Simple Spectrum Analyzer using device's microphone. If you want source code, send me a email: ).
Commanche is the official developer of the app which can be found in the Entertainment category. Over 1K users downloaded and installed Spectrum Analyzer and its most recent release was on Jan 23, 2017. According to the latest info, this application is rated 3 on Google Play.
Get the setup package of Spectrum Analyzer 1.0 that is free and check out users' reviews on Droid Informer. The app is built for Android 4.0 and higher. The download of the app doesn't contain viruses or any kind of malware.