Standing waves build up between parallel walls. A standing wave occurs when the sound overlaps in the direction it just came from. This will usually have ill effects during tracking.
Sometimes in speaker cabinets a note that is played seems louder than the others. A standing wave is building up inside the cabinet. Or in that rooom you have played in might have parallel walls.
As for 90 deg corners low frequencies build up more than higher ones. Thus an engineer will have to design one mare wall with an angle of less than 55 and no more than 35 as long as that wall is not parallel to any other wall.
As for resonators. Sounds that hit it are just redirected to another location.
Standing waves are created when you have two parallel facing walls. There will be a particular set of frequencies that are reinforced by the distance between the walls (the sound makes exactly one round trip on each cycle of the speaker and the pressure fronts pile up). This is what happens in bathrooms- you probably know one where the deep tones of your voice are tremendously supported (doesn't everybody sing in the shower?). Most rooms have three pairs of parallel surfaces, and the dimensions are usually just right to affect music. An eight foot ceiling, for instance, reinforces 70 hz. ( This is called a room mode.)
This phenomenon can be prevented by designing the room with nonparallel walls. It can be cured in existing rooms by making one of the walls absorptive or by breaking up the flat surfaces. When sound is reflected off a rounded or complex surface, it is diffused. Diffusion spreads the reverberant sound evenly throughout a room, which not only prevents standing waves but also eliminates "dead spots"-- places where components of the sound are missing.
We can break up flat surfaces by hanging large objects called diffusers. The shapes chosen for diffusers are really a matter of taste and cost. Avoid concave curves, which focus sound instead of dispersing it, but otherwise pyramids, lattices, or computer designed random surfaces all work well. The depth of a diffuser determines the lowest frequency that will be affected. A diffuser one foot deep will scatter sound down to 160 hz