Once installed, Noizio lives in your menubar.
Adjust the sliders until you have the right mix, and then get to work.
Noizio is free and available now in the Mac App Store.
Hit the link below to try it out.
Noizio(Free) | Mac App Store viaNoizio
Noizio Plays Coffee Shop and Other Ambient Sounds to Help You Focus
Mac: There are plenty of tools to help you focus with a little ambient sound, whether it's raindrops falling, the buzz of a coffee shop, or so
