What is Reddit's opinion of Room Acoustics?

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If you want to learn this properly, you'll have to study, there are some nice textbooks (in English) that can get you started.

There's also a lot to read on the Internet, but much of it is ill informed and/or skewed by local norms (Vietnam is not the only noisy place, but it's differently noisy to many others).

As for sound absorption in a cafe, there's three main principles I advise my clients:

1. Direct noise. People speaking and eating will always be heard a certain distance, you want to hear people across your table, but maybe not from two tables over. Consider putting distance, or barriers, to isolate adjacent guest parties. Classically, you can use a booth style architecture, but you could as well use book cases and other clever ways to break up direct ("sight") lines. Just remember that people sitting will be at a different height (have been to places who used wood screens with openings at seating level, defeating the entire point)

During covid a lot of places set up acrylic screens, these will also work.

1. Noise absorption. Any and all sounds inside will bounce around until absorbed by something. The ideal placement for absorbents is on the first surface they noise will reflect from. That means for chair scraping and shoe shuffling, you might want absorption on the underside of tables, or the front of your cafe counter. For speech, it will mean at head height on the back and side walls, both for standing and sitting heights.

Absorbents are basically any porous material, and most often it's hygiene and/or looks who dictate exactly what material is used. Commercial products commonly use mineral wool, but you can use heavy textiles, books, cushioned furniture. Also, people are great absorbents, they're just also noisy.

Often you'll want to put the absorbents up behind/in something, just remember to make it porous, so the sound can get through. Perforated wood/metal panels are common, but you can use wire mesh or a thin t-shirt cloth as well. Just remember that any hard stuff, like glue or paint, will diminish the effect, often quite drastically.

Bonus point: different depths of absorbent will catch different frequencies of noise. A 4 cm absorbent (4 cm of mineral wool in a holder) will catch almost all audible noise. If you have trouble with bass or traffic noise, you will want some more specific treatment with deeper absorbents. This also ties in with the next major point.

1. Diffusion. In most cafés we want a nice background ambiance, we want to know we're among people, but not be interrupted by them. As a reference, high end hotel lobbies are often good at this, you want the murmur of activity, but not hear any actual words. As a bonus, diffusion both masks/drowns out some noise, and makes absorption easier.

Diffusion is basically breaking up sounds so that we no long recognise them as a specific sound, so that it registers only as background noise.

Diffusion is done by making your surfaces acoustically uneven. You want to break up flat areas to a mix between 0.5 - 15 cm. Often this is done by having stuff on your walls, shelves with objects, decorative things, tiling (bricks, slate, wood in uneven, but interesting shapes), cabinets, serving stations, window niches, etc. Everything helps. Just remember that you want at least some breaks to be at head height, and to avoid too large flat areas (E.g. covering your bookcase with glass doors defeats the point).

A fancy way of doing this is to have absorbents of different thicknesses, there's commercial products that do this, but you can easily achieve the same effect by having a stuffed couch, a bookcase with books of different sizes as well as other objects, having your wall absorbents of different thickness, etc.