What is the difference between a soundbar, 5.1 and Dolby Atmos?

They describe increasing levels of spatial audio capability, and the labels are used loosely enough that a box saying "Atmos" may deliver very little of it.

Stereo (2.0) — two channels, left and right.

2.1 — stereo plus a subwoofer for low frequencies. The .1 always denotes the low-frequency effects channel, which is why the notation works this way.

5.1 — the standard surround configuration: front left, centre, front right, surround left, surround right, plus a subwoofer. The centre channel carries most dialogue, which is why a proper centre speaker improves speech clarity more than any other single upgrade.

7.1 adds two rear surrounds.

Dolby Atmos is a different concept rather than simply more speakers. Traditional surround assigns sound to channels; Atmos treats sounds as objects with positions in three-dimensional space, and the playback system renders those objects to whatever speakers are actually present. It adds height — written as 5.1.2 or 7.1.4, where the final digit is the number of height channels.

Where the marketing gets loose. A soundbar advertising Atmos may have:

Genuine upward-firing drivers bouncing sound off the ceiling — which works reasonably in a room with a flat, hard ceiling of ordinary height, and poorly otherwise.

Only virtualisation — psychoacoustic processing creating an impression of height from forward-firing drivers. Effects vary from convincing to negligible, and it is frequently what "Atmos support" means.

Practical guidance:

A decent soundbar with a real subwoofer beats built-in TV speakers by an enormous margin, and that first step delivers most of the available improvement.

Dialogue clarity is what most people actually want. A centre channel or a bar with a dedicated dialogue mode addresses it.

Check the connection. Atmos in lossless formats requires eARC, not ARC.

Room and placement matter more than channel count.