horn lining or TL

There is a product that can be applied to the inner surface of horns or tapered drive lines to improve performance. I am not referring to rumble dampener.
I am thinking, for example, of the rebounds of acoustic waves against the walls. it probably doesn't exist otherwise it would have already been done by the producers, but you never know and it doesn't cost anything to ask.
Thank you
 
There is a product that can be applied to the inner surface of horns or tapered drive lines to improve performance. I am not referring to rumble dampener.
I am thinking, for example, of the rebounds of acoustic waves against the walls. it probably doesn't exist otherwise it would have already been done by the producers, but you never know and it doesn't cost anything to ask.
Thank you

A horn and a transmission line require rather different things.

A transmission line needs a fairly precise amount of light stuffing to be present in order to let the low frequencies through while absorbing the high frequencies. Sticking foam to the walls and partially filling the line rather than filling the whole line with loose stuffing is a fairly good way to achieve this because it can be precise and it won't settle and change the amount of absorption. The wavelength is so long in a transmission line that the thin boundary layer next wall has no effect, indeed the fact sound will travel at a different speed through the foam/stuffing won't cause any significant issues. There is nothing critical going on unlike in a horn.

With a horn one ideally wants a totally hard frictionless wall with the wavefront exactly at 90 degrees to the wall or else there will be unwanted reflections. This isn't possible in practise but modern geometries are a significant step forward compared to the honky horns with lots of reflections from a few decades back. Anything that adds friction or softens the impedance of the wall will move away from this ideal and could be expected to make thing worse. However, if the wavefront is significantly far from ideal initially it is feasible that friction and/or softening the wall might bring about a tiny improvement but I have my doubts it would be worthwhile compared to improving the geometry.
 
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It needs a problem to which it is an optimum solution. In terms of engineering a bit of fluff behind the tweeter will absorb high frequency sound effectively for low cost and in a fairly small amount of space (varies a bit with effectiveness). Metamaterials can also achieve the same thing but for higher cost and with more space. However, unlike a bit of fluff, metamaterials can be a way to positively distinguish speakers and increase their value beyond the additional cost of including them if the marketing sticks in the eyes of most consumers. In the case of activated carbon for example it didn't really but this time the signs seem to be more positive. The value might fade away with time but then again it might not.
 
Cessaro had/have a ribbed surface machined into the upper mid horn.
The size of the ribs would be deliberate to the frequency in question.
I guess deeper frequencies would require too large ribs.

I experimented with a fine sand finish on my upper mid horns; 1200 to 11000Hz.
Not on the immediate throat area, that should be smooth, but about 2" out and beyond.
This I like, both on the tractrix 400Hz and Le Cléac'h 550Hz horns, I now use.
Mid frequencies might like horns to be very smooth and slippery?
Bass deep frequencies don't care much about imperfections, deep bass can go around sharp corners like in a tapped horn no problem.
 
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