Tune. Then trust the ear.
Chromatic, or by open strings — guitar, bass, ukulele, violin. Uses your mic.
Tuner · uses your mic
Works for any monophonic source — sing a note, bow a string, play a single piano key. The mic permission is asked once and stays on while you're on this page.
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Frequency
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Cents
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Reference
Tip — for low bass notes, get the mic close. For violin and high notes, anywhere in the room works.