Time coherency (MTM Project)

[djarchow]

When I made the hilbert-bode transforms, I had to use 3,7cm delay for the woofer, and 0,9cm for the tweeter to get the phases match. So this would mean, that the difference in acoustic centers are around 2,8cm? And by pushing the tweeter enclosure back, they would be aligned at the measuring point.

No, it just means that given the rolloffs you chose in the hilbert screen, you had to adjust the delay that much. Choose a different set of rolloffs and that delay will change.

I normally set my left time marker in the MLS screens to be just before the start of the rise of the tweeter impulse. Then when measuring the woofer, set that left time marker to be the same time as the tweeter time marker. Thus you have removed all of the delay from the mic to the tweeter and the woofer phase is relative to that.

Here is how the phase responses looks, without any change to the tweeter location, and with the Xo described earlier:

Phases

They seem to align quite well, right?

They align well up to about 3k, but this isn't a good thing since you want 3rd order acoustic slopes the inter driver/xo phase should be 90 degrees apart.

I would also like to know the difference in the MLS measurement screen, between the two window types, rectangular and blackman-harris? In every SE guide, the blackman-harris is suggested. But when I make my measurements (typically with 4-5,8ms window), I get much better result with rectangular window near the lowest window frequency. With blackman-harris, the response starts to slope down before the window edge. I'm a bit confused with this.

The rectangular window gives you better resolution at lower frequencies at the expense of more noise in the high frequencies from reflections. With the 5.7 ms window you will have valid data down to about 175 Hz but your resolution will not be great. So you only have valid data points at 175 Hz, 350 Hz, 525 Hz etc. Did you do a nearfield measurement of the woofer, apply the baffle effects in SE and then merge with the far field data?

Regards,

Dennis

 

[Twisted85]

They align well up to about 3k, but this isn't a good thing since you want 3rd order acoustic slopes the inter driver/xo phase should be 90 degrees apart.

OK, I read about it from the Loudspeaker Design Cookbook and devided to change the slopes to 4th order L-R. And here how the responses look like:

4th order

The phases seem to align quite nicely. The Z offset plays quite small role as the frequency is so low.

I also used a parallel notch filter to flatten the 2KHz peak. This speaker can sound a just bit bass heavy in a room (as the FR is now pretty flat), or what do you think? But this is just a experimental project, nothing I'm gonna finish.

The rectangular window gives you better resolution at lower frequencies at the expense of more noise in the high frequencies from reflections. With the 5.7 ms window you will have valid data down to about 175 Hz but your resolution will not be great. So you only have valid data points at 175 Hz, 350 Hz, 525 Hz etc. Did you do a nearfield measurement of the woofer, apply the baffle effects in SE and then merge with the far field data?

Yes, I did.

Here are the differences between the two windows, I measured:

Windowtypes

The window was 5,8ms, but the time of the first reflection (floor) arrival was over 6ms. How come the rectangular window can be more sensitive to reflections as the time window rules them out? Or does baffle diffractions play any rule here?

 

[djarchow]

If the right time marker is prior to the first reflection, than either will work fine, though the blackmann harris will have less resolution in the low frequencies than the rectangular. But because of the tapered window, if you move your right tie marker out past your first reflection, you will see the benefit of the blackman window.

Regards,

Dennis