Obviously, any information directed at people with no knowledge in area is simplified and possibly also intentionally incorrect.
The simple articles on the internet and brochures assume you are stupid and you need safe way to set up your speakers. So that's why they ask you to place them far from wall. It would be stupid to try to explain physics of wave propagation and interference to people who need to read a leaflet to be able to set up their speakers.
But if you have bit more knowledge it is not difficult to control your situation while also having speakers closer to the wall.
Sound is distorted when you receive multiple waves that are out of phase and especially, as it usually happens, when different wavelengths are received at different phases (some in phase, some completely out of phase).
While it is not possible to remove all reflections in a closed room (and some reflection is actually welcome because we want to feel being in a room rather than void) it is possible to make sure that the ratio of the wave from the speaker to the one from reflections is high enough that we do not perceive them as distortions.
When a speaker is placed close to a wall, any energy radiated from the back of the speaker is reflected. There is also some energy from the front of the speaker, but because of the speaker itself only little of it reaches the wall behind and by the time it is reflected in the direction of the listener there is just not enough energy to be perceived as distortion.
It is important that a lot of this is due to relative distances of sound sources (or reflections). If you sit 1m from the speaker that is 50cm from the wall, the sound from the speaker travels 1m but the reflection travels 2m to reach you. This means the reflection is attenuated by 6dB by distance alone (assuming omnidirectional emission from the speaker and that it is perfectly reflected from the wall).
When a speaker is emitting very close to the wall and uses back port to emit bass, that back port is very close to the wall and not only gets distorted (due to being closer to the wall than wave length) but also reflected with a power very close to the original wave but out of phase, depending on frequency. That's what gives horrible result.
Back ported speakers sound better because they elliminate sound of moving air, but they have disadvantage in placement close to walls. Just use front ported speakers when placing close to wall (see bookshelf speakers) which are designed exactly for that use case.
More tips:
- don't place speakers close to the wall if you are going to sit far from them, because this causes relative ratios to be close
- it is ok to place front ported speakers close to the wall if you are going to sit close to them (for on your desk)
- don't just focus on what's in front of you, if there is a wall behind you it is going to cause reflections that can cause powerful distortions if you are relatively close to it compared to the distance of speakers.
- you can put something soft and permeable behind your speaker and behind your head to reduce high frequency distortions
- you can play with some types of shapes around speakers and behind your head to reduce low frequency reflections distorting your sound. By having different parts of the reflection arrive at different times you reduce perceived effect of the distortion.