SynthScaper requires a quite enough CPU performance for processing when all oscillators and all voices are active. At the maximum using it will be 3 osc x 3 layers x 6 voices = 54 voices. And can playback up to 9 samples (3 osc x 3 layers) at the same time. And also filters, lfo and envelopes for each oscillator. That's a lot.
Therefore this app can works only on 64 bit devices.
I recommended use at least iPhone 6 / iPad Mini 4 / iPad Air 2 and higher because CPU A8 / A8X is enough not only for this app with maximum use but also for work with several another app at the same time thru Inter-App audio.
But you can start SynthScaper even on first 64 devices like iPad Air / iPad Mini 2 or 3 with
CPU A7. It will works. But maybe you will need change some settings in this app for performance optimization especially when you use it with another apps.
read built-in description (use button "?" on the main screen) section "About" - "Performance optimization". For all other devices info from this section also will be useful.
[ More about SynthScaper - Soundscapes synthesizer ]