*** ATTENTION! *** 1. Double Bass is a *SOUND PLUGIN* for another app called Functional Ear Trainer. It's not a stand-alone application!!! Install FET first. 2. Moreover you need to have an in-app purchase "Sound Plugins" to enable them. To activate Double Bass: 0. Install it. 1. Run Functional Ear Trainer. 2. Click on the "gear" icon in the Main Menu. 3. Choose "Double Bass" under the "Sounds" setting. 🎶 Enjoy! :)