SpectrumGen is a spectral synthesizer with a minimalistic pixel interface. It's a bit like the PixelWave, but now you can draw the spectrum of sound!
Also available for iOS.

Key features:
• it is possible to draw the spectrum and play it at the same time;
• export to WAV;
• eight predefined timbres;
• changeable number of octaves (from 2 to 8);
• USB MIDI input (Android 6+).

Dark colors - low frequencies (bass). Bright colors - high frequencies.

Known solutions for some problems:
https://warmplace.ru/android