/S max flat alignment in a box program (~3.67 ft^3/28.7 Hz Fb), then converted it into a straight ML-TL:

L = 46"
zdriver = 18.1"
zport = 43.5"
So, SL = 137.94"^2
density = 0.2 lbs/ft^3
rport = 1.5"
Lport = 3.5"

Sims aneochoically flat and as smooth as a baby's bottom with minimal stuffing. ;) Of course it is only as accurate as the specs are, so as always, YMMV.

GM
 

q-rz

Member
2005-06-30 1:30 pm
Vienna
hi gm,

thanks for your reply. there are my specs for the driver:
Fs = 28 Hz
Vas = 113L
Re = 5.7 ohms
Qms = 1.6
Qes = 0.52
Qts = 0.39
Le = 0.9 mH/1 kHz
Sd = 235 cm^2
Xmax = +/- 4 mm
SPL sens = 89 dB/1W/m
Pe = 80 W
BL = 6.6 N/A

my experimental tl geometry was:
Length = 80"
Ssmall = Slarge = 2*Sd
Port diameter = 2.6"
Port length = 8"
driver offset = 0.4

i got [email protected]~28hz with these parameters. setting another (very small) Ssmall or Slarge values (So and Sl i think, i'm not at home now), 0.1*Sd for example, causes no great difference.

i thought maybe there is something wrong with my mathcad or with the worksheet i have downloaded.

greetings,
peter
 
Greets!

OK, you used the 'Le' from another version and the whole driver to calc Sd, but you compensated for this by increasing BL so it sims identical in my ML-TL example, ergo the specs I used will sim the same in yours.

Plugging your numbers in I get a ~18 Hz Fb with the same F5 as you, but when I plug in 0.1 Sd I get an indescribably bad response with an Fb that is somewhere below 10 Hz and a ~68 Hz F5, so something's amiss at your end.

The fact that you get the correct response one time and not when you change it implies you are having the same problem I had constantly with the demo version 8 and to a lesser extent with my licensed copy of MathCad Pro 2001, which requires that the spreadsheet be closed/reopened. A minor aggravation with a licensed copy, but a real pain in the #$% to keep inputting it ~each time you do a change in the demo version.

GM