Is that Piano Forte an IEM, correct? It's must be a cool project! I have made something similar with the Mrice E100 housing, once it has a similar horn shape it was cool to see the influence on the sound depending on the driver, I got amazing results at treble extension (using 300Ohms and H180 drivers) - like cymbals most realistic. However, the fit becomes more tricky than the MX500 shell to get the full potential. I'd imagine this Piano Forte has a similar idea and probably an amazing sound.
Out of curiosity, I've heard from Henry (Cyperus) using metal housing isn't a good idea sound-wise, I'd guess due to the sound would reflect much more inside than plastic or wood causing a trade-off, which makes sense since it's really rare see a speaker or instrument using another thing besides wood and non-metal materials. But maybe for earbuds or IEMs the sound can get a pleasant timbre for some with the right tuning, like MrZ/Toneking lineup.
My next projects I really want to try some wood shells and figure out if there's a big difference in the timbre. The hard call now is to find out a person with some CNC machine or a skilled luthier, lol.
I think most people consider the Piano Forte to be an IEM, but it doesn't use silicone tips or have a seal at all, so it is kind of a hybrid between an earbud and IEM. It's presentation of the sound and also comfort are much more in line with an earbud than any IEM I've had. I haven't heard an original PF yet, but even this DIY has shown how impressive the soundstage and out of head feeling of these is, so much so that I bought a pair of the Lab II.
It seems to me that the PF housings cause any driver to have a rolled off treble. Aside from soundstage, I would say that is their defining characteristic more than any "coloration" like some people say, just my opinion from tinkering with different drivers and tunings so far. I don't think it will really be possible to get a flat and well extended response from these shells, but that isn't to say they will be missing anything. I think a more mid-centric response is inherent in the design of the shell. Interestingly, I find this much more pleasurable in the DIY PF than I would in a really mid-centric earbud. I think since the PF tips funnel sound so well into your ears, It allows for excellent detail retrieval, which makes the treble roll off just feel laid back more than severely lacking to me.
As far as my opinion on housing material goes, I guess I just don't think there should be any rules like metal is worse than plastic, plastic is worse than wood, etc. I think it is likely different materials cause different responses given the same dimensions and design of housings were created, but I think it would be possible to have an excellent design made from anything really if you understand the properties of the material. Speaker cabinets would be a good example of this. Sure most cabinets are made of wood, but not all wood is created equal. There are solid wood cabinets, mdf, birch ply, etc, and of course the thickness used and bracing design matters as well. There are all kinds of proven designs depending on what you choose. It is impossible to eliminate reflections and coloration completely, but a good understanding of your material and design will allow you to shift coloration to different frequencies. Most speaker cabinet designers decide to either shift to high frequencies with a super stiff cabinet, or try and get them into the lower frequencies and reduce their SPL with a thin, more flexible design. The goal is usually to get them out of the midrange where the human ear picks up the most detail.
Its hard to say for sure, but I suspect this is not nearly as big a deal with earbuds as it is with speaker cabinets. Speakers move way more air, and have much more mechanical movement, and I would think the transfer of energy from driver to cabinet would be a much greater percentage in a speaker than earbud. Maybe I'm wrong though, when you consider how close the earbuds are to your ears it might be just as relevant.
I've already become a big fan of the metal IEM tip design of these PF's. I have been thinking about making some computer models of something similar to have 3D printed for more DIY fun. I think something like a Dunu Titan 1 design using metal tips, and large enough to use 15.4mm earbud drivers could be a cool project. It would take a lot of time, trial and error though.
