Little help needed - Vifa MG18 + Vifa DX25 - 2.5way

[guitvinny]

Hi everyone,

I'm about to build my first pair of speaker (beside being a EE student, I have fairly good woodworking skills).

I want a ported 2.5 way with Vifa MG18 + Vifa DX25 (I already got the drivers) floorstanding pair of speaker.. Quite an ambitious project !

I'm beginning the reading of loudspeaker cookbook and introduction to loudspeaker desing, I started playing with WinISD, but I still got few questions.


- For a 2.5 way, I saw designs with the 3 drivers enclosed in the same volume, others with the driver acting as a sub in a separate volume. For sake of simplicity of tuning, I think it's better to have only a single volume of air. I'm I wrong? What's better and what are the trade off?

- Using WinISD, tuning for 40Hz, it gave me a volume of ~52 L. Should I follow this recommendation, would I make a mistake ? If not.. where I start ?

- For the port : What diameter should I use ? I was thinking 3".. should I go smaller ? Does it matter, in witch way ? From WinISD : If I use a 3" port it should be ~ 10cm.. isn't small ? Should I use theses values ?

- For damping : Is there a rule of thumb to follow for filling enclosure with polyester ? Is it done 'by ear' ?

Thanks in advance !


Here's the link to the datasheets of the drivers :

DX25 : http://www.solen.ca/pdf/vifa/dx25tg0504.pdf
MG18 : http://www.solen.ca/pdf/vifa/mg18wk0908.pdf

[jerome69]

Hello,

52L@40Hz seems to me OK !

Use the largest port you can, why ? because you avoid turbulence in the port. Turbulence causes distortion at high levels.
If you can use 4"diameter ~ 9" long , it's better. But 3" is good.
It's better to use flared port.

The damping, you can see here. This tutorial is well done.

Hope that helps !

[guitvinny]

Thanks for the link to the damping tutorial, very instructive.

Still, should I enclose the 3 drivers in the same volume or should it be separated ? Also, I saw few design with the tweeter being off-axis, what is the benefit of this practise ? Finally, what should be the the vertical separation between tweeter-mid and mid-sub ?

Thanks, I hope I don't bother with my newcomer questions !

[guitvinny]

A bit more help would be greatly appreciated !.. please !

[Jay_WJ]

As for the enclosure volume separation, see Zaph's article:

http://www.zaphaudio.com/commonenclosuretest.html

Maximum allowed tweeter-midrange spacing depends on your crossover frequency.

Offsetting a tweeter is to mitigate baffle diffraction effects on its response.

[guitvinny]

Hi,

I've just made the 3d modeling of the speakers

http://entertainment.webshots.com/ph...68130724erdMlo

http://entertainment.webshots.com/ph...68130724JLMXIG

Please, have a quick look and tell me if there is any issues that I should check before going on (like drivers positions, bracing, else..). It's my first build, I'm very glad for every information you can provide me !

Greetings

[Inductor]

Looks like your drivers have a big dip followed by a step of +/- 3dB at ~1KHz before the xover with the tweeter.
Port diam. min. is 2" or 5 cm for a calculated real excursion around Fb of Xc=4,17mm.

[guitvinny]

Hi,

Thank a lot for your replies,

- I intend to use 3" port, ~ 10 cm

- Is it me or the speaker parameter in winISD differ from these given by Vifa ? (ex : Vas in winisd is 32,9 L and in Vifa datasheet is more like 45 L, it goes the same for fs, Qts..etc ). For simulation, I've used the parameters from manufacturer that I've entered by hand. Should I use these provided by winISD ? With my parameter, at 32 L@40hz, I got a overshoot of .6dB @ ~120Hz

I've just attach the driver parameters to the post. Please have a look, maybe I've done some errors.

[Inductor]

I just noticed that i used an old driver on WinISD. But don't forget to insert 2x drivers and under signal, give the input of max. amplification (?), and then check for cone excursion (Xmax.?) on the graphics window, if it's acceptable. That's were you are controlling max. output for the system and extension. Post results.

[guitvinny]

Hi,

I've just re-check everything with cone extrusion in mind, with 35 W of power. (I'm not sure if it's a good source but I saw there : http://www.oregondv.com/Vifa_Speakers_MG18WK09-08.htm that the Xmax is 4,65mm). So I played with different volume (40, 45, 52 L) all tuned at 35Hz. Finally, I think (please tell me if I'm wrong), that these drivers would be more suited in 40 L enclosure, tuned at 35Hz. I should be able to get a f3 of ~50Hz.

You can have a look at my results : (by the way, the orange one is 40L at 35Hz

1- Transfer function