Microphone of my smartphone (to determine car engine rpm )

[SteveJG]

Perhaps

Tuner - Frequency Detector, Musical Note.

Tuner Captures audio in real time, analyzes its frequency and determines the corresponding pitch (musical note) based on equal temperament. [tuner] StartTuning Starts audio capture and tone detection. [component_method (41)] StopTuning Stops audio capture and tone detection. [component_method (42)] ToneDetected Fired whenever a tone is detected in the captured audio. [component_event (22)] frequency: The detected frequency in Hertz (Hz). note: The corresponding musical note, such as…

or use the magnifying glass in the upper right hand corner of this page and look for FFT .