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Old 18th June 2005, 05:50 PM   #1
adolphe is offline adolphe  Canada
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Default offset tweeter in MTM?

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?
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Old 18th June 2005, 06:13 PM   #2
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Default Edge diffraction

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.
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Old 18th June 2005, 06:22 PM   #3
Zaph is offline Zaph  United States
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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:

Click the image to open in full size.

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
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Old 18th June 2005, 08:14 PM   #4
adolphe is offline adolphe  Canada
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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.
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Old 18th June 2005, 09:44 PM   #5
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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.
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Old 19th June 2005, 05:13 PM   #6
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Default Zaph OT

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.
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Old 19th June 2005, 06:52 PM   #7
Zaph is offline Zaph  United States
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Default Re: Zaph OT

Quote:
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.
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Old 19th June 2005, 07:35 PM   #8
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Default With apologies to adolphe

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,
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Old 2nd June 2013, 11:35 PM   #9
FE3T is offline FE3T  Norway
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I know this is an old tread, but since this already exist i se no reason to create a New tread.

My question are:
With offset tweeters it should be possible to get the woofers closer together and thereby make it possible to chose a higher xover frequenzy. are there any disadvantages to do this? I am looking into using 8" woofers and having the drivers in a line seems to resulting in excessive C-C distances for a usefull xover. especially with my Audax 34mm tweets wich have a 130mm faceplate.
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Old 3rd June 2013, 12:35 AM   #10
system7 is offline system7  United Kingdom
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Actually, An Audax 34mm tweeter will crossover sufficiently low, around 1.8kHZ, that the error in terms of wavelength should be manageable with an inline MTM configuration, shouldn't it?

I think you worry too much...
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