Time Alignment for WMTW/WWMT project ?

[atm98]

I want to understand the time-alignment issue before starting this project, so here it goes:

At the crossover frequency, the tweeter should have a phase advance due to the XO. If so, then the speaker layout should attempt to bring the mid closer to the listening position, and place the tweeter further away. I am assuming this is the reason for the tweeter offset of .75" from centerline. However, if I end up placing the speakers toward the listening position, there is virtually no difference between the mid and tweeter in horizontal distance to listening position.

(I assume my listening position is 38" above the floor; same height as the tweeter.)

The vertical difference 5.25" between mid/tweeter accounts for almost all the time delay. This appears to be counterproductive as the mid lags the tweeter. It appears to me that a better design option is to recess the tweeter, or place the tweeter below the mid.

Ultimately, I have 3 questions:
1. Does the .75" tweeter offset really make a difference?
2. Should I consider switching the location of the mid and tweeter?
3. On the WMTW, what is the difference if I place above vs. below the TV (listening level)? - Yes I know I could probably just flip the speaker over.

And advice or experience will greatly help. Thanks.

Austin

[Landroval]

1. Does the .75" tweeter offset really make a difference?

It is offset to decrease the effect of baffle diffractions.

[cjd]

X-axis offset is to minimise diffraction at one given frequency. This is important to understand - diffraction can wreak havoc on a design.

Z-axis offset is something that some designers use to time-align drivers (i.e. they tilt the baffle back so the tweeter sits farther back than the leading edge of the mid - acoustic centers are vertically aligned, however).

Additionally, you'll note that crossover simulation software allows you to SET z-axis, which means that when you design you'll include that difference and correct for it in the crossover (note that if you use real measurements with the mic at the same spot for ALL the drivers - i.e. measure them in-cabinet without moving the cabinet or mic as you measure each individual driver - you do not have to do this z-axis setting, as the measurements reflect it already).

Tweeter on top vs. on bottom in a TM tends to give some "tilt" to the response pattern - it usually tilts to the tweeter if I remember right.

A bigger issue to consider for a center is vertical alignment across the whole front stage - a tweeter on the bottom puts it that much farther below the tweeter axis on your mains.

[atm98]

Thanks Chris,
That puts me back on track. I should probably spend more time looking into the diffraction issue.

Austin

[dlr]

One detail

X-axis offset is to minimise diffraction at one given frequency.

The only case for which there would be a single frequency to affect would be for a driver centered on a circular baffle. Any other baffle will have multiple frequencies at which diffraction is significant and which will change due to x- or y-axis changes in position on the baffle. Some may even be worsened. IMO use of the BDS is essential prior to cutting baffle openings.

[JonP]

The vertical difference 5.25" between mid/tweeter accounts for almost all the time delay. This appears to be counterproductive as the mid lags the tweeter. It appears to me that a better design option is to recess the tweeter, or place the tweeter below the mid.

Sounds like everybody covered everything, but in case it wasn't obvious to you, any kind of "step" (like recessing the tweeter into the baffle) would create huge diffraction effects. Typical attempts to bring the tweeter physically back closer to the acoustic center of the mid are in angling the whole flat baffle with flush mounted drivers, thereby not adding any new steps or angles to the basic baffle geometry. Of course, now you have to deal with your drivers slightly off axis to the listener...