... What do you mostly hear in a room with a lot of reflections? It's the comb filtering, flutter-echo or echo if it's a large venue. ...
Flutter echo is really bad on small apartment rooms with bare walls, really bad, it seems to be the dominant source of "noise" basically. Really long sustaining robot sound if you clap hands, really the most easiest to perceive so most dominant distraction. It makes conversation difficult for example, the whole room could seem very noisy due to flutter echo. This is fun and easy practical test in your livingroom everyone, try standing up clap your hands loud and it's quite easy to trigger the flutter echo in a typical apartment room with bare parallel walls, you must be between at least two bare walls to have it.
Now, clap your hands near floor level while having your ears below furnishing level. The flutter echo isn't nearly as bad due to furniture breaking sound between the parallel walls! Flutter echo is very local effect in a way, very easy to take out by bookshelf with stuff, or almost anything really, that reflects / absorbs mid range wavelengths, and it's easy to test, just clap your hands in various locations in your room, between bare/ obstructed walls.
ps. anyone trying to imagine the reflections, here is some handy tips to have a perspective to it:
Blink of an eye lasts about 100ms and sound travels 343m/s so in a blink of an eye sound travels about 34m. So, in a small room whose walls are 5m apart sound reflects roughly seven times within blink of an eye. I can easily blink my eyes multiple times before hand clap flutter dies away, but on a spot where there is no flutter I hear hardly any "reverb", so there is basically no "late reverberation" in my apartment living room, or at least it's very low in level compared to the clap and flutter.
To my understanding late reverberation would be roughly at the same SPL through out the room, measured at any position, as per definition. Assuming late reverberation makes good acoustic environment, I would have to enhance it somehow, while simultaneously try and kill the early reflections somehow to shift relative balance of sounds in the room from early to late reflections, right, based on the hand clap sound test. This is why one should not absorb too much as it would also kill the late reverberation there is hardly any to begin with ( in a domestic room ). Hence it makes sense to try and only kill worst offenders, earliest loudest early specular reflections, and try and scatter rest of the sound to shift sound energy from early to later arriving (to ear).
Big enough surface area of acoustic treatment is required to cover necessary bandwidth (flutter for example), which I do not know but would assume vocal harmonic range so below 1kHz and up to get some clarity to sound. This means important sound wavelengths from say 34cm and shorter so the acoustic treatment needs to be at least this big in size to have any effect, as well as with effective thickness. More like from Schroeder frequency up which is about 1m in wavelenght or a small room but this big acoustic treatment starts to kill the "late reverberation" so some compromise somewhere. I'm sure professional acousticians have a lot to say to this and formalize / correct me, but this is kinda an example of a simplified process for audio hobbyist to tackle things in practical level.