* People just usually choose not to pay extra because it's not actually that important to them*

Do they choose this, though? Or are they either renting (absolutely no choice) or not monied enough to upgrade their house in this way? Is this something that your average builder offers the average person building a home? (My parents didn't have that choice when building a house in the 90s)

The technologies involved in providing inter-room sound isolation aren't exotic/unknown in general (due to significant demand from some types of office-style commercial buildings and upscale/premium apartments/condos), nor do they exceed the familiar capabilities of your normal stud-framed wall crew that regularly does insulated exterior/garage-facing walls.

So as long as you're in a place where common construction crews have regular experience building thermally insulated stud-framed walls, you'd at most have to look for an architect that has experience designing cross-tenant sound isolation who is otherwise a suitable choice for designing the kind of home you desire.

Their existing experience allows to get decent economics for the design workload of even your one-off non-catalogue customization. And considering there are few differences between sound isolation and thermal isolation (except that heat can also be stopped by IR-opaque foam rolling in for air pressure in the evacuated space between two foils (AFAIK that is much better at shielding heat flux than sound's pressure waves, due to the inherent concentration of air pressure into the stiff foam, as there is no gas pressure inside the pores to handle static air pressure forces, while heat conduction doesn't focus like it and heat radiation is interrupted by the opaque foam walls that act as "vacuum super insulation"[0]), and sound can also be stopped by thick heavy stone walls), the architect would at most spec slightly different insulation and likely somewhat different mounting brackets/pads for connecting the drywall to the frame.

The end result is no need for non-trivial engineering that a local architect should be able to satisfyingly do from experience, and a design that is just a little weird in overall layout/structure, instead of exceeding the construction crew's scope of experience in any particular detail.

[0]: https://en.wikipedia.org/wiki/Multi-layer_insulation

What materials are commonly used? People in older buildings definitely have sound issues

I was told lowering they ceiling and creating an air gap is effective

In my experience it's newer buildings that tend to have paper thin walls. In my current apartment(late 70s) I very rarely hear my neighbors.

At my last apartment I could literally hear my neighbors cough. Absolutely miserable. I wish there were some sort of measurable standard rating for noise transmittance through walls that landlords are obligated to disclose to potential renters.

New buildings have strict fire separation requirements that pretty much enforces sound proofing. All new build high rises I have stayed in have had total silence.

Seen many buildings where the concrete walls soundproof the rooms but plumbing ducts in bathrooms transmit sound like crazy - as in you can hear conversations from people several floors down. The inspection hatch cover made of thin sheet metal seems to act like a microphone exciting the air in the duct causing the other covers to act like speakers

It’s really difficult, especially in the bass region. Things like double acoustically isolated walls that are really expensive.

Usually thick insulation because you need mass to block bass.

Like rockwool: https://www.rockwool.com/north-america/products-and-applicat...

Poured concrete, or whatever you might do that is as close to thick, airtight, poured concrete barriers as possible.

Eliminate all holes, and it just becomes a question of mass.

Mass Loaded Vinyl Soundproofing

I think when you say people you mean “builders.” I think people, I.e., the people who actually live in the homes, prefer quiet.

It's both, isn't it? The paper talks about measuring the effect of having both the front and the back bare or coated.