App Nap is a new power saving feature in Mac OS X 10.9 Mavericks. When an application is completely hidden behind other windows and is not currently executing a task like playing music or checking for e-mail, Mavericks will deem that app “inactive” and put it to sleep to save power and free up your computer’s resources. This can have an adverse effect on your audio applications.
Luckily, you can easily turn off App Nap for individual applications. To do so, simply find the application for which you want to disable App Nap in Finder. Then, right-click on that application and choose Get Info. From that window, you can check the box that says “Prevent App Nap.”