I thought I would share the app I use when I do my core work. It takes all the guess work out of timing. I know there are many different ones but just in case someone needed an app to buzz then when to change for the core work. I set mine to 30 second intervals. I found this one because it is easy to use, free and you can have your own music. Also it is on android and Apple.
Interval Timer:Timing for HIIT Training, Workouts by Deltaworks