Arrowhead is a loud place. I remember it being unnaturally loud for the preseason game last year vs. SF on the first few plays.
I remember in the mid '90s watching an MNF game where the ref was asking the fans to quiet down otherwise the Chiefs would get a penalty.
It is absolutely for real, but at the same time nothing has been as consistent as the challenge of playing at Century Link for opposing teams.
I have a different take on this sound thing. It's quite possible that outright SPL isn't everything, but maybe the SPL at certain frequencies might matter more. It would be interesting to take an RTA (Real Time Analyzer) like one used to EQ sound systems and see what the curve looks like at both of these stadiums. My guess is the curve is more favorable at the Link.
If I had to guess, it's possible Century Link has more SPL in the 2-3khz and upper treble, like 15-20khz. These sorts of ranges overboosted in audio are more annoying to listen to and are fatiguing.
Either that or there's something else going on. I'm an electrical engineer and huge audiophile (both home and car) and I try to explain to people that sometimes you cannot measure everything, or even if you can, correlating it to what is "better" in real life isn't as easy as you think.
Like for instance there are numerous quality products that don't "measure" particularly well that sound better IMO than products that have nicer #s for distortion, signal to noise ratio, etc.
Psychoacoustics plays a role in audio listening for enjoyment and most likely plays a role in how hard it is to hear.
For a test, if you can get your hands on a test tone generator, play around with the different frequencies. Certain frequencies at a fixed volume are harder on the ears than others.
Another thing I'm involved in is intelligibility for fire alarm system design. This is the engineering of a voice communication system in a building where the occupants can understand what is being said. It is different from audibility which is merely being able to hear it. Comprehension is a harder task.
So there are different principles at task to make a sound system more intelligible than others. Understanding acoustics is quite in depth, and IMO is a lot more complicated than merely focusing on SPL.
This is the first time I've given this stadium loudness issue any real thought and now I'm convinced that what C-Link has going on has less to do with SPL and more to do with the acoustics of the stadium.
It probably would be very effective even at 10-15 dB lower.
The bottom line is the same eye test we have to evaluate QBs in light of what the passer rating, QBR, PFF, etc. says is the same one we can use to evaluate which teams are struggling more offensively at any particular stadium.