Not sure which forum to write this on -- picked this one. Last night saw the great Shawn Mullins at a small venue with probably no more than 100 people - It was him and one other guy on electric and acoustic.....as he played there was a distinct back beat,  - pretty bassy, subtle, but in perfect tempo with his playing....I thought it might be someone foot tapping, but not. So my question is - what do you think was generating this? there was a laptop involved at the sound system booth - so thought perhaps it was from there --- or maybe from a stomp box they had on stage???

Could have been an audio file played on the laptop. A stomp box type of looper could have done the same thing. Impossible to say. I did find credit to different people in a couple of his discography pages for loops, so he does have that going on.... though there is no way to tell what was playing those loops. You could always try to contact him through his website.

 I took you advice and wrote my question on the Shawn Mullins website and got a nice response for those interested: 

Hey Charles, thanks for coming. We really enjoyed that venue and the food! What you hearing is a the sound of Shawn hitting the bridge of his guitars (where the pick-up is located) with the area between his wrist and palm. I extunate that sound on certain songs simply by increasing the 50Hz on the channel strip. It's all organic, no computers are used except for me to monitor the response of the sound system and keep these things under control. Too much of the low frequency would cause the sysem to go into a feedback loop so the computer helps me get the maximum out of any system before that happens. 


Some people use something that I have heard being referred to as a boot bar. It's an electric device that is placed at peoples feet to create the same effect but it's typically used on downbeats like a kick drum. 


-kip
Shawn Mullins Production Mgr / Sound Engineer