The period is defined as the time needed to complete one vibration, and it is the reciprocal of the frequency:
[tex]T= \frac{1}{f} [/tex]
where f is the frequency.
The frequency of the instruments is f=440 Hz, so according to the previous formula, the period of one vibration is
[tex]T= \frac{1}{f}= \frac{1}{440 Hz}=2.27 \cdot 10^{-3} s=2.27 ms [/tex]
