Hornloading the Heil AMT1

[badman]

ESS AMT-1 Air motion transformers ?!

I've posted this on the Geddes Waveguide thread and the thread linked above, but this subject probably deserves its own thread. The design started out with the MDF edgeround projecting into the throat, but the projection worked out to be greater than anticipated so was ground down. The gap between the assembly and the heil will be coupled with foam window gasketing to make the transition smooth but removable, and the triangular piece in contact with the AMT frame has bondo filling the gap to the rest of the horn flare. The expansion is straightwalled with a roundover to a short flat section then a round-back. According to the behavior of horns with compression drivers, and the simulations run by our own Mige0 here: Dipole Horn, Dipole Directivity Control Device
this should maintain CD exceedingly well. I'm undecided about whether to include the top section or not, it seems like it's unnecessary as there's already dipole cancellation and the directionality imposed by the 4" vertical dimension taking place. Perhaps just a soft absorber to dampen any reflections off the top edges of the MDF?

Let me know what you think!

[mige0]

Originally Posted by badman

oops missed attachment- it's hard to tell but the continuity between horn and heil is very good.

Ahh, haven't noticed you already have built it.

Anyway. Give it a try "as is" and possibly follow my suggestions later on if you think it's worth the effort.

I have only seen drawings of the ESS so far but what I remember the roughly 90 deg front is *not* the contour you are looking for to match.

What IMO is to look for is the *internal* contour and this one IRC is plain 0 deg.

So - your own contour should start out with a smooth transition to that 0deg - opening up to the 90deg (included) you prefer.

Michael

[badman]

Hm, upon reviewing, I see what you're saying. Perhaps I'll kerfcut the dickens out of a few pieces of ply and give one of the more constricted throat profiles a shot. In retrospect, I wish I'd simply used kerfcutting to make this assembly. I'm not convinced that the more constricted throat is desirable however, as I see the increasing directivity with frequency of that profile style to be a negative. We'll find out when I test this one!


Any thoughts on the vertical component?

ESS Great Heil Air-Motion Transformer | Parts-Express.com

[mige0]

In general - with what dimension you start out into the horn has nothing (or at least not that much) to do with the directivity you end up with - if you meant it that way.
It has more to do with the "top end" control of the horn / waveguide.

So - if you are in OS style waveguide you could start with a simple roundover into the conical section right at the place where the slits end - making no use of the 90deg (incl) angel part of the Heil at all.

Originally Posted by badman

Any thoughts on the vertical component?

Not at the time being - I have some ideas but have to check out first which is most unlikely to happen this year.


I'd very much appreciate if you could do some measurements even before doing any "improvements" and post the IR files.
This would give me - and others - the opportunity to do some analysis I'm particularly interested in.


Michael

[badman]

Hm. I guess only measures will tell us. My measurement setup is rough (it's generally enough to tell me whether a concept is working as intended or not, which is sufficient for me), but I can certainly confirm if it's imposing directivity as intended. One follows the other, so far as horn action and directivity are concerned. I certainly hope that I'm getting reasonable cylindrical propagation, as these things have been something of a pain to get up and running

The horn action on the Beyma TPL-150H doesn't seem to be at the mercy of a constricted throat, however, it appears to have the coverage angle defined by the angle of incidence in the throat, as it's a straight-wall profile until the termination roundover.

[Robh3606]

Any thoughts on the vertical component?

Seem's it's already fixed and narrow. You find any polars on the Heil by it's self?

Rob

[cuibono]

Cool looking project! Can't wait to see some measurements. I'm currently back in LA, so if you want some help, I've got a functional measurement setup. How wide is the whole thing?

[badman]

ESS AMT-1 Air motion transformers ?!

This is the best (horizontal) polar I know of. I have a suspicion that the 1k hump represents a dipole peak before the bottom end rolloff. I am hoping that the peaky polars are largely due to interference patterns from the harsh termination of the "horn".

[badman]

I'd appreciate working with you on running measures, I'll get in touch when they're ready.

[mige0]

Originally Posted by badman

ESS AMT-1 Air motion transformers ?!

This is the best (horizontal) polar I know of. I have a suspicion that the 1k hump represents a dipole peak before the bottom end rolloff. I am hoping that the peaky polars are largely due to interference patterns from the harsh termination of the "horn".

People get easily impressed by the fancy look of - and the countless legends about the Heil speaker.

But from a mere speaker design point of view any modern AMT is light years ahead.

Meaning – for a new design (contrary to retro-design) and especially for any waveguide / horn / diffraction alignment design you are far better off with the almost perfect wavefront (in the frequency range of interest) released by AMT's from Beyma, Mundorf, etc.

But as you already have the Heil at hand – just give it a try.
There are pretty nice applets out there to visualize (more or less) how the released wavefront of the Heil might look like, but its possibly not of much help for you beyond what I've said.


Michael