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A student has asked if these horns are known to have an "engineered" overtone series that runs flat in the range in the staff and below.

Can anyone confirm or offer a solution?

I suspect psycho-acoustics to be partially operative here as well.

That is not my experience at all. Mine is pretty much what I'd expect from a brass instrument - 5th partial a tad low, 6th a tad high, 7th funky.