Final answer:
A piano tuner uses a 440 Hz tuning fork when tuning a piano. The tuning is done by striking the tuning fork and piano key at the same time, then listening for a 'beat frequency'. This process is repeated until the beats disappear, indicating that the piano string has been tuned correctly.
Step-by-step explanation:
A piano tuner would typically use a 440 Hz tuning fork when tuning a piano. This particular frequency, also referred to as 'A4' note, is conventionally used as a standard for musical pitch.
In tuning a piano, the tuner adjusts the piano string until the sound it produces matches the pitch of the tuning fork - an instant when the piano and the tuning fork resonate at the exact same frequency. This is done by striking the tuning fork and piano key at the same time, then listening for the 'beat frequency'. If there is a beat frequency, this means that the piano and tuning fork are not yet in harmony.
The tuner then tightens or loosens the piano string, strikes the key again and listens for the beats. This process is repeated until the beats disappear, indicating that the piano string has been tuned correctly to the desired frequency.
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