1. Double Bass is a *SOUND PLUGIN* for another app called Functional Ear Trainer. It's not a stand-alone application!!! Install FET first: https://play.google.com/store/apps/details?id=com.kaizen9.fet.android
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.