Off-axis response of AMT/ribbon?

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Datasheets of most of AMT/ribbon didn't show the off-axis response, so what I wanna know is, how're their off-axis performance.

Do they have significantly worse off-axis response than those common tweeters (domes, ring radiators)? The "common" here I mean is mainstream, not common-priced whatsoever.
 
Those things are planar radiators, so any directivity modeling program can give rather reliable results, eg. The Edge. Many of them have more ripples above 10KkHz than simulations show because of the mask/frame where membrane is. Because they mostly are rectangular, vertical dispersion is usually different (and more aggressive) than horizontal.
I can easily find directivity plots of them with google.

This is for Mundorf AMT, quite lots of smoothing applied https://www.madisoundspeakerstore.c...mundorf-amt164um2.1-r-air-motion-tweeter-amt/

Here group test done by the Zaph Zaph|Audio
 
If you explore the Madisound site for their offering of Mundorf AMTs you can see the graphs for several different models.

All AMTs will have excellent horizontal dispersion plots (very wide and flat vs. frequency) while the vertical dispersion is more limited by the length (height) of the driver. The vertical dispersion plots show reduced dispersion for longer drivers as frequency increases while smaller AMTs have better vertical dispersion across frequency. Compare the horizontal and vertical dispersion plots for the Mundorf AMT17D1.1 and say the Mundorf AMT164UM2.3 to understand these properties. Other manufacturers' AMTs will have similar performance.

For AMT (and ribbon tweeters) drivers I position the driver so that I listen at ear height (tweeter height at 36-39 inches for me) when I'm sitting. Although standing and listening would be OK, critical listening would be better at tweeter height.
 
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I can't speak for ribbons very much but I find the off-axis response of AMT's to sound quite good, despite the rolloff. Horizontally they may seem a little limited, but not grossly so, compared to say a DX25 or XT25 Peerless/Vifa tweeter.

Vertically it's tighter, but that's a good thing.

One interesting thing I've measured is that the FR of an AMT at 9' is significantly flatter than that of an XT25 ring radiator, though at 1' they measure very close. The XT25 shows significant ripples in response, especially in the low end at 9'. This translates into smoothness and clarity methinks.
 
"The XT25 shows significant ripples in response, especially in the low end at 9'."
you've just seen more baffle edge diffraction and/or nearby boundary reflections with the ring radiator because it has lower directivity."

Both have no directivity in low end! Was the 9' measurement done indoors? Then it could be "more boundary reflections with dome" Line arrays keep similar response to far-farfield because of radically different pattern, but difference between single planars and domes I don't believe that this works.

If we believe in Edge (and why shouldn't we), the difference of 25mm circle and 25x75 planar is minimal. A dome is not that different animal, but sure it radiates more to 90¤.
 

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Hi KSTR and Juhazi,

Sorry, yes, that was the whole point and I guess I just assumed it was so obvious I didn't need to t ype it. :) Let me restate it.


"The lower directivity of the ring radiator showed measurable frequency response ripples in room which the AMT was immune to."

Is that more clear? :D :D

Best,


Erik
 
Directivity of an AMT/ribbon only depends on the dimensions of the radiating area. Here is an example.


Dayton AMTPro-4 without faceplate

Horizontal (3.8 cm wide):
attachment.php


Vertical (14.5 cm long):
attachment.php



The Beyma TPL-150 looks very similar without the horn. It is just a bit shorter.

It is generally not true that ribbons have a wide hroizontal dispersion. The Dayton for example starts to beam above 8 kHz which is a result of its pretty large width of 3.8 cm (1.5"). Below that the enclosure's dimension are responsible for directivity (in this case the naked driver). Build in a larger enclosure the dispersion will change, but the beaming above 8 kHz will not.
 

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