iOS Musician Apps (115) Supported Technologies is Inter-App Audio (IAA), Instrument Type is Synth

SpringSound is a physical-modeling audio synthesizer that produces sounds by simulating interactions and vibrations of a set of activators and masses connected to springs.

Professional Virtual Analogue Synthesizer.

Pure Synth® Platinum is a full-blown rompler/synthesizer for your iOS device. Neither the synth engine or the sample engine is an afterthought: Both engines are fully capable and feature-rich. The romper section contains a well rounded assortment of analog, synth, string, digital, and acoustic instruments.

AudioKit Synth One is a completely free & open-source professional iOS Synthesizer. 

Pixound provides a deep musical experience that lets you instantly create music using any picture.

Shoom is an expressive XY pad synthesizer. Or, more precisely, three identical synthesizers in one app.

Xynthesizr is a streamlined yet capable 32-step matrix sequencer/monotimbral synthesizer with generative features and MIDI.

Just a Toy Synthesizer. Included: 2 Oscillators, LPF, Delay effector, Ribbon controller.

KQ MiniSynth is a polyphonic modular synthesizer.

Attack Drums is a synthesizer designed mainly for drum sounds, but the powerful sound engine can also be used for everything else - including vocals.

Phase84 is a phase distortion synthesizer.

Fed up with standardized presets and instruments? Feel lost in overcomplex filters and effects? Don’t wanna sound like everyone else? BLEASS is just the right instrument for you: it unleashes your inner musical creativity by bringing music production and performance to its very core: play, perform, compose beats and melodies in a new yet intuitive way thanks to its multitrack sequencer, synthesizer and drum machine.

This synthesizer emulates male voice vowels in a unique way.

Tines emulates an electric piano.

Syndt is a polyphonic synthesizer with a lot of possibilities.

Baervaag brings FM synthesis to the masses! An easy to use but still tweakable synthesizer, accessible to everyone.

Epic 64-voice Virtual Analog / PCM iPad Synth. Inspired by the Digital Synths of the 80s and 90s.

AudioKit Bass 808 Synth is perfect for earth-shaking sub bass lines and creating your own custom electronic drum sounds – It’s the ultimate tool for creating your own 808 Kick basses!

Cube Synth is a groundbreaking new software instrument giving you the power and flexibility of additive synthesis together with easy editing and morphing capabilities.

Tera Synth is a Modular Analog Synthesizer.