Crossover suckout question.

[marchel]

Dear All,

Just recently I've been playing around with speaker workshop, Doing Xo simulation.

I made a 2way mtm design that produces a reasonably flat freq. response in the simulation. The woofer offset is -1.2 inch, using 7" woofer and a dome tweeter, It means that the baffle is flat.

I then reversed the polarity of the tweeter to see the Xo suckout, The notch is a good minus 50db more or less from the average response of the speaker, It's a good sign.

But...

The center of the notch is not at the center of XO freq. The center of the notch is at around 1880hz, but the Xo freq is at around 1580hz...

Is this ok ?

Does the notch need to be at the center of the XO freq?

If so , What can I adjust to move it to the center .

The XO is LR4 acoustic.

Thank you.

[Dave Bullet]

What that tells you is you do not have phase tracking through the crossover. Turn on phase on both the woofer and tweeter in your crossover chart. The phase dotted lines for each driver should "line up" through the crossover region.

How did you generate phase for your drivers? Maybe if you can post an image here (with phase on) that would confirm things.

[marchel]

Note: I'm no expert in speaker workshop, I merely copied the design of Jay_WJ from his peerless 2way design LINK , And then converted it to become a MTM 2way.

Below is a screen shot of an in phase XO with no suckout.

[marchel]

And here is the inverted polarity with the suckout.

PS , Please excuse the poor image quality, and I would like to thank Jay_WJ for his work.

[BHTX]

It's not too difficult to make out with a bit of effort, especially after downloading it and zooming a bit...
But what's wrong with your print screen key?

[sreten]

Hi,

This will always happen for for near LR4 acoustic with offsets, you need
to introduce some assymetry to get a null at the c/o point due to offsets.

2.5 way ? : http://www.diyaudio.com/forums/showt...87#post1371987

edit :also note that the null point will vary strongly with the vertical
axis and optimising it on-axis is not a particularly useful exercise.
The in-phase response over a range of vertical angles is much more
important and if the smoothest response has some phase mismatch
at the c/o point this is not as such a bad thing. Some topologies e.g.
subtractive crossovers require phase mismatching by definition.

/sreten.

[marchel]

Hello BHTX,

I've been pressing the print screen key and nothing happens , And I'm sure my keyboard is ok.

Hello Sreten,

So you're saying that the design is ok or acceptable?

[Eva]

Don't forget to verify the notch in the real speaker system once it is assembled, and tune the real crossover if necessary. Simulation is not that accurate.

[sreten]

Originally posted by marchel


Hello Sreten,

So you're saying that the design is ok or acceptable?

Hi,

I'm saying that the vertical lobing and phase behaviour issues of an
MTM are somewhat different to a MT. For an MTM vertically up or
down the path difference between mid and treble becomes more
similar before at higher angles it begins increasing again.
(If you assume the more axial unit contributes more.)

I'm saying that for an MTM vertical lobing is one of the most important
issues and this may be optimum with some phase mismatch at the c/o
point. To accurately model this you need off axis responses of the
drivers noting that for a MTM on the tweeter axis at 1m the bass
units responses are not exactly the same as on the axis of the
bass unit.

/sreten.

[2K]

For your simulations you are using the response of one woofer instead of the combined response of both to show the coss point. The null uses both woofers and is probably close to where it should be.