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I put some modifications on my spearpoint horn. One seals the air leak at the metal-wood interface with a wide rubber-band. The other stiffens the horn using two struts with bumpers on the ends.

What a curiosity... Really it improves the sound? Those struts is one of the things I thought to add at the big tinplate sides of my hmv194, to see if the sound improves. Thanks for the photo. Are they plastic tubes?

Inigo wrote:What a curiosity... Really it improves the sound? Those struts is one of the things I thought to add at the big tinplate sides of my hmv194, to see if the sound improves. Thanks for the photo. Are they plastic tubes?

They are wooden dowels. They can be stained and finished. Not sure if they are making a big difference yet.

I think the rubber band did make a difference.

The rubber, of only for the air leak sealing at that point, must make a difference!

The dowels in the horn make no difference or maybe a negative difference. Don't bother...

I'd make an educated guess... Your horn cross sections are circular. This the best shape for stiffness against symmetric air pressure, so likely this horn wouldn't need the dowels. Besides that it's made from wood, also pretty inert, at least much more than metal horns. These facts are in concordance with your results. No improvement with the dowels the horn doesn't need them.
Besides that, these extraordinary horns like yours are reported to provide a very good sound transfer and impedance matching with the air in the room. They are long, and similar in flare to exponential horn. They are much better than the internal horns of the Victrolas (non orthophonic ones). So don't expect improvements in sound modifying them.
Perhaps something can be done with the exhibition.
Have you ever tried your Gramophone with a no4 soundbox? If not, give it a try and you'll discover how good is the sound system (tonearm+horn).
So if you manage to adjust the exhibition to the best point, it will sound great, although not with so much bass as with the no4. But the exhibition is the original one, and you'll want to use it, am i wrong?
I this because I've done many experiments with the exhibition, in the line of what was done by the EMG/Expert people. Their first expert soundboxes were but a modified Exhibition.
I'm preparing a descriptive document about my experiments, which I will post herein. I've not arrived to a definitive decision, but I've found infinite adjusting possibilities, very interesting. My problem is that I only have one Exhibition and one exhibition junior, and would need at least one or two more exhibitions to be able to compare between them. But they are so expensive... I've seen some in eBay, etc, but they cost as much as my HMV Style III tabletop gramophone complete!

The bracing struts remind me of something I read in an early 1930s edition of The Gramophone, about an idea called the Wild Tautener (the same Wild who was a soundbox maestro, and amongst other things, modified and tuned Meltropes). The performance of EMG horns was supposedly improved by tightly fixing several strands of cord or elastic over the face of the bell.

Here's the page about Wild on Chunny's fantastic website: http://www.gramophonemuseum.com/wild.html

Well I love the idea of the rubber band, and have several on the way to me! It should be a huge help with my HMV 31a which has a gap (currently filled with grease) the size of Scenectady. Thanks for the tip.