offset tweeter in MTM?

[adolphe]

Why in some MTM (D'Appolito) configurations are the tweeter offset from the centerline of the woofers? A specific example is the Eros design. And on the topic of the Eros design, why is the array offset from the centerline of the cabinet?

[Ed LaFontaine]

Diffraction is something that occurs when a sound wave moving across a flat surface encounters an edge. Diffraction produces secondary waves that interfere with the fidelity of the original signal. (We don't want that)
If the distance from cabinet edge to driver is the same in more than one direction, the resulting diffraction of signal is reinforced. This is especially true for frequencies which have a direct relationship with the distance the wave travels to the edge.

The offset is an attempt to make the frequencies at which diffraction occurs do so over a broader range. The thought here is that the resulting diffraction has less of an effect than if the drivers are centered.

An alternate view favors centering the drivers and treating with "baffle step compensation". Search for that one.

Hope that helps.

[Zaph]

To add to BG's post, a centered tweeter smoothes out very nicely off axis, while an offset tweeter can get a little more ragged.

So basically, where you choose to put the tweeter may depend on where you want the primary listening axis. For example, on this MTM:



I centered the tweeter and I recommend the primary listening axis to be a few degrees off horizontal. Not only does the response smooth out off axis, a slightly rising top octave on the tweeter is flattened out.

More info here: Vifa XG18 / Seas 27TDFC MTM

[adolphe]

Thanks for the info guys.

Zaph, I think I'm going to center everything. I will also be using these speakers for home theatre, so not everyone listening/watching will be in the idle position. So by the sounds of it, I would yield a better sound by designing it to be heard slightly off axis.

[BobEllis]

do a search for the program called "the edge" by SAVANTE (IIRC)- it is a diffraction simulator that will allow you to see the FR effect of centered vs. offset drivers. It accurately predicted the response ripples of my project - smoother response with an offset. It'll help you make an informed decision on tweeter placement.

Personally, I lean towards offset tweeters, but a always, YMMV.

[Ed LaFontaine]

John, I just read your post using metal domes with a wave guide. E-mail me when/if you decide to take the next step.

[Zaph]

Originally posted by bg40403
John, I just read your post using metal domes with a wave guide. E-mail me when/if you decide to take the next step.

I don't know if you saw it, but there was a discussion on Madisound a while back. Mad Post

I do intend to use the waveguide sooner or later. Not necessarily with a metal dome though I'm sure the results would also be good in that case. I've got an extra pair of TDFC's laying around which I may use. I also have various 7" Seas paper drivers which may be good mates. (I'll know after I test them) I have to finish a couple current projects first though.

Note: I generally don't hand out my email address until I get to know someone a little bit. (sorry) I have been meaning to update my email page to explain why, maybe I'll get to it this weekend.

[Ed LaFontaine]

for hijacking his thread...

John,
I don't have the test set-up you have. I do have a wood lathe and several blanks ready for turning into waveguides for evaluation. I had been considering the 27TAC/GW for it's low FR, though Zalytron doesn't list it today. Another dome of interest is the RS28A.
I started this here:
Tweeter faceplate as part of the cabinet
So, as I said, when you move along with this Please let me know.

Regards,