Question

how to get dB value for 31.5 Hz?

Answer 1

1 decibel (dB) means sound pressure level and can be expressed as

$$I=10\text{ log}\frac{S_1}{S_2},$$

where $\text{log}$ represents decimal logarithm and $S_1, S_2$ - the intensity of two sounds. Therefore, without additional information we cannot convert 31.5 Hz to dB.