Mundorf AMT 27d

[annerholm]

Hi,
I've been looking at these Mundorf AMT 27d tweeters as a possible candidate for an OB project. Impressive specs and a possible low xo could make it possible to pair them with double 6.5" woofers and a ripole sub. IMO these could also solve some problems when trying to achieve constant directivity in a dipole set up, and also keep it a 3-way and not 4-way (incl sub).
Here it is: http://www.lautsprechershop.de/pdf/mundorf/mundorf_amt27d11.pdf
Some questions still:
Impedance 12 ohm - high isn't it? Why?
If pairing it with two 6.5" woofers in a MMT xo would be nice to have down at 1600Hz - possible?

If anyone have experience from these it would be really interesting to know, or just to hear your general opinon! Or if you have used som of the other Mundorf AMT in the dipole series.

Best wishes,
Anders

 

[annerholm]

Any comment appreciated

 

[Keith Taylor]

For a transducer of that height the vertical radiation pattern was always going to be narrow, something the plots confirm. The sound of the AMT is going to change between standing up and sitting down.

Some people may be able to live with this: I am not among them.

Keith

 

[annerholm]

But wouldn't that apply to the smaller ones in the same Mundorf series? Basically it's just the height that differs, otherwise they have the same construction (except different bandwith of course). In the case of vertical dispersion the 27d would be superior to those. Or am I missing something here - is there anything that gives a higher (longer) AMT a more narrow vertical dispersion?
The other ones:
http://www.lautsprechershop.de/pdf/mundorf/mundorf_amt17d21.pdf
http://www.lautsprechershop.de/pdf/mundorf/mundorf_amt23d11.pdf
http://www.lautsprechershop.de/pdf/mundorf/mundorf_amt25d11.pdf

 

[phazer99]

Or am I missing something here - is there anything that gives a higher (longer) AMT a more narrow vertical dispersion?

Yes, as the sound wavelength becomes shorter relative to the dimension of the driver it starts acting more like a line source than a point source. The relation between driver height and vertical dispersion is clearly noticable in the vertical SPL plots of the different drivers you link to. Whether this is good or bad depends on your application.

If you gonna XO close to the driver FS you need a really steep (high order) HP filter to minimize the effects of harmonic distortion. I would use digital filters and bi-amping. Looks like an interesting project, please post progress reports

 

[annerholm]

Ok, I think I get it. Thanks for the feedback.
If I compare to the smaller AMT 25d (about half the size), that one has a very good vertical dispersion at 5 deg. Compared to the 27d it would have a wider vertical dispersion already from about 1m, and a lot a more at 3m. Maybe that one would be better?
Problem is, it wouldn't be able to xo as low as I thought, when pairing it with two 6.5" (which I'm quite keen to do). Also the 25d is not as good as the 27d in keeping a true dipole pattern as high up as possible because it's wider dimension. Maybe it's pick my poision... Or?

phazer99: yes, the setup would be with to Hypex AS2.100d, and sub out.

 

[phazer99]

You should also consider using the Dayton Audio AMTPRO-4 instead, as it's much cheaper and has lower Fs (but worse efficiency and power handling).

For greater vertical dispersion (and less distortion) you can always stack two of them vertically.

 

[solhaga]

For greater vertical dispersion (and less distortion) you can always stack two of them vertically.

How can that be? Would that be more or less the same as having one long?

 

[phazer99]

How can that be? Would that be more or less the same as having one long?

Yes, this is the principle of the line array system. If placed close enough the drivers (almost) form a line source.

 

[solhaga]

But wouldn't it then be less vertical dispersion with two AMT on top of each other and not greater?

 

[phazer99]

But wouldn't it then be less vertical dispersion with two AMT on top of each other and not greater?

Not for high frequencies. A bit oversimplified the dispersion pattern of a finite line source goes from spherish to cylinderish as the wavelength becomes smaller and smaller relative to the length of the source. So if you place two line sources on top of each other you are basically building a longer line source and higher dispersion cylinder. The two sources should be placed as close to each other to avoid uneven dispersion. Also ideally the length of the line source should be much greater than the wave length of the sound it reproduces otherwise the dispersion pattern will look more like that of a point source.

 

[solhaga]

Maybe it is a matter of wording.

To me a taller line array means a smaller dispersion, from Wiki:

"The taller the stack is, the narrower the vertical dispersion will be and the higher the sensitivity will be on-axis"

 

[annerholm]

Maybe I'm wrong here, but in this case isn't it about the actual dispersion at the ends of the driver? If I understood phaser99 right, a shorter AMT driver will start to act as a line source higher up in frequence than a taller driver. So, when stacked, even if it acts together it will still have this dispersion at the ends according to the/it's driver size?
I mean, for ex. compared between a 240mm tall driver and 2x120m.

Or if I put it like this: Should 2xAMT25d be better than one AMT27d in terms of vertical dispersion?

 

[solhaga]

I would say that one 27d is better as it can be hard to get two 25ds close enough in order to consider them as one line source. Another down side with two 25ds apart from each other is that combing will occur.

 

[phazer99]

Maybe it is a matter of wording.

To me a taller line array means a smaller dispersion, from Wiki:

"The taller the stack is, the narrower the vertical dispersion will be and the higher the sensitivity will be on-axis"

Yes, maybe I use incorrect wording. The dispersion off-axis of a specific frequency will be less the longer the line source, but the actual on-axis area with more even frequency response will be larger. I think a focused vertical dispersion about 0.5 - 1 m tall in the listening position would be good as it eliminates most floor/ceiling reflections while still being tall enough to cover all common listening positions.

 

[solhaga]

Yes, maybe I use incorrect wording. The dispersion off-axis of a specific frequency will be less the longer the line source, but the actual on-axis area with more even frequency response will be larger. I think a focused vertical dispersion about 0.5 - 1 m tall in the listening position would be good as it eliminates most floor/ceiling reflections while still being tall enough to cover all common listening positions.

Great! Then we have the same view.

I think your last sentence is the main reason for domestic use of line arrays.

 

[annerholm]

For greater vertical dispersion (and less distortion) you can always stack two of them vertically.

Ok, I think I got it wrong. You probably meant just compared to one driver on axis. So even if a shorter driver has better vertical disperion (due to it's size), that will increase when stacked? So a line source of a certain length - regardless of if it's one or many drivers - has theoretically always the same vertical dispersion?

 

[Keith Taylor]

But wouldn't it then be less vertical dispersion with two AMT on top of each other and not greater?

Indeed it would. Anders, we are talking about basic acoustic principles. The kind you would encounter in an acoustics text book.

For every transducer there are physical dimensions to the radiating surfaces, which, in the case of the AMT are unequal between horizontal and vertical.

The other important dimension is the "acoustic size", meaning the physical dimension in relation to the wavelength we are considering. Some texts designate this relationship ka. ka can vary from fractional values through unity (where the wavelength and radiator dimension are equal) to double digits. A source with dimensions much smaller than the wavelength is described as "acoustically small" All woofers would fit this description. A source with dimensions much bigger than the wavelength is "acoustically large".

The AMT we are discussing is acoustically small in the horizontal plane but
large over much of its range in the vertical plane. Acoustically small = broad directivity. Acoustically large = narrow directivity. Making a few assumptions such as pistonic motion; it does not matter who made the speaker driver, its directivity characteristics have been largely defined by the dimensions.

Keith

 

[phazer99]

I agree with that one 27D should be better than two 25D in a line source application. I think a (questionable?) rule of thumb is a driver distance of at most half the wavelength in a line array and at 15 kHz this would mean about 1 cm which might be hard to achieve.

Regarding vertical dispersion of the 27D, it looks acceptable at +/- 3 degrees off-axis which would give you a listening window about 0.5 m tall at 3 m listening distance. Sounds ok to me.

 

[annerholm]

Thanks Keith, good explanations, cleared up things up here in my brain;-)

I just made an estimation considering that the AMT25d has almost the same SPL on axis as at 5 degrees. It's diaphragm seems to be about 90mm tall. Then, at a 3m listening distance it would have a good response that is about 600mm tall.
If you compare this to the AMT27d that has really bad SPL at 5 degrees, and has a diaphragm that is about 200mm tall. Which would make for a good response that is 200mm tall (or if optimistic at 2,5 degress about 450mm tall..).

So the smaller 25d (just one) would give a taller vertical response than the bigger 27d if I'm correct.
But then the 25d is harder to xo and acoustically small...

(phaser99: didn't see your post before posting this)