The diffuse field is created by reflected sound, mainly in the mid and high frequency ranges, and ideally distributes it evenly across all frequencies and directions. With a clever choice of speaker placement, the diffuse field even contributes to a better “sense of space” when listening – especially with reflections from the side walls.
In contrast, room modes (often referred to as “room resonances”) are created by structured, standing superpositions of the floor, ceiling, and walls. These are also referred to as “standing waves.” You can imagine this as a bathtub in which the water sloshes back and forth. The decisive factor here is that the wavelength (λ) of a frequency matches the room dimensions (λ/2 corresponds exactly to the distance between the reflecting surfaces). For example, in a room with a length of 4.2 m, there is an axial pronounced mode at around 41 Hz – higher modes then at multiples such as 82 Hz, 124 Hz, and so on. These room modes cause certain low frequencies to be particularly loud or almost inaudible depending on their position in the room – resulting in uneven bass that fluctuates noticeably when the listener moves. This cannot often be remedied 100 percent; depending on the conditions in the room, sound absorbers on the walls, a different speaker setup with a changed distance to the wall, or electronic room correction, which adjusts the levels accordingly, are recommended. Translated with DeepL.com (free version)