Originally Posted by citsur86
For that matter, can you give a definition of both.
A null is simply a cancellation, which can be due to the direct sound from a speaker being out of phase with its reflection from a nearby boundary OR a subwoofer and speaker being out of phase at the crossover frequency OR a room resonance that has certain locations in the room where the sound waves bouncing back and forth are meeting out of phase. In all cases, we're talking about a cancellation that is phase-based (as opposed to something level-based, like a dip that is deliberately designed into a speaker's frequency response, which might look like a mild cancellation in measurements but is not).
Room modes are also called room resonances. Like when you blow across an empty Coke bottle and get the air inside to resonate (make that booooohhhh sound). Enlarge that chamber to the size of your room and you can still get the air to resonate (of course at different frequencies than the empty bottle). As sound waves bounce around your room, they'll be meeting each other in phase, combining constructively to create peaks as well as meeting each other out of phase, combing destructively to create nulls.
Since different frequencies have different wavelengths, their sound waves bouncing between opposite boundaries will create peaks & nulls at various locations. And those locations will change with each bounce EXCEPT at frequencies whose wavelenghts happens to match or are multiples of a room dimension. Since those waves complete a cycle with each bounce, they always line up with themselves with each return trip. Which means the locations of their peaks & nulls don't change and can be predicted with a room mode calculator. Because those resonances appear to be standing still, they're sometimes referred to as standing waves.
That's one of the things that separate room modes (aka room resonances, aka standing waves) from other frequencies: their peaks & nulls remain in the same predictable locations instead of changing locations with each bounce (like non-modal frequencies do). And because their wavelengths match room dimensions, they continue to bounce back and forth (resonate) long after other frequencies have lost energy and naturally decayed.