Polk Audio Signa S4 vs Sonos Arc Ultra

The Signa S4 is the most complete home-cinema package here. It is a genuine 3.1.2 layout: three forward channels, two drivers aimed at the ceiling for Atmos height, and a separate wireless subwoofer included rather than sold as an upgrade. Neither Sonos pick includes a sub at all.

VoiceAdjust is the feature that earns its keep daily. It raises the center channel independently of everything else, which is precisely the fix for the modern mixing habit that buries dialogue under score and effects. BassAdjust does the same job at the other end. The bar is slim enough at around 2.36 inches tall not to block the bottom of a television on a stand.

The compromises are structural. Surround effects are virtual, produced by processing rather than by speakers behind you, so they place sound wider rather than actually behind. The slim cabinet limits how loud the bar goes on its own before it hardens, which the subwoofer masks at the bottom and not in the middle. The sub is another box needing floor space and a mains socket. And there is no Wi-Fi, so music from a phone arrives over Bluetooth at Bluetooth quality.

The Arc Ultra is the bar to buy if the bar is the system. Sonos puts 14 drivers behind 15 class-D amplifiers in a 9.1.4 layout: seven tweeters, six midrange woofers and one Sound Motion woofer, which is the genuinely new part. Instead of one large magnet, it uses four small opposing motors driving the cone from its corners, which lets a shallow driver shift far more air than its depth would normally allow. Sonos claims roughly double the bass of the original Arc.

What that buys in practice is a bar that does not need a subwoofer to sound complete in a normal living room, and a center channel clear enough that reviewers single out dialogue as the biggest improvement over the original Arc. It is a full Sonos speaker, so it streams, groups and Trueplay-tunes like the Beam.

The caveats are worth knowing before you spend. There are no HDMI inputs at all: everything runs through the television and back over eARC, and on a television without an eARC port the Sonos optical adapter is a separate purchase and drops you to compressed surround. It is long, so it needs a wide stand or a wall, and in a small room the extra width mostly makes the bass boomy rather than better. Adding Sonos rears and a sub to complete the system costs more again than the bar. And it is locked to the Sonos app, so the app's reliability is your soundbar's reliability.