Heil air Motion transformer directivity with horn

mpoz

Member
2016-05-04 12:26 am
Hi all,

I'm wondering if it is possible to use a Heil air motion transformer (tweeter) in a manner analogous to a compression driver, such that the mouth of the horn has a smaller area than the tweeter.

My goal here is not an increase in efficiency so much as controlling directivity to a higher frequency than would be possible if the mouth of the horn were larger than the tweeter. Air motion transformers are generally fairly large and if I use a horn that has a mouth larger than the air motion transformer itself, I'll lose directional control fairly quickly and have beaming at high frequencies.

I doubt it's possible considering the shape/motion of air motion transformers, but as I don't know a lot about them I'm hoping I'm mistaken. Or, if it's not possible, is there any other alternative to this issue?

Maybe it'd be helpful to say that I'm specifically thinking of designing a horn for the Beyma TPL-150 (rather than using Beyma's own horn), but I'm interested in the general theory as much as the specific application.

Thanks!
 
"My goal here is [...] controlling directivity"

How? Are you trying to narrow the horizontal response or widen the vertical response?

If the former, then Beyma's own horn should do the job.

If the latter, you could try blocking the top and bottom of the driver with foam, so that it is effectively shorter / more of a point source, at high frequencies. I think the shorter AMTs and other planars have better HF precisely because they are shorter.
 

mpoz

Member
2016-05-04 12:26 am
"My goal here is [...] controlling directivity"

How? Are you trying to narrow the horizontal response or widen the vertical response?

Actually neither -- it's to increase the directivity control up to a higher frequency range. IIRC, the Beyma's directivity control at higher frequencies is limited by the size of the pleated tweater (which is rather large for a tweeter).

hollowboy said:
If the latter, you could try blocking the top and bottom of the driver with foam, so that it is effectively shorter / more of a point source, at high frequencies.

This sounds like an interesting idea, but will it cause other problems (standing waves, diffraction, etc.)?
 

mpoz

Member
2016-05-04 12:26 am
Did you mean to write 'control directivity down to a lower frequency'? Also with regards to your first post a properly implemented waveguide should not have a beaming problem at high frequencies even with the larger mouth.

Sorry, somehow missed this.

I actually meant to control directivity up to a higher frequency. The highest frequency with directivity control is determined by the size of the throat, correct? So essentially I'm trying to decrease the size of the throat smaller than the size of the driver itself (similar to a compression driver, except with a Heil air motion transformer -- in this case the Beyma TPL-150).
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
A smaller throat may help efficiency at higher frequencies. A source of a finite size will have an effect on directivity. If the source is rectangular the longer dimension could be seen to have higher directivity. The dimension having an effect on how low this directivity is held. Adding a waveguide beyond the source can hold this directivity down lower. Broadly speaking you could view the finite size of the radiator as representing similar to a waveguide that has a mouth of that size and shape.
 
Hawthorne Audio use to make a large OB that used the big Heil in an MTM configuration that was horn loaded. I don't see it any longer on their website but I listened to it for an hour or so at a fest and really liked them.
 

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mrk7

Member
2011-03-25 8:49 am
GC
to increase the directivity control up to a higher frequency range.

Generally, reducing the size of the aperture 'presented' to the room should increase the beaming frequency (i.e. smaller aperture maintains wider dispersion up higher).

will it cause other problems (standing waves, diffraction, etc.)?

As far as I know, both those issues are a function of wavelengths, enclosure/horn entry geometry and the opacity of materials used. If you create an enclosure (i.e. band pass/compression chamber) you can expect standing wave modes to occur if soundwaves are able to form and interact (add constructively or destructively to create peaks and dips). Usually full, half and quarter wavelengths are considered to be important.

Also, I think the form of the soundwaves (e.g. cylindrical for planars) creates another variable to be considered.