How can a subwoofer expand the soundstage?

I thought its because the subwoofer is omnidirectional in the lower region and thus adds to and expands the soundstage that the conventional speakers, whose sonic signature is directional, can't possibly convey. So, together, you get both directional and omni-directional cues which gives a larger soundstage beyond the mere boundary of the listening room and somewhat takes you to the recording venue instead. Unless you have already a gargantuan full ranger, most speakers can use the help of a good sub or two in this department.

Regards,

Kenobi

I second Kenk168

Roxy54 is on the right track. Notes at any particular frequency have both higher and lower harmonics. A system that isn't truly full range can't totally reproduce the lower harmonics. Adding a sub will allow reproduction of these harmonics which helps with the ambient info and spatial cues as mentioned by Roxy54. So, even music that doesn't seem to have much low frequency info can still be enhanced by the sub giving a better soundstage, imho. That's also the same reason why tweeters with excellent top end extension add to the spatial qualities of many recordings.

Rockyboy… I experienced the same phenomenon you describe when I switched my Theta Casablanca to run a sub in 2 channel Stereo mode with my Usher Be 718’s. Prior to that I was only running the sub for 5.1 home theater. Considering the fact that I already was crossing the loss pass signal to a phantom sub, I really wasn’t expecting image and sound stage to improve. The best way to describe the change is to say it was like setting the Usher’s free.

Stanwall, I am vey surprised to hear the subwoofer manufacturer recommends running the main speaker.
My understanding is there are tremendous benefits to be had by crossing over somewhere around or under 100Hz to a powered sub, even on quite good fairly 'full range' speaker.
Perhaps others can comment.