Soundboards vary in performance and will affect the results you get with SigJenny.  A soundboard's clock frequency will affect the upper frequency that SigJenny can generate and a soundboard's mixer device will determine how accurately the output level can be set.  Current soundboards pretty much come in two flavours; plain vanilla with a clock frequency of 44.1kHz limiting SigJenny to a highest frequency of 20kHz, and the posher boards, like the Soundblaster Audigy, that have a 96KHz clock frequency enabling SigJenny to achieve to 40kHz.