Carrie Symonds' flat builder appears not to have a basic grasp of acoustic engineering, or he'd know that sound waves transfer through solid mediums and thickness of walls is not enough for decent acoustic isolation, only attenuation.
Good practical article on this here from @soundonsoundmag magazine.
He may also be unaware of the high level of sensitivity of an iPhone's contender microphones, and of the extent to which this is further enhanced by compression and other algorithms in the iPhone.
Here's a good demo of that by @thatpedalshow (from 18:15) https://youtu.be/sePQePcP8Ho?t=1095 …
For non-guitarists/audio folk, they are going from something you could hardly hear over a normal volume TV to the kind of volume that most people would cover their ears for in a small room, but the iPhone makes the quiet sounds much louder and the loud sounds much quieter.
Sorry, I'm in the planning process to build a new small studio, so happen to be spending a lot of time on questions like that at the moment.
Sorry for the typo. Should have been 'condenser' mic, not 'contender', though in this case it was perhaps an anti-contender condenser mic.
PS, here’s a thread from an architect on this.
PPS, thanks to the excellent link @SamuelJohnsnon gave, I now know that iPhones actually contain a MEMS microphone, rather than an electret/condenser mic.
Interesting article comparing the two technologies here for people who, like me, regularly read articles headlined “Microphone of the Month”, “large diagphram vs small diaphragm mics as drum overheads” and similar. https://www.cui.com/blog/comparing-mems-and-electret-condenser-microphones …
And in case you missed it in the replies, here’s an acoustician’s view (thread):
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