The relation between the time and frequency of a sound wave is given by

The relation between time and frequency of a sound wave is given by $\mathrm{f}=\frac{1}{\mathrm{T}}$.
Frequency is defined as the number of oscillations per second, and the time period is defined as the time taken to complete one oscillation.
Suppose the time period of a sound wave is represented by $\mathrm{T}$, and the frequency of the wave is represented by $\mathrm{f}$. 

In that case, it is derived that $\mathrm{f}$ is inversely proportional to the time period of the wave $\mathrm{T}$, and thus, the relation, $\mathrm{f}=\frac{1}{\mathrm{T}}$ is the right equality.