Why is off-axis horizontal MTM bad?

azira · 12th March 2004, 04:23 PM

I'm starting my research on building a center channel. It seems like the most popular designs are MTM (2/2.5way) or WMTW (3-way) type of designs. I also read a lot that MTMs have pretty crummy off-axis behavior. It seems like people are refering to nodes and lulls created by the 2 element "line array" of the Ms for this conclusion. But if the CC is horizontal, wouldn't it mean that you are always listening on-axis? My listening angle would be something like +/- 30deg.
thx.
--
Danny

sreten · 12th March 2004, 04:47 PM

At c/o a vertical 2-way MTM can be treated as a 3 element line array.

Horizontal dispersion is not a problem, but vertically due to the
3 elements dispersion has more nulls than a MT 2-way.
(Typically 4 versus 2, and the angle of the 1st null is smaller)

When turned on its side, so the vertical dispersion becomes
the horizontal dispersion basically the extra nulls are not
a good idea in terms of horizontal seating position coverage.

For a horizontal box :

--T--
WMW

Is the way to do it, have vertical MT array with woofers either side.

sreten.

officeboy · 12th March 2004, 05:15 PM

As you can see.. It's not that pretty, unless you’re really off axis.

azira · 12th March 2004, 05:52 PM

Ok, I just missed something. I've also been reading up on line arrays and I thought the papers indicated increased sensitivity on-axis with line arrays while the lulls started appearing off-axis. The sound field becomes more like a cylinder so any point vertically on-axis should be the same as any ohter point. Is this wrong? That's the way it's indicated in the vpr excel program.

sreten · 12th March 2004, 06:19 PM

Is only true for a perfect line array, e.g. a ribbon or similar.

Or lots of units at frequencies with wavelengths greater than the spacing.

sreten.

Bill Fitzpatrick · 12th March 2004, 06:26 PM

Quote:

Originally posted by officeboy
As you can see.. It's not that pretty, unless you’re really off axis.

Where is that ripple tank program located. I want to play.

officeboy · 12th March 2004, 06:29 PM

http://www.falstad.com/mathphysics.html

I can spend hours playing with these things.

azira · 12th March 2004, 06:41 PM

Quote:

Originally posted by sreten
Is only true for a perfect line array, e.g. a ribbon or similar.

Or lots of units at frequencies with wavelengths greater than the spacing.

sreten.

But when the wavelength starts to equal or become shorter than the spacing, the dispersion pattern is on both axises?

sreten · 12th March 2004, 06:50 PM

Quote:

Originally posted by azira

But when the wavelength starts to equal or become shorter than the spacing, the dispersion pattern is on both axises?

Oh yes,

sreten.

Timn8ter · 12th March 2004, 06:51 PM

It's interesting how much it cleans up when you add a 4th source at nearly the same point as the center source. This seems to simulate what would happen if you add a mid-ranger under/over the tweeter. This app is way

12th March 2004, 04:23 PM	#1
azira diyAudio Member Join Date: Dec 2003 Location: Near Seattle	Why is off-axis horizontal MTM bad? I'm starting my research on building a center channel. It seems like the most popular designs are MTM (2/2.5way) or WMTW (3-way) type of designs. I also read a lot that MTMs have pretty crummy off-axis behavior. It seems like people are refering to nodes and lulls created by the 2 element "line array" of the Ms for this conclusion. But if the CC is horizontal, wouldn't it mean that you are always listening on-axis? My listening angle would be something like +/- 30deg. thx. -- Danny

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12th March 2004, 04:47 PM	#2
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	At c/o a vertical 2-way MTM can be treated as a 3 element line array. Horizontal dispersion is not a problem, but vertically due to the 3 elements dispersion has more nulls than a MT 2-way. (Typically 4 versus 2, and the angle of the 1st null is smaller) When turned on its side, so the vertical dispersion becomes the horizontal dispersion basically the extra nulls are not a good idea in terms of horizontal seating position coverage. For a horizontal box : --T-- WMW Is the way to do it, have vertical MT array with woofers either side. sreten.

12th March 2004, 05:52 PM	#4
azira diyAudio Member Join Date: Dec 2003 Location: Near Seattle	Ok, I just missed something. I've also been reading up on line arrays and I thought the papers indicated increased sensitivity on-axis with line arrays while the lulls started appearing off-axis. The sound field becomes more like a cylinder so any point vertically on-axis should be the same as any ohter point. Is this wrong? That's the way it's indicated in the vpr excel program.

12th March 2004, 06:19 PM	#5
sreten diyAudio Member Join Date: Nov 2003 Location: Brighton UK	Is only true for a perfect line array, e.g. a ribbon or similar. Or lots of units at frequencies with wavelengths greater than the spacing. sreten.

12th March 2004, 06:29 PM	#7
officeboy diyAudio Member Join Date: Nov 2003 Location: NW Washington	http://www.falstad.com/mathphysics.html I can spend hours playing with these things.

12th March 2004, 06:51 PM	#10
Timn8ter diyAudio Member Join Date: Jul 2003 Location: Seattle, WA	It's interesting how much it cleans up when you add a 4th source at nearly the same point as the center source. This seems to simulate what would happen if you add a mid-ranger under/over the tweeter. This app is way

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