Generate a 440 Hz tone
Download a WAV file holding a 440 Hz sine at -6 dBFS, ten seconds long. That frequency is A4, the reference pitch that ISO 16 fixes and that most tuners assume.
Tuning by beats
Play the tone alongside the note you are tuning. When the two pitches differ, the combined sound pulses at the difference between them: 2 Hz apart produces two pulses a second. Adjust until the pulsing slows and stops.
The method is more precise than judging pitch directly, because the beat rate is easy to hear even when the interval is far too small to identify by ear.
Why a pure sine
A sine carries one frequency and nothing else, so the only beating comes from the fundamental. A richer waveform adds harmonics that beat against your instrument’s harmonics at the same time, producing several overlapping rates and a much vaguer point of agreement.
Other reference pitches
440 Hz is standard but not universal. Many European orchestras tune to 442 or 443 Hz. Baroque and period ensembles commonly use 415 Hz, close to a semitone lower. Some tuning traditions use 432 Hz. Set any of these in the full Tone Generator.
The edge fade
The file fades in and out over 10 ms, which removes the click a tone starting at full amplitude would otherwise produce. It does not change the pitch or the steady level.