iOS Developer for Baseball App

We have a live iOS app for baseball hitters (Attack Tee — Smart Hitting) and need one new feature built: Timing Training. The hitter's phone mounts in our patented holder on top of a batting tee, inches from the ball. The app plays a countdown, then measures when the bat hits the ball, to the millisecond, and tells the hitter whether they were early, on time, or late. The core of this job is audio. The phone's mic must reliably detect the crack of the bat and timestamp it accurately. Everything else is standard UI work. If you have not done real-time audio work in iOS, this is not the right job for you. What the audio module must do: AVAudioEngine input tap in measurement mode (no AGC, no noise suppression) Timestamp contact from the sample offset in the buffer, aligned to the host clock — not at callback time Detect by transient onset, not volume threshold (the app plays its own sounds) Compensate audio output latency so the cues the hitter hears are correctly aligned Target accuracy: ±5 ms against a reference signal What we provide: A complete, working interactive prototype with all logic, math, layout, and copy — this is your spec A detailed SOW with the timing model, feature list, and acceptance criteria Design assets and brand fonts Existing app codebase Structure: Milestone 1 is the audio module only, paid on hitting the accuracy criteria. We do this first because it's the only part that can genuinely fail. Milestones 2–4 are the UI and session logic. Required: Swift, AVFoundation/AVAudioEngine, real-time audio processing, shipped iOS apps. Nice to have: DSP or onset-detection experience, baseball knowledge. Please include in your proposal: A real-time audio feature you've shipped, and what the timing accuracy requirement was How you'd timestamp a transient to millisecond accuracy on iOS How you'd stop the app's own audio output from false-triggering the detector