The only good free dual n-back app

There appear to a number of paid dual n-back apps out there, but this is the only one Ive found that is solely ad-supported. Its minimal, doesnt let you change sound sets or do any other customization but its free! And it works great. Totally solid!
I would love the ability to change sound sets though, as a musician Id like to be able to use notes of the scale instead of letters.

JQ the cub about Dual N-Back, v2.3.6