I am by no means an expert, but also not keen on potentially breaking copyright based on a single forum post by a guy I don't know - no offense meant in any way.
I understand your concerns.
I'm not saying you're wrong, but I'm not convinced. A measurement is not done solely by a machine, in fact two different people might measure headphones wildly different and get wildly different results, even if using the same "machine" and same headphones.
>A person who finds and records a particular fact does not create that fact; he or she merely discovers its existence
Is this really the case for headphone measurements? If so, does this mean I can rip any headphone measurement on any site, and simply put it all in a single database?.
When facts are established or discoveries made, human are always involved, as the devices" autonomy" is too limited, the purpose, arrangements, parameter configurations, etc. are set by humans, nevertheless copyright never applies to facts, science, discoveries (but the devices themselves or the processes for running them could be seen as inventions which might be protected by patents if fulfilling certain requirements).
The closest you can get are copyrights for photographs and for industrial designs (dimensions and arrangements of the pattern of a device). Because one would not be able to recognize the author, artist behind the result, this is not really specific for the author, artist, and so for me this is wrong, an abusive extension of the copyright. I understand this was justified because there is an aesthetic endeavour, the photograph taking a picture of a landscape or of a portrait being compared to the painter painting the same, i.e. as producing "works of arts"(aesthetically) unlike what happens in pure fact-finding and science.
As long as there is no "artistic" intent but only technical fact finding, graphs representings measurements cannot fall under copyright, and the less the numerical values represented on the graphs.
There is one thing though about which you should be careful:
In the USA copyright was extended to cover the selective creation of databases, and through the TRIPS agreement the USA managed to impose this and many other recent US extensions of "intellectual property" to many other countries (e.g. in Europe).
While facts such as measurements are unprotectable, creating through selective work a database of such facts can be considered to be a "creative" work if it "constitutes an original work of authorship" and the database could then be copyrighted. So someone could create a database of measurements, and apply and obtain a copyright for it, even if he didn't do the measurements himself. The individual facts (measurements) remain unprotected and all can always be extracted, but copying the whole copyrighted database would be a violation of the obtained copyright.
So as long as you are not replicating an entire database created by creative selection and for which copyright could be obtained, you are fine. I am not aware of any copyrighted database re. headphone or IEM measurements (probably not worth it for big companies).
But even if someone managed to copyright such a database, you would still be able to extract all the factual records inside (as long as there is no issue of privacy, e.g. names and addresses of customers, which does not apply here) and use or reproduce all of them in a different manner, you would only not be allowed to replicate the whole database.